phosphorescence and sulphide of zinc 367 physiological effects of high frequency 162, 394 polyphase systems 26 polyphase transformer 109 pyromagnetic generators 429 regulator for rotary current motors 45 resonance, electric, phenomena of 340 "resultant attraction" 7 rotating field transformers 9 rotating magnetic field 9 royal institution lecture 124 scope of lectures 119 single phase motor 76 single circuit, self-starting synchronizing motors 50 spinning filament effects 168 streaming discharges of high tension coil 155, 163 synchronizing motors 9 telegraphy without wires 246 transformer with shield between primary and secondary 113 thermo-magnetic motors 424 thomson, j. j., on vacuum tubes 397, 402, 406 thomson, sir w., current accumulator 471 transformers: alternating 7 magnetic shield 113 polyphase 109 rotating field 9 tubes: coated with yttria, etc. 187 coated with sulphide of zinc, etc. 290, 367 unipolar generators 465 unipolar generator, forbes 468, 474 yttria, coated tubes 187 zinc, tubes coated with sulphide of 367 warning: this book of one hundred years ago describes experiments which are too dangerous to attempt by either adults or children. it is published for historical interest only. the "how-to-do-it" books electricity for boys [illustration: fig. 1. work bench] the "how-to-do-it" books electricity for boys a working guide, in the successive steps of electricity, described in simple terms with many original illustrations by j. s. zerbe, m.e. author of carpentry for boys practical mechanics for boys [illustration: printer's mark] the new york book company new york copyright, 1914, by the new york book company contents introductory page 1 i. electricity considered. brief historical events page 5 the study of electricity. first historical accounts. bottling electricity. discovery of galvanic electricity. electro-motive force. measuring instruments. rapidity of modern progress. how to acquire the vast knowledge. the means employed. ii. what tools and apparatus are needed page 11 preparing the workshop. uses of our workshop. what to build. what to learn. uses of the electrical devices. tools. magnet-winding reel. iii. magnets, coils, armatures, etc. page 18 the two kinds of magnets. permanent magnets. electro-magnets. magnetism. materials for magnets. non-magnetic material. action of a _second_ magnet. what north and south pole mean. repulsion and attraction. positives and negatives. magnetic lines of force. the earth as a magnet. why the compass points north and south. peculiarity of a magnet. action of the electro-magnet. exterior magnetic influence around a wires carrying a current. parallel wires. iv. frictional, voltaic or galvanic and electro-magnetic electricity page 29 three electrical sources. frictional electricity. leyden jar. voltaic or galvanic electricity. voltaic pile; how made. plus and minus signs. the common primary cell. battery resistance. electrolyte and current. electro-magnetic electricity. magnetic radiation. different kinds of dynamos. direct current dynamos. simple magnet construction. how to wind. the dynamo fields. the armature. armature windings. mounting the armature. the commutator. commutator brushes. dynamo windings. the field. series-wound field. shunt-wound. compound-wound. v. how to detect and measure electricity page 49 measuring instruments. the detector. direction of current. simple current detector. how to place the detector. different ways to measure a current. the sulphuric acid voltameter. the copper voltameter. the galvanoscope electro-magnetic method. the calorimeter. the light method. the preferred method. how to make a sulphuric acid voltameter. how to make a copper voltameter. objections to the calorimeter. vi. volts, amperes, ohms and watts page 60 understanding terms. intensity and quantity. voltage. amperage meaning of watts and kilowatt. a standard of measurement. the ampere standard. the voltage standard. the ohm. calculating the voltage. vii. push buttons, switches, annunciators, bells and like apparatus page 65 simple switches. a two-pole switch. double-pole switch. sliding switch. reversing switch. push buttons. electric bells. how made. how operated. annunciators. burglar alarm. wire circuiting. circuiting system with two bells and push buttons. the push buttons, annunciators and bells. wiring up a house. viii. accumulators, storage or secondary batteries page 82 storing up electricity. the accumulator. accumulator plates. the grid. the negative pole. connecting up the plates. charging the cells. the initial charge. the charging current. ix. the telegraph page 90 mechanism in telegraph circuit. the sending key. the sounder. connecting up the key and sounder. two stations in circuit. the double click. illustrating the dot and the dash. the morse telegraph code. example in use. x. high-tension apparatus, condensers, etc. page 98 induction. low and high tension. elastic property of electricity. the condenser. connecting up a condenser. the interrupter. uses of high-tension coils. xi. wireless telegraphy page 104 telegraphing without wires. surging character of high-tension currents. the coherer. how made. the decoherer. the sending apparatus. the receiving apparatus. how the circuits are formed. xii. the telephone page 110 vibrations. the acoustic telephone. sound waves. hearing electricity. the diaphragm in a magnetic field. a simple telephone circuit. how to make a telephone. telephone connections. complete installation. the microphone. light contact points. how to make a microphone. microphone, the father of the transmitter. automatic cut-outs for telephones. complete circuiting with transmitters. xiii. electrolysis, water purification, electroplating page 123 decomposing liquids. making hydrogen and oxygen. purifying water. rust. oxygen as a purifier. composition of water. common air not a good purifier. pure oxygen a water purifier. the use of hydrogen in purification. aluminum electrodes. electric hand purifier. purification and separation of metals. electroplating. plating iron with copper. direction of current. xiv. electric heating. thermo-electricity page 135 generating heat in a wire. resistance of substances. signs of connectors. comparison of metals. a simple electric heater. how to arrange for quantity of current used. an electric iron. thermo-electricity converting heat directly into electricity metals. electric, positive, negative. thermo-electric coupler. xv. alternating currents, choking coil, transformer page 145 direct current. alternating current. the magnetic field. action of a magnetized wire. the movement of a current in a charged wire. current reversing itself. self-induction. brushes in a direct current dynamo: alternating, positive and negative poles. how an alternating current dynamo is made. the windings. the armature wires. choking coils. the transformer. how the voltage is determined. voltage and amperage in transformers. xvi. electric lighting page 161 early conditions. fuels. reversibility of dynamo. electric arc. mechanism to maintain the arc. resistance coil. parallel carbons for making arc. series current. incandescent system. multiple circuit. subdivision of electric light. the filament. the glass bulb. metallic filaments. vapor lamps. directions for improvements. heat in electric lighting. curious superstitions concerning electricity. magnetism. amber. discovery of the properties of a magnet. electricity in mountain regions. early beliefs as to magnetism and electricity. the lightning rod. protests against using it. pliny's explanation of electricity. xvii. power, and various other electrical manifestations page 175 early beliefs concerning the dynamo. experiments with magnets. physical action of dynamo and motor. electrical influence in windings. comparing motor and dynamo. how the current acts in a dynamo. its force in a motor. loss in power transmission. the four ways in which power is dissipated. disadvantages of electric power. its advantages. transmission of energy. high voltages. the transformer. step-down transformers. electric furnaces. welding by electricity. merging the particles of the joined ends. xviii. x-ray, radium and the like page 184 the camera and the eye. actinic rays. hertzian waves. high-tension apparatus. vacuum tubes. character of the ultra-violet rays. how distinguished. the infra-red rays. their uses. x-rays not capable of reflection. not subject to refraction. transmission through opaque substances. reducing rates of vibration. radium. radio-activity. radio-active materials. pitchblende. a new form of energy. electrical source. healing power. problems for scientists. list of illustrations fig. 1. work bench frontispiece page 2. top of magnet-winding reel 14 3. side of magnet-winding reel 14 4. journal block 15 5. plain magnet bar 19 6. severed magnet 20 7. reversed magnets 21 8. horseshoe magnet 22 9. earth's magnetic lines 23 10. two permanent magnets 24 11. magnets in earth's magnetic field 24 12. armatures for magnets 25 13. magnetized field 26 14. magnetized bar 26 15. direction of current 27 16. direction of induction current 28 17. frictional-electricity machine 30 18. leyden jar 32 19. galvanic electricity. crown of cups 33 20. voltaic electricity 34 21. primary battery 36 22. dynamo field and pole piece 39 23. base and fields assembled 41 24. details of the armature, core 42 25. details of the armature, body 42 26. armature journals 43 27. commutator 43 28. end view of armature, mounted 44 29. top view of armature on base 45 30. field winding 47 31. series-wound 47 32. shunt-wound 48 33. compound-wound 48 34. compass magnet, swing to the right 50 35. magnetic compass 50 36. magnet, swing to the left 50 37. indicating direction of current 51 38. the bridge of the detector 52 39. details of detector 53 40. cross-section of detector 54 41. acid voltameter 56 42. copper voltameter 56 43. two-pole switch 66 44. double-pole switch 66 45. sliding switch 67 46. rheostat form of switch 68 47. reversing switch 69 48. push button 70 49. electric bell 71 50. armature of electric bell 72 51. vertical section of annunciator 72 52. front view of annunciator 72 53. horizontal section of annunciator 72 54. front plate of annunciator 72 55. alarm switch on window 76 56. burglar alarm on window 76 57. burglar alarm contact 77 58. neutral position of contact 78 59. circuiting for electric bell 79 60. annunciators in circuit 80 61. wiring system for a house 80 62. accumulator grids 83 63. assemblage of accumulator grids 85 64. connecting up storage battery in series 87 65. parallel series 88 66. charging circuit 88 67. telegraph sending key 91 68. telegraph sounder 92 69. a telegraph circuit 94 70. induction coil and circuit 99 71. illustrating elasticity 100 72. condenser 101 73. high-tension circuit 102 74. current interrupter 103 75. wireless-telegraphy coherer 105 76. wireless sending-apparatus 107 77. wireless receiving-apparatus 108 78. acoustic telephone 111 79. illustrating vibrations 111 80. the magnetic field 112 81. section of telephone receiver 114 82. the magnet and receiver head 115 83. simple telephone connection 116 84. telephone stations in circuit 117 85. illustrating light contact points 118 86. the microphone 119 87. the transmitter 119 88. complete telephone circuit 121 89. device for making hydrogen and oxygen 124 90. electric-water purifier 127 91. portable electric purifier 129 92. section of positive plate 130 93. section of negative plate 130 94. positive and negative in position 130 95. form of the insulator 130 96. simple electric heater 137 97. side view of resistance device 139 98. top view of resistance device 139 99. plan view of electric iron 140 100. section of electric iron 141 101. thermo-electric couple 143 102. cutting a magnetic field 146 103. alternations, first position 148 104. alternations, second position 148 105. alternations, third position 148 106. alternations, fourth position 148 107. increasing alternations, first view 149 108. increasing alternations, second view 149 109. connection of alternating dynamo armature 150 110. direct current dynamo 151 111. circuit wires in direct current dynamo 152 112. alternating polarity lines 154 113. alternating current dynamo 155 114. choking coil 157 115. a transformer 158 116. parallel carbons 164 117. arc-lighting circuit 165 118. interrupted conductor 166 119. incandescent circuit 167 120. magnetic action in dynamo, 1st 177 121. magnetic action in dynamo, 2d 177 122. magnetic action in dynamo, 3d 178 123. magnetic action in dynamo, 4th 178 124. magnetic action in motor, 1st 179 125. magnetic action in motor, 2d 179 126. magnetic action in motor, 3d 180 127. magnetic action in motor, 4th 180 introductory electricity, like every science, presents two phases to the student, one belonging to a theoretical knowledge, and the other which pertains to the practical application of that knowledge. the boy is directly interested in the practical use which he can make of this wonderful phenomenon in nature. it is, in reality, the most successful avenue by which he may obtain the theory, for he learns the abstract more readily from concrete examples. it is an art in which shop practice is a greater educator than can be possible with books. boys are not, generally, inclined to speculate or theorize on phenomena apart from the work itself; but once put them into contact with the mechanism itself, let them become a living part of it, and they will commence to reason and think for themselves. it would be a dry, dull and uninteresting thing to tell a boy that electricity can be generated by riveting together two pieces of dissimilar metals, and applying heat to the juncture. but put into his hands the metals, and set him to perform the actual work of riveting the metals together, then wiring up the ends of the metals, heating them, and, with a galvanometer, watching for results, it will at once make him see something in the experiment which never occurred when the abstract theory was propounded. he will inquire first what metals should be used to get the best results, and finally, he will speculate as to the reasons for the phenomena. when he learns that all metals are positive-negative or negative-positive to each other, he has grasped a new idea in the realm of knowledge, which he unconsciously traces back still further, only to learn that he has entered a field which relates to the constitution of matter itself. as he follows the subject through its various channels he will learn that there is a common source of all things; a manifestation common to all matter, and that all substances in nature are linked together in a most wonderful way. an impulse must be given to a boy's training. the time is past for the rule-and-rote method. the rule can be learned better by a manual application than by committing a sentence to memory. in the preparation of this book, therefore, i have made practice and work the predominating factors. it has been my aim to suggest the best form in which to do the things in a practical way, and from that work, as the boy carries it out, to deduce certain laws and develop the principles which underlie them. wherever it is deemed possible to do so, it is planned to have the boy make these discoveries for himself, so as to encourage him to become a thinker and a reasoner instead of a mere machine. a boy does not develop into a philosopher or a scientist through being told he must learn the principles of this teaching, or the fundamentals of that school of reasoning. he will unconsciously imbibe the spirit and the willingness if we but place before him the tools by which he may build even the simple machinery that displays the various electrical manifestations. chapter i the study of electricity. historical there is no study so profound as electricity. it is a marvel to the scientist as well as to the novice. it is simple in its manifestations, but most complex in its organization and in its ramifications. it has been shown that light, heat, magnetism and electricity are the same, but that they differ merely in their modes of motion. first historical account.--the first historical account of electricity dates back to 600 years b. c. thales of miletus was the first to describe the properties of amber, which, when rubbed, attracted and repelled light bodies. the ancients also described what was probably tourmaline, a mineral which has the same qualities. the torpedo, a fish which has the power of emitting electric impulses, was known in very early times. from that period down to about the year 1600 no accounts of any historical value have been given. dr. gilbert, of england, made a number of researches at that time, principally with amber and other materials, and boyle, in 1650, made numerous experiments with frictional electricity. sir isaac newton also took up the subject at about the same period. in 1705 hawksbee made numerous experiments; also gray, in 1720, and a welshman, dufay, at about the same time. the germans, from 1740 to 1780, made many experiments. in 1740, at leyden, was discovered the jar which bears that name. before that time, all experiments began and ended with frictional electricity. the first attempt to "bottle" electricity was attempted by muschenbr[oe]ck, at leyden, who conceived the idea that electricity in materials might be retained by surrounding them with bodies which did not conduct the current. he electrified some water in a jar, and communication having been established between the water and the prime conductor, his assistant, who was holding the bottle, on trying to disengage the communicating wire, received a sudden shock. in 1747 sir william watson fired gunpowder by an electric spark, and, later on, a party from the royal society, in conjunction with watson, conducted a series of experiments to determine the velocity of the electric fluid, as it was then termed. benjamin franklin, in 1750, showed that lightning was electricity, and later on made his interesting experiments with the kite and the key. discovering galvanic electricity.--the great discovery of galvani, in 1790, led to the recognition of a new element in electricity, called galvanic or voltaic (named after the experimenter, volta), and now known to be identical with frictional electricity. in 1805 poisson was the first to analyze electricity; and when [oe]rsted of copenhagen, in 1820, discovered the magnetic action of electricity, it offered a great stimulus to the science, and paved the way for investigation in a new direction. ampere was the first to develop the idea that a motor or a dynamo could be made operative by means of the electro-magnetic current; and faraday, about 1830, discovered electro-magnetic rotation. electro-magnetic force.--from this time on the knowledge of electricity grew with amazing rapidity. ohm's definition of electro-motive force, current strength and resistance eventuated into ohm's law. thomson greatly simplified the galvanometer, and wheatstone invented the rheostat, a means of measuring resistance, about 1850. then primary batteries were brought forward by daniels, grove, bunsen and thomson, and electrolysis by faraday. then came the instruments of precision--the electrometer, the resistance bridge, the ammeter, the voltmeter--all of the utmost value in the science. measuring instruments.--the perfection of measuring instruments did more to advance electricity than almost any other field of endeavor; so that after 1875 the inventors took up the subject, and by their energy developed and put into practical operation a most wonderful array of mechanism, which has become valuable in the service of man in almost every field of human activity. rapidity of modern progress.--this brief history is given merely to show what wonders have been accomplished in a few years. the art is really less than fifty years old, and yet so rapidly has it gone forward that it is not at all surprising to hear the remark, that the end of the wonders has been reached. less than twenty-five years ago a high official of the united states patent office stated that it was probable the end of electrical research had been reached. the most wonderful developments have been made since that time; and now, as in the past, one discovery is but the prelude to another still more remarkable. we are beginning to learn that we are only on the threshold of that storehouse in which nature has locked her secrets, and that there is no limit to human ingenuity. how to acquire the vast knowledge.--as the boy, with his limited vision, surveys this vast accumulation of tools, instruments and machinery, and sees what has been and is now being accomplished, it is not to be wondered at that he should enter the field with timidity. in his mind the great question is, how to acquire the knowledge. there is so much to learn. how can it be accomplished? the answer to this is, that the student of to-day has the advantage of the knowledge of all who have gone before; and now the pertinent thing is to acquire that knowledge. the means employed.--this brings us definitely down to an examination of the means that we shall employ to instil this knowledge, so that it may become a permanent asset to the student's store of information. the most significant thing in the history of electrical development is the knowledge that of all the great scientists not one of them ever added any knowledge to the science on purely speculative reasoning. all of them were experimenters. they practically applied and developed their theories in the laboratory or the workshop. the natural inference is, therefore, that the boy who starts out to acquire a knowledge of electricity, must not only theorize, but that he shall, primarily, conduct the experiments, and thereby acquire the information in a practical way, one example of which will make a more lasting impression than pages of dry text. throughout these pages, therefore, i shall, as briefly as possible, point out the theories involved, as a foundation for the work, and then illustrate the structural types or samples; and the work is so arranged that what is done to-day is merely a prelude or stepping-stone to the next phase of the art. in reality, we shall travel, to a considerable extent, the course which the great investigators followed when they were groping for the facts and discovering the great manifestations in nature. chapter ii what tools and apparatus are needed preparing the workshop.--before commencing actual experiments we should prepare the workshop and tools. since we are going into this work as pioneers, we shall have to be dependent upon our own efforts for the production of the electrical apparatus, so as to be able, with our home-made factory, to provide the power, the heat and the electricity. then, finding we are successful in these enterprises, we may look forward for "more worlds to conquer." by this time our neighbors will become interested in and solicit work from us. uses of our workshops.--they may want us to test batteries, and it then becomes necessary to construct mechanism to detect and measure electricity; to install new and improved apparatus; and to put in and connect up electric bells in their houses, as well as burglar alarms. to meet the requirements, we put in a telegraph line, having learned, as well as we are able, how they are made and operated. but we find the telegraph too slow and altogether unsuited for our purposes, as well as for the uses of the neighborhood, so we conclude to put in a telephone system. what to build.--it is necessary, therefore, to commence right at the bottom to build a telephone, a transmitter, a receiver and a switch-board for our system. from the telephone we soon see the desirability of getting into touch with the great outside world, and wireless telegraphy absorbs our time and energies. but as we learn more and more of the wonderful things electricity will do, we are brought into contact with problems which directly interest the home. sanitation attracts our attention. why cannot electricity act as an agent to purify our drinking water, to sterilize sewage and to arrest offensive odors? we must, therefore, learn something about the subject of electrolysis. what to learn.--the decomposition of water is not the only thing that we shall describe pertaining to this subject. we go a step further, and find that we can decompose metals as well as liquids, and that we can make a pure metal out of an impure one, as well as make the foulest water pure. but we shall also, in the course of our experiments, find that a cheap metal can be coated with a costly one by means of electricity--that we can electroplate by electrolysis. uses of the electrical devices.--while all this is progressing and our factory is turning out an amazing variety of useful articles, we are led to inquire into the uses to which we may devote our surplus electricity. the current may be diverted for boiling water; for welding metals; for heating sad-irons, as well as for other purposes which are daily required. tools.--to do these things tools are necessary, and for the present they should not be expensive. a small, rigidly built bench is the first requirement. this may be made, as shown in fig. 1, of three 2-inch planks, each 10 inches wide and 6 feet long, mounted on legs 36 inches in height. in the front part are three drawers for your material, or the small odds and ends, as well as for such little tools as you may accumulate. then you will need a small vise, say, with a 2-inch jaw, and you will also require a hand reel for winding magnets. this will be fully described hereafter. you can also, probably, get a small, cheap anvil, which will be of the greatest service in your work. it should be mounted close up to the work bench. two small hammers, one with an a-shaped peon, and the other with a round peon, should be selected, and also a plane and a small wood saw with fine teeth. a bit stock, or a ratchet drill, if you can afford it, with a variety of small drills; two wood chisels, say of 3/8-inch and 3/4-inch widths; small cold chisels; hack saw, 10-inch blade; small iron square; pair of dividers; tin shears; wire cutters; 2 pairs of pliers, one flat and the other round-nosed; 2 awls, centering punch, wire cutters, and, finally, soldering tools. [illustration: _fig. 2. top view_ magnet-winding reel] [illustration: _fig. 3. side view_ magnet-winding reel] if a gas stove is not available, a brazing torch is an essential tool. numerous small torches are being made, which are cheap and easily operated. a small soldering iron, with pointed end, should be provided; also metal shears and a small square; an awl and several sizes of gimlets; a screwdriver; pair of pliers and wire cutters. from the foregoing it will be seen that the cost of tools is not a very expensive item. this entire outfit, not including the anvil and vise, may be purchased new for about $20.00, so we have not been extravagant. magnet-winding reel.--some little preparation must be made, so we may be enabled to handle our work by the construction of mechanical aids. [illustration: _fig. 4. journal block._] first of these is the magnet-winding reel, a plan view of which is shown in fig. 2. this, for our present work, will be made wholly of wood. select a plank 1-1/2 inches thick and 8 inches wide, and from this cut off two pieces (a), each 7 inches long, and then trim off the corners (b, b), as shown in fig. 4. to serve as the mandrel (c, fig. 2), select a piece of broomstick 9 inches long. bore a hole (d) in each block (a) a half inch below the upper margin of the block, this hole being of such diameter that the broomstick mandrel will fit and easily turn therein. place a crank (e), 5 inches long, on the outer end of the mandrel, as in fig. 3. then mount one block on the end of the bench and the other block 3 inches away. affix them to the bench by nails or screws, preferably the latter. on the inner end of the mandrel put a block (f) of hard wood. this is done by boring a hole 1 inch deep in the center of the block, into which the mandrel is driven. on the outer face of the block is a square hole large enough to receive the head of a 3/8-inch bolt, and into the depression thus formed a screw (g) is driven through the block and into the end of the mandrel, so as to hold the block (f) and mandrel firmly together. when these parts are properly put together, the inner side of the block will rest and turn against the inner journal block (a). the tailpiece is made of a 2" ã� 4" scantling (h), 10 inches long, one end of it being nailed to a transverse block (i) 2" ã� 2" ã� 4". the inner face of this block has a depression in which is placed a v-shaped cup (j), to receive the end of the magnet core (k) or bolt, which is to be used for this purpose. the tailpiece (h) has a longitudinal slot (l) 5 inches long adapted to receive a 1/2-inch bolt (m), which passes down through the bench, and is, therefore, adjustable, so it may be moved to and from the journal bearing (a), thereby providing a place for the bolts to be put in. these bolts are the magnet cores (k), 6 inches long, but they may be even longer, if you bore several holes (n) through the bench so you may set over the tailpiece. with a single tool made substantially like this, over a thousand of the finest magnets have been wound. its value will be appreciated after you have had the experience of winding a few magnets. order in the workshop.--select a place for each tool on the rear upright of the bench, and make it a rule to put each tool back into its place after using. this, if persisted in, will soon become a habit, and will save you hours of time. hunting for tools is the unprofitable part of any work. chapter iii magnets, coils, armatures, etc. the two kinds of magnet.--generally speaking, magnets are of two kinds, namely, permanent and electro-magnetic. permanent magnets.--a permanent magnet is a piece of steel in which an electric force is exerted at all times. an electro-magnet is a piece of iron which is magnetized by a winding of wire, and the magnet is energized only while a current of electricity is passing through the wire. electro-magnet.--the electro-magnet, therefore, is the more useful, because the pull of the magnet can be controlled by the current which actuates it. the electro-magnet is the most essential of all contrivances in the operation and use of electricity. it is the piece of mechanism which does the physical work of almost every electrical apparatus or machine. it is the device which has the power to convert the unseen electric current into motion which may be observed by the human eye. without it electricity would be a useless agent to man. while the electro-magnet is, therefore, the form of device which is almost wholly used, it is necessary, first, to understand the principles of the permanent magnet. magnetism.--the curious force exerted by a magnet is called magnetism, but its origin has never been explained. we know its manifestations only, and laws have been formulated to explain its various phases; how to make it more or less intense; how to make its pull more effective; the shape and form of the magnet and the material most useful in its construction. [illustration: _fig 5._ plain magnet bar] materials for magnets.--iron and steel are the best materials for magnets. some metals are non-magnetic, this applying to iron if combined with manganese. others, like sulphur, zinc, bismuth, antimony, gold, silver and copper, not only are non-magnetic, but they are actually repelled by magnetism. they are called the diamagnetics. non-magnetic materials.--any non-magnetic body in the path of a magnetic force does not screen or diminish its action, whereas a magnetic substance will. in fig. 5 we show the simplest form of magnet, merely a bar of steel (a) with the magnetic lines of force passing from end to end. it will be understood that these lines extend out on all sides, and not only along two sides, as shown in the drawing. the object is to explain clearly how the lines run. [illustration: _fig. 6._ severed magnet] action of a severed magnet.--now, let us suppose that we sever this bar in the middle, as in fig. 6, or at any other point between the ends. in this case each part becomes a perfect magnet, and a new north pole (n) and a new south pole (s) are made, so that the movement of the magnetic lines of force are still in the same direction in each--that is, the current flows from the north pole to the south pole. what north and south poles mean.--if these two parts are placed close together they will attract each other. but if, on the other hand, one of the pieces is reversed, as in fig. 7, they will repel each other. from this comes the statement that likes repel and unlikes attract each other. repulsion and attraction.--this physical act of repulsion and attraction is made use of in motors, as we shall see hereinafter. it will be well to bear in mind that in treating of electricity the north pole is always associated with the plus sign (+) and the south pole with the minus sign (-). or the n sign is positive and the s sign negative electricity. [illustration: _fig. 7._ reversed magnets] positives and negatives.--there is really no difference between positive and negative electricity, so called, but the foregoing method merely serves as a means of identifying or classifying the opposite ends of a magnet or of a wire. magnetic lines of force.--it will be noticed that the magnetic lines of force pass through the bar and then go from end to end through the atmosphere. air is a poor conductor of electricity, so that if we can find a shorter way to conduct the current from the north pole to the south pole, the efficiency of the magnet is increased. this is accomplished by means of the well-known horseshoe magnet, where the two ends (n, s) are brought close together, as in fig. 8. the earth as a magnet.--the earth is a huge magnet and the magnetic lines run from the north pole to the south pole around all sides of the globe. [illustration: _fig. 8._ horseshoe magnet] the north magnetic pole does not coincide with the true north pole or the pivotal point of the earth's rotation, but it is sufficiently near for all practical purposes. fig. 9 shows the magnetic lines running from the north to the south pole. why the compass points north and south.--now, let us try to ascertain why the compass points north and south. let us assume that we have a large magnet (a, fig. 10), and suspend a small magnet (b) above it, so that it is within the magnetic field of the large magnet. this may be done by means of a short pin (c), which is located in the middle of the magnet (b), the upper end of this pin having thereon a loop to which a thread (d) is attached. the pin also carries thereon a pointer (e), which is directed toward the north pole of the bar (b). [illustration: _fig. 9._ earth's magnetic lines] you will now take note of the interior magnetic lines (x), and the exterior magnetic lines (z) of the large magnet (a), and compare the direction of their flow with the similar lines in the small magnet (b). the small magnet has both its exterior and its interior lines within the exterior lines (z) of the large magnet (a), so that as the small magnet (b) is capable of swinging around, the n pole of the bar (b) will point toward the s pole of the larger bar (a). the small bar, therefore, is influenced by the exterior magnetic field (z). [illustration: _fig. 10._ two permanent magnets] [illustration: _fig. 11._ magnets in the earth's magnetic field] let us now take the outline represented by the earth's surface (fig. 11), and suspend a magnet (a) at any point, like the needle of a compass, and it will be seen that the needle will arrange itself north and south, within the magnetic field which flows from the north to the south pole. peculiarity of a magnet.--one characteristic of a magnet is that, while apparently the magnetic field flows out at one end of the magnet, and moves inwardly at the other end, the power of attraction is just the same at both ends. in fig. 12 are shown a bar (a) and a horseshoe magnet (b). the bar (a) has metal blocks (c) at each end, and each of these blocks is attracted to and held in contact with the ends by magnetic influence, just the same as the bar (d) is attracted by and held against the two ends of the horseshoe magnet. these blocks (c) or the bar (d) are called armatures. through them is represented the visible motion produced by the magnetic field. [illustration: _fig. 12._ armatures for magnets] action of the electro-magnet.--the electro-magnet exerts its force in the same manner as a permanent magnet, so far as attraction and repulsion are concerned, and it has a north and a south pole, as in the case with the permanent magnet. an electro-magnet is simply a bar of iron with a coil or coils of wire around it; when a current of electricity flows through the wire, the bar is magnetized. the moment the current is cut off, the bar is demagnetized. the question that now arises is, why an electric current flowing through a wire, under those conditions, magnetizes the bar, or _core_, as it is called. [illustration: _fig. 13._ magnetized field] [illustration: _fig. 14._ magnetized bar] in fig. 13 is shown a piece of wire (a). let us assume that a current of electricity is flowing through this wire in the direction of the darts. what actually takes place is that the electricity extends out beyond the surface of the wire in the form of the closed rings (b). if, now, this wire (a) is wound around an iron core (c, fig. 14), you will observe that this electric field, as it is called, entirely surrounds the core, or rather, that the core is within the magnetic field or influence of the current flowing through the wire, and the core (c) thereby becomes magnetized, but it is magnetized only when the current passes through the wire coil (a). [illustration: _fig. 15._ direction of current] from the foregoing, it will be understood that a wire carrying a current of electricity not only is affected within its body, but that it also has a sphere of influence exteriorly to the body of the wire, at all points; and advantage is taken of this phenomenon in constructing motors, dynamos, electrical measuring devices and almost every kind of electrical mechanism in existence. exterior magnetic influence around a wire carrying a current.--bear in mind that the wire coil (a, fig. 14) does not come into contact with the core (c). it is insulated from the core, either by air or by rubber or other insulating substance, and a current passing from a to c under those conditions is a current of _induction_. on the other hand, the current flowing through the wire (a) from end to end is called a _conduction_ current. remember these terms. in this connection there is also another thing which you will do well to bear in mind. in fig. 15 you will notice a core (c) and an insulated wire coil (b) wound around it. the current, through the wire (b), as shown by the darts (d), moves in one direction, and the induced current in the core (c) travels in the opposite direction, as shown by the darts (d). [illustration: _fig. 16._ direction of induction current] parallel wires.--in like manner, if two wires (a, b, fig. 16) are parallel with each other, and a current of electricity passes along the wire (a) in one direction, the induced current in the wire (b) will move in the opposite direction. these fundamental principles should be thoroughly understood and mastered. chapter iv frictional, voltaic or galvanic, and electro-magnetic electricity three electrical sources.--it has been found that there are three kinds of electricity, or, to be more accurate, there are three ways to generate it. these will now be described. when man first began experimenting, he produced a current by frictional means, and collected the electricity in a bottle or jar. electricity, so stored, could be drawn from the jar, by attaching thereto suitable connection. this could be effected only in one way, and that was by discharging the entire accumulation instantaneously. at that time they knew of no means whereby the current could be made to flow from the jar as from a battery or cell. frictional electricity.--with a view of explaining the principles involved, we show in fig. 17 a machine for producing electricity by friction. [illustration: _fig. 17._ friction-electricity machine] this is made up as follows: a represents the base, having thereon a flat member (b), on which is mounted a pair of parallel posts or standards (c, c), which are connected at the top by a cross piece (d). between these two posts is a glass disc (e), mounted upon a shaft (f), which passes through the posts, this shaft having at one end a crank (g). two leather collecting surfaces (h, h), which are in contact with the glass disc (e), are held in position by arms (i, j), the arm (i) being supported by the cross piece (d), and the arm (j) held by the base piece (b). a rod (k), u-shaped in form, passes over the structure here thus described, its ends being secured to the base (b). the arms (i, j) are both electrically connected with this rod, or conductor (k), joined to a main conductor (l), which has a terminating knob (m). on each side and close to the terminal end of each leather collector (h) is a fork-shaped collector (n). these two collectors are also connected electrically with the conductor (k). when the disc is turned electricity is generated by the leather flaps and accumulated by the collectors (n), after which it is ready to be discharged at the knob (m). in order to collect the electricity thus generated a vessel called a leyden jar is used. leyden jar.--this is shown in fig. 18. the jar (a) is of glass coated exteriorly at its lower end with tinfoil (b), which extends up a little more than halfway from the bottom. this jar has a wooden cover or top (c), provided centrally with a hole (d). the jar is designed to receive within it a tripod and standard (e) of lead. within this lead standard is fitted a metal rod (f), which projects upwardly through the hole (d), its upper end having thereon a terminal knob (g). a sliding cork (h) on the rod (f) serves as a means to close the jar when not in use. when in use this cork is raised so the rod may not come into contact, electrically, with the cover (c). the jar is half filled with sulphuric acid (i), after which, in order to charge the jar, the knob (g) is brought into contact with the knob (m) of the friction generator (fig. 17). voltaic or galvanic electricity.--the second method of generating electricity is by chemical means, so called, because a liquid is used as one of the agents. [illustration: _fig. 18._ leyden jar] galvani, in 1790, made the experiments which led to the generation of electricity by means of liquids and metals. the first battery was called the "crown of cups," shown in fig. 19, and consisting of a row of glass cups (a), containing salt water. these cups were electrically connected by means of bent metal strips (b), each strip having at one end a copper plate (c), and at the other end a zinc plate (d). the first plate in the cup at one end is connected with the last plate in the cup at the other end by a conductor (e) to make a complete circuit. [illustration: _fig. 19._ galvanic electricity. crown of cups] the cell and battery.--from the foregoing it will be seen that within each cup the current flows from the zinc to the copper plates, and exteriorly from the copper to the zinc plates through the conductors (b and e). a few years afterwards volta devised what is known as the voltaic pile (fig. 20). voltaic pile--how made.--this is made of alternate discs of copper and zinc with a piece of cardboard of corresponding size between each zinc and copper plate. the cardboard discs are moistened with acidulated water. the bottom disc of copper has a strip which connects with a cup of acid, and one wire terminal (a) runs therefrom. the upper disc, which is of zinc, is also connected, by a strip, with a cup of acid from which extends the other terminal wire (b). [illustration: _fig. 20._ voltaic electricity] _plus and minus signs._--it will be noted that the positive or copper disc has the plus sign (+) while the zinc disc has the minus (-) sign. these signs denote the positive and the negative sides of the current. the liquid in the cells, or in the moistened paper, is called the _electrolyte_ and the plates or discs are called _electrodes_. to define them more clearly, the positive plate is the _anode_, and the negative plate the _cathode_. the current, upon entering the zinc plate, decomposes the water in the electrolyte, thereby forming oxygen. the hydrogen in the water, which has also been formed by the decomposition, is carried to the copper plate, so that the plate finally is so coated with hydrogen that it is difficult for the current to pass through. this condition is called "polarization," and to prevent it has been the aim of all inventors. to it also we may attribute the great variety of primary batteries, each having some distinctive claim of merit. the common primary cell.--the most common form of primary cell contains sulphuric acid, or a sulphuric acid solution, as the electrolyte, with zinc for the _anode_, and carbon, instead of copper, for the _cathode_. the ends of the zinc and copper plates are called _terminals_, and while the zinc is the anode or positive element, its _terminal_ is designated as the positive pole. in like manner, the carbon is the negative element or cathode, and its terminal is designated as negative pole. fig. 21 will show the relative arrangement of the parts. it is customary to term that end or element from which the current flows as positive. a cell is regarded as a whole, and as the current passes out of the cell from the copper element, the copper terminal becomes positive. [illustration: _fig. 21._ primary battery] battery resistance, electrolyte and current.--the following should be carefully memorized: a cell has reference to a single vessel. when two or more cells are coupled together they form a _battery_. _resistance_ is opposition to the movement of the current. if it is offered by the electrolyte, it is designated "internal resistance." if, on the other hand, the opposition takes place, for instance, through the wire, it is then called "external resistance." the electrolyte must be either acid, or alkaline, or saline, and the electrodes must be of dissimilar metals, so the electrolyte will attack one of them. the current is measured in amperes, and the force with which it is caused to flow is measured in volts. in practice the word "current" is used to designate ampere flow; and electromotive force, or e. m. f., is used instead of voltage. electro-magnetic electricity.--the third method of generating electricity is by electro-magnets. the value and use of induction will now be seen, and you will be enabled to utilize the lesson concerning magnetic action referred to in the previous chapter. magnetic radiation.--you will remember that every piece of metal which is within the path of an electric current has a space all about its surface from end to end which is electrified. this electrified field extends out a certain distance from the metal, and is supposed to maintain a movement around it. if, now, another piece of metal is brought within range of this electric or magnetic zone and moved across it, so as to cut through this field, a current will be generated thereby, or rather added to the current already exerted, so that if we start with a feeble current, it can be increased by rapidly "cutting the lines of force," as it is called. different kinds of dynamo.--while there are many kinds of dynamo, they all, without exception, are constructed in accordance with this principle. there are also many varieties of current. for instance, a dynamo may be made to produce a high voltage and a low amperage; another with high amperage and low voltage; another which gives a direct current for lighting, heating, power, and electroplating; still another which generates an alternating current for high tension power, or transmission, arc-lighting, etc., all of which will be explained hereafter. in this place, however, a full description of a direct-current dynamo will explain the principle involved in all dynamos--that to generate a current of electricity makes it necessary for us to move a field of force, like an armature, rapidly and continuously through another field of force, like a magnetic field. direct-current dynamo.--we shall now make the simplest form of dynamo, using for this purpose a pair of permanent magnets. [illustration: _fig. 22._ dynamo field and pole piece] simple magnet construction.--a simple way to make a pair of magnets for this purpose is shown in fig. 22. a piece of round 3/4-inch steel core (a), 5-1/2 inches long, is threaded at both ends to receive at one end a nut (b), which is screwed on a sufficient distance so that the end of the core (a) projects a half inch beyond the nut. the other end of the steel core has a pole piece of iron (c) 2" ã� 2" ã� 4", with a hole midway between the ends, threaded entirely through, and provided along one side with a concave channel, within which the armature is to turn. now, before the pole piece (c) is put on, we will slip on a disc (e), made of hard rubber, then a thin rubber tube (f), and finally a rubber disc (g), so as to provide a positive insulation for the wire coil which is wound on the bobbin thus made. how to wind.--in practice, and as you go further along in this work, you will learn the value, first, of winding one layer of insulated wire on the spool, coating it with shellac, and then putting on the next layer, and so on; when completely wound, the two wire terminals may be brought out at one end; but for our present purpose, and to render the explanation clearer, the wire terminals are at the opposite ends of the spool (h, h'). the dynamo fields.--two of these spools are so made and they are called the _fields_ of the dynamo. we will next prepare an iron bar (i), 5 inches long and 1/2 inch thick and 1-1/2 inches wide, then bore two holes through it so the distance measures 3 inches from center to center. these holes are to be threaded for the 3/4-inch cores (a). this bar holds together the upper ends of the cores, as shown in fig. 23. [illustration: _fig. 23._ base and fields assembled] we then prepare a base (j) of any hard wood, 2 inches thick, 8 inches long and 8 inches wide, and bore two 3/4-inch holes 3 inches apart on a middle line, to receive a pair of 3/4-inch cap screws (k), which pass upwardly through the holes in the base and screw into the pole pieces (c). a wooden bar (l), 1-1/2" ã� 1-1/2", 8 inches long, is placed under each pole piece, which is also provided with holes for the cap screws (k). the lower side of the base (j) should be countersunk, as at m, so the head of the nut will not project. the fields of the dynamo are now secured in position to the base. [illustration: _fig. 24._ details of the armature, core _fig. 25._ details of the armature, body] the armature.--a bar of iron (fig. 24), 1" ã� 1" and 2-1/4 inches long, is next provided. through this bar (1) are then bored two 5/16-inch holes 1-3/4 inches apart, and on the opposite sides of this bar are two half-rounded plates of iron (3) (fig. 25). armature winding.--each plate is 1/2 inch thick, 1-3/4 inches wide and 4 inches long, each plate having holes (4) to coincide with the holes (2) of the bar (1), so that when the two plates are applied to opposite sides of the bar, and riveted together, a cylindrical member is formed, with two channels running longitudinally, and transversely at the ends; and in these channels the insulated wires are wound from end to end around the central block (1). mounting the armature.--it is now necessary to provide a means for revolving this armature. to this end a brass disc (5, fig. 26) is made, 2 inches in diameter, 1/8 inch thick. centrally, at one side, is a projecting stem (6) of round brass, which projects out 2 inches, and the outer end is turned down, as at 7, to form a small bearing surface. [illustration: _fig. 26._ journals _fig. 27._ commutator, armature mountings] the other end of the armature has a similar disc (8), with a central stem (9), 1-1/2 inches long, turned down to 1/4-inch diameter up to within 1/4 inch of the disc (7), so as to form a shoulder. the commutator.--in fig. 27 is shown, at 10, a wooden cylinder, 1 inch long and 1-1/4 inches in diameter, with a hole (11) bored through axially, so that it will fit tightly on the stem (6) of the disc (5). on this wooden cylinder is driven a brass or copper tube (12), which has holes (13) opposite each other. screws are used to hold the tube to the wooden cylinder, and after they are properly secured together, the tube (12) is cut by a saw, as at 14, so as to form two independent tubular surfaces. [illustration: _fig. 28._ end view armature, mounted] these tubular sections are called the commutator plates. [illustration: _fig. 29._ top view of armature on base] in order to mount this armature, two bearings are provided, each comprising a bar of brass (15, fig. 28), each 1/4 inch thick, 1/2 inch wide and 4-1/2 inches long. two holes, 3 inches apart, are formed through this bar, to receive round-headed wood screws (16), these screws being 3 inches long, so they will pass through the wooden pieces (i) and enter the base (j). midway between the ends, each bar (15) has an iron bearing block (17), 3/4" ã� 1/2" and 1-1/2 inches high, the 1/4-inch hole for the journal (7) being midway between its ends. commutator brushes.--fig. 28 shows the base, armature and commutator assembled in position, and to these parts have been added the commutator brushes. the brush holder (18) is a horizontal bar made of hard rubber loosely mounted upon the journal pin (7), which is 2-1/2 inches long. at each end is a right-angled metal arm (19) secured to the bar (18) by screws (20). to these arms the brushes (21) are attached, so that their spring ends engage with the commutator (12). an adjusting screw (22) in the bearing post (17), with the head thereof bearing against the brush-holder (18), serves as a means for revolubly adjusting the brushes with relation to the commutator. dynamo windings.--there are several ways to wind the dynamos. these can be shown better by the following diagrams (figs. 30, 31, 32, 33): the field.--if the field (a, fig. 30) is not a permanent magnet, it must be excited by a cell or battery, and the wires (b, b') are connected up with a battery, while the wires (c, c') may be connected up to run a motor. this would, therefore, be what is called a "separately excited" dynamo. in this case the battery excites the field and the armature (d), cutting the lines of force at the pole pieces (e), so that the armature gathers the current for the wires (c, c'). [illustration: _fig. 30._ field winding] [illustration: _fig. 31._ series-wound] series-wound field.--fig. 31 shows a "series-wound" dynamo. the wires of the fields (a) are connected up in series with the brushes of the armature (d), and the wires (g, g') are led out and connected up with a lamp, motor or other mechanism. in this case, as well as in figs. 32 and 33, both the field and the armature are made of soft gray iron. with this winding and means of connecting the wires, the field is constantly excited by the current passing through the wires. shunt-wound field.--fig. 32 represents what is known as a "shunt-wound" dynamo. here the field wires (h, h) connect with the opposite brushes of the armature, and the wires (i, i') are also connected with the brushes, these two wires being provided to perform the work required. this is a more useful form of winding for electroplating purposes. [illustration: _fig. 32._ shunt-wound _fig. 32._ compound-wound] compound-wound field.--fig. 33 is a diagram of a "compound-wound" dynamo. the regular field winding (j) has its opposite ends connected directly with the armature brushes. there is also a winding, of a comparatively few turns, of a thicker wire, one terminal (k) of which is connected with one of the brushes and the other terminal (k') forms one side of the lighting circuit. a wire (l) connects with the other armature brush to form a complete lighting circuit. chapter v how to detect and measure electricity measuring instruments.--the production of an electric current would not be of much value unless we had some way by which we might detect and measure it. the pound weight, the foot rule and the quart measure are very simple devices, but without them very little business could be done. there must be a standard of measurement in electricity as well as in dealing with iron or vegetables or fabrics. as electricity cannot be seen by the human eye, some mechanism must be made which will reveal its movements. the detector.--it has been shown in the preceding chapter that a current of electricity passing through a wire will cause a current to pass through a parallel wire, if the two wires are placed close together, but not actually in contact with each other. an instrument which reveals this condition is called a _galvanometer_. it not only detects the presence of a current, but it shows the direction of its flow. we shall now see how this is done. for example, the wire (a, fig. 35) is connected up in an electric circuit with a permanent magnet (b) suspended by a fine wire (c), so that the magnet (b) may freely revolve. [illustration: _fig. 34._ _fig. 35._ _fig. 36._ to the right, compass magnet, to the left] for convenience, the magnetic field is shown flowing in the direction of the darts, in which the dart (d) represents the current within the magnet (b) flowing toward the north pole, and the darts (e) showing the exterior current flowing toward the south pole. now, if the wire (a) is brought up close to the magnet (b), and a current passed through a, the magnet (b) will be affected. fig. 35 shows the normal condition of the magnetized bar (b) parallel with the wire (a) when a current is not passing through the latter. direction of current.--if the current should go through the wire (a) from right to left, as shown in fig. 34, the magnet (b) would swing in the direction taken by the hands of a clock and assume the position shown in fig. 34. if, on the other hand, the current in the wire (a) should be reversed or flow from left to right, the magnet (b) would swing counter-clock-wise, and assume the position shown in fig. 36. the little pointer (g) would, in either case, point in the direction of the flow of the current through the wire (a). [illustration: _fig. 37._ indicating direction of current] simple current detector.--a simple current detector may be made as follows: prepare a base 3' ã� 4' in size and 1 inch thick. at each corner of one end fix a binding post, as at a, a', fig. 37. then select 20 feet of no. 28 cotton-insulated wire, and make a coil (b) 2 inches in diameter, leaving the ends free, so they may be affixed to the binding posts (a, a'). now glue or nail six blocks (c) to the base, each block being 1" ã� 1" ã� 2", and lay the coil on these blocks. then drive an l-shaped nail (d) down into each block, on the inside of the coil, as shown, so as to hold the latter in place. [illustration: _fig. 38._ the bridge] now make a bridge (e, fig. 38) of a strip of brass 1/2 inch wide, 1/16 inch thick and long enough to span the coil, and bend the ends down, as at f, so as to form legs. a screw hole (g) is formed in each foot, so it may be screwed to the base. midway between the ends this bridge has a transverse slot (h) in one edge, to receive therein the pivot pin of the swinging magnet. in order to hold the pivot pin in place, cut out an h-shaped piece of sheet brass (i), which, when laid on the bridge, has its ends bent around the latter, as shown at j, and the crossbar of the h-shaped piece then will prevent the pivot pin from coming out of the slot (h). [illustration: _fig. 39._ details of detector] the magnet is made of a bar of steel (k, fig. 39) 1-1/2 inches long, 3/8 inch wide and 1/16 inch thick, a piece of a clock spring being very serviceable for this purpose. the pivot pin is made of an ordinary pin (l), and as it is difficult to solder the steel magnet (k) to the pin, solder only a small disc (m) to the pin (l). then bore a hole (n) through the middle of the magnet (k), larger in diameter than the pin (l), and, after putting the pin in the hole, pour sealing wax into the hole, and thereby secure the two parts together. near the upper end of the pin (l) solder the end of a pointer (o), this pointer being at right angles to the armature (k). it is better to have a metal socket for the lower end of the pin. when these parts are put together, as shown in fig. 37, a removable glass top, or cover, should be provided. this is shown in fig. 40, in which a square, wooden frame (p) is used, and a glass (q) fitted into the frame, the glass being so arranged that when the cover is in position it will be in close proximity to the upper projecting end of the pivot pin (l), and thus prevent the magnet from becoming misplaced. [illustration: _fig. 40._ cross section of detector] how to place the detector.--if the detector is placed north and south, as shown by the two markings, n and s (fig. 37), the magnet bar will point north and south, being affected by the earth's magnetism; but when a current of electricity flows through the coil (b), the magnet will be deflected to the right or to the left, so that the pointer (o) will then show the direction in which the current is flowing through the wire (r) which you are testing. the next step of importance is to _measure_ the current, that is, to determine its strength or intensity, as well as the flow or quantity. different ways of measuring a current.--there are several ways to measure the properties of a current, which may be defined as follows: 1. the sulphuric acid voltameter.--by means of an electrolytic action, whereby the current decomposes an acidulated solution--that is, water which has in it a small amount of sulphuric acid--and then measuring the gas generated by the current. 2. the copper voltameter.--by electro-chemical means, in which the current passes through plates immersed in a solution of copper sulphate. 3. the galvanoscope.--by having a coil of insulated wire, with a magnet suspended so as to turn freely within the coil, forming what is called a galvanoscope. 4. electro-magnetic method.--by using a pair of magnets and sending a current through the coils, and then measuring the pull on the armature. 5. the power or speed method.--by using an electric fan, and noting the revolutions produced by the current. 6. the calorimeter.--by using a coil of bare wire, immersed in paraffine oil, and then measuring the temperature by means of a thermometer. [illustration: _fig. 41._ acid voltameter] [illustration: _fig. 42._ copper voltameter] 7. the light method.--lastly, by means of an electric light, which shows, by its brightness, a greater or less current. the preferred methods.--it has been found that the first and second methods are the only ones which will accurately register current strength, and these methods have this advantage--that the chemical effect produced is not dependent upon the size or shape of the apparatus or the plates used. how to make a sulphuric acid voltameter.--in fig. 41 is shown a simple form of sulphuric acid voltameter, to illustrate the first method. a is a jar, tightly closed by a cover (b). within is a pair of platinum plates (c, c), each having a wire (d) through the cover. the cover has a vertical glass tube (e) through it, which extends down to the bottom of the jar, the electrolyte therein being a weak solution of sulphuric acid. when a current passes through the wires (d), the solution is partially decomposed--that is, converted into gas, which passes up into the vacant space (f) above the liquid, and, as it cannot escape, it presses the liquid downwardly, and causes the latter to flow upwardly into the tube (e). it is then an easy matter, after the current is on for a certain time, to determine its strength by the height of the liquid in the tube. how to make a copper voltameter.--the second, or copper voltameter, is shown in fig. 42. the glass jar (a) contains a solution of copper sulphate, known in commerce as blue vitriol. a pair of copper plates (b, b') are placed in this solution, each being provided with a connecting wire (c). when a current passes through the wires (c), one copper plate (b) is eaten away and deposited on the other plate (b'). it is then an easy matter to take out the plates and find out how much in weight b' has gained, or how much b has lost. in this way, in comparing the strength of, say, two separate currents, one should have each current pass through the voltameter the same length of time as the other, so as to obtain comparative results. it is not necessary, in the first and second methods, to consider the shapes, the sizes of the plates or the distances between them. in the first method the gas produced, within a given time, will be the same, and in the second method the amount deposited or eaten away will be the same under all conditions. disadvantages of the galvanoscope.--with the third method (using the galvanoscope) it is necessary, in order to get a positively correct reading instrument, to follow an absolutely accurate plan in constructing each part, in every detail, and great care must be exercised, particularly in winding. it is necessary also to be very careful in selecting the sizes of wire used and in the number of turns made in the coils. this is equally true of the fourth method, using the electro-magnet, because the magnetic pull is dependent upon the size of wire from which the coils are made and the number of turns of wire. objections to the calorimeter.--the calorimeter, or sixth method, has the same objection. the galvanoscope and electro-magnet do not respond equally to all currents, and this is also true, even to a greater extent, with the calorimeter. chapter vi volts, amperes, ohms and watts understanding terms.--we must now try to ascertain the meaning of some of the terms so frequently used in connection with electricity. if you intended to sell or measure produce or goods of any kind, it would be essential to know how many pints or quarts are contained in a gallon, or in a bushel, or how many inches there are in a yard, and you also ought to know just what the quantity term _bushel_ or the measurement _yard_ means. intensity and quantity.--electricity, while it has no weight, is capable of being measured by means of its intensity, or by its quantity. light may be measured or tested by its brilliancy. if one light is of less intensity than another and both of them receive their impulses from the same source, there must be something which interferes with that light which shows the least brilliancy. electricity can also be interfered with, and this interference is called _resistance_. voltage.--water may be made to flow with greater or less force, or velocity, through a pipe, the degree of same depending upon the height of the water which supplies the pipe. so with electricity. it may pass over a wire with greater or less force under one condition than another. this force is called voltage. if we have a large pipe, a much greater quantity of water will flow through it than will pass through a small pipe, providing the pressure in each case is alike. this quantity in electricity is called _amperage_. in the case of water, a column 1" ã� 1", 28 inches in height, weighs 1 pound; so that if a pipe 1 inch square draws water from the bottom it flows with a pressure of 1 pound. if the pipe has a measurement of 2 square inches, double the quantity of water will flow therefrom, at the same pressure. amperage.--if, on the other hand, we have a pipe 1 inch square, and there is a depth of 56 inches of water in the reservoir, we shall get as much water from the reservoir as though we had a pipe of 2 square inches drawing water from a reservoir which is 28 inches deep. meaning of watts.--it is obvious, therefore, that if we multiply the height of the water in inches with the area of the pipe, we shall obtain a factor which will show how much water is flowing. here are two examples: 1. 28 inches = height of the water in the reservoir. 2 square inches = size of the pipe. multiply 28 ã� 2 = 56. 2. 56 = height of the water in the reservoir. 1 square inch = size of the pipe. multiply 56 ã� 1 = 56. thus the two problems are equal. a kilowatt.--now, in electricity, remembering that the height of the water corresponds with _voltage_ in electricity, and the size of the pipe with _amperage_, if we multiply volts by amperes, or amperes by volts, we get a result which is indicated by the term _watts_. one thousand of these watts make a kilowatt, and the latter is the standard of measurement by which a dynamo or motor is judged or rated. thus, if we have 5 amperes and 110 volts, the result of multiplying them would be 550 watts, or 5 volts and 110 amperes would produce 550 watts. a standard of measurement.--but with all this we must have some standard. a bushel measure is of a certain size, and a foot has a definite length, so in electricity there is a recognized force and quantity which are determined as follows: the ampere standard.--it is necessary, first, to determine what an ampere is. for this purpose a standard solution of nitrate of silver is used, and a current of electricity is passed through this solution. in doing so the current deposits silver at the rate of 0.001118 grains per second for each ampere. the voltage standard.--in order to determine the voltage we must know something of _resistance_. different metals do not transmit a current with equal ease. the size of a conductor, also, is an important factor in the passage of a current. a large conductor will transmit a current much better than a small conductor. we must therefore have a standard for the _ohm_, which is the measure of resistance. the ohm.--it is calculated in this way: there are several standards, but the one most generally employed is the _international ohm_. to determine it, by this system, a column of pure mercury, 106.3 millimeters long and weighing 14.4521 grams, is used. this would make a square tube about 94 inches long, and a little over 1/25 of an inch in diameter. the resistance to a current flow in such a column would be equal to 1 ohm. calculating the voltage.--in order to arrive at the voltage we must use a conductor, which, with a resistance of 1 ohm, will produce 1 ampere. it must be remembered that the volt is the practical unit of electro-motive force. while it would be difficult for the boy to conduct these experiments in the absence of suitable apparatus, still, it is well to understand thoroughly how and why these standards are made and used. chapter vii push buttons, switches, annunciators, bells and like apparatus simple switches.--we have now gone over the simpler or elementary outlines of electrical phenomena, and we may commence to do some of the practical work in the art. we need certain apparatus to make connections, which will be constructed first. a two-pole switch.--a simple two-pole switch for a single line is made as follows: a base block (a, fig. 43) 3 inches long, 2 inches wide and 3/4 inch thick, has on it, at one end, a binding screw (b), which holds a pair of fingers (c) of brass or copper, these fingers being bent upwardly and so arranged as to serve as fingers to hold a switch bar (d) between them. this bar is also of copper or brass and is pivoted to the fingers. near the other end of the base is a similar binding screw (e) and fingers (f) to receive the blade of the switch bar. the bar has a handle (g) of wood. the wires are attached to the respective binding screws (b, e). double-pole switch.--a double-pole switch or a switch for a double line is shown in fig. 44. this is made similar in all respects to the one shown in fig. 43, excepting that there are two switch blades (a, a) connected by a cross bar (b) of insulating material, and this bar carries the handle (c). [illustration: _fig. 43._ two-pole switch] [illustration: _fig. 44._ double-pole switch] other types of switch will be found very useful. in fig. 45 is a simple sliding switch in which the base block has, at one end, a pair of copper plates (a, b), each held at one end to the base by a binding screw (c), and having a bearing or contact surface (d) at its other end. at the other end of the base is a copper plate (e) held by a binding screw (f), to the inner end of which plate is hinged a swinging switch blade (g), the free end of which is adapted to engage with the plates (a, b). [illustration: _fig. 45._ sliding switch] sliding switch.--this sliding switch form may have the contact plates (a, b and c, fig. 46) circularly arranged and any number may be located on the base, so they may be engaged by a single switching lever (h). it is the form usually adopted for rheostats. reversing switch.--a reversing switch is shown in fig. 47. the base has two plates (a, b) at one end, to which the parallel switch bars (c, d) are hinged. the other end of the base has three contact plates (e, f, g) to engage the swinging switch bars, these latter being at such distance apart that they will engage with the middle and one of the outer plates. the inlet wires, positive and negative, are attached to the plates (a, b, respectively), and one of the outlet wires (h) is attached to the middle contact plate (f), while the other wire is connected up with both of the outside plates. when the switch bars (c, d) are thrown to the left so as to be in contact with e, f, the outside plate (e) and the middle plate (f) will be positive and negative, respectively; but when the switch is thrown to the right, as shown in the figure, plate f becomes positive and plate e negative, as shown. [illustration: _fig. 46._ rheostat form of switch] push buttons.--a push button is but a modified structure of a switch, and they are serviceable because they are operating, or the circuit is formed only while the finger is on the button. [illustration: _fig. 47._ reversing switch] in its simplest form (fig. 48) the push button has merely a circular base (a) of insulating material, and near one margin, on the flat side, is a rectangular plate (b), intended to serve as a contact plate as well as a means for attaching one of the wires thereto. in line with this plate is a spring finger (c), bent upwardly so that it is normally out of contact with the plate (b), its end being held by a binding screw (d). to effect contact, the spring end of the finger (c) is pressed against the bar (b), as at e. this is enclosed in a suitable casing, such as will readily suggest itself to the novice. electric bell.--one of the first things the boy wants to make, and one which is also an interesting piece of work, is an electric bell. to make this he will be brought, experimentally, in touch with several important features in electrical work. he must make a battery for the production of current, a pair of electro-magnets to be acted upon by the current, a switch to control it, and, finally, he must learn how to connect it up so that it may be operated not only from one, but from two or more push buttons. [illustration: _fig. 48._ push button] how made.--in fig. 49 is shown an electric bell, as usually constructed, so modified as to show the structure at a glance, with its connections. a is the base, b, b' the binding posts for the wires, c, c the electro-magnets, c' the bracket for holding the magnets, d the armature, e the thin spring which connects the armature with the post f, g the clapper arm, h the bell, i the adjusting screw on the post j, k the wire lead from the binding post b to the first magnet, l the wire which connects the two magnets, m the wire which runs from the second magnet to the post j, and n a wire leading from the armature post to the binding post b'. [illustration: _fig. 49._ electric bell] the principle of the electric bell is this: in looking at fig. 49, you will note that the armature bar d is held against the end of the adjusting screw by the small spring e. when a current is turned on, it passes through the connections and conduits as follows: wire k to the magnets, wire m to the binding post j, and set screw i, then through the armature to the post f, and from post f to the binding post b'. [illustration: _fig. 50._ armature of electric bell] electric bell--how operated.--the moment a current passes through the magnets (c, c), the core is magnetized, and the result is that the armature (d) is attracted to the magnets, as shown by the dotted lines (o), when the clapper strikes the bell. but when the armature moves over to the magnet, the connection is broken between the screw (i) and armature (d), so that the cores of the magnets are demagnetized and lose their pull, and the spring (e) succeeds in drawing back the armature. this operation of vibrating the armature is repeated with great rapidity, alternately breaking and re-establishing the circuit, by the action of the current. in making the bell, you must observe one thing, the binding posts (b, b') must be insulated from each other, and the post j, or the post f, should also be insulated from the base. for convenience we show the post f insulated, so as to necessitate the use of wire (n) from post (f) to binding post (b'). the foregoing assumes that you have used a cast metal base, as most bells are now made; but if you use a wooden base, the binding posts (b, b') and the posts (f, j) are insulated from each other, and the construction is much simplified. it is better, in practice, to have a small spring (p, fig. 50) between the armature (d) and the end of the adjusting screw (i), so as to give a return impetus to the clapper. the object of the adjusting screw is to push and hold the armature close up to the ends of the magnets, if it seems necessary. if two bells are placed on the base with the clapper mounted between them, both bells will be struck by the swinging motion of the armature. an easily removable cap or cover is usually placed over the coils and armature, to keep out dust. a very simple annunciator may be attached to the bell, as shown in the following figures: [illustration: _figs. 51-54._ annunciator] annunciators.--make a box of wood, with a base (a) 4" ã� 5" and 1/2 inch thick. on this you can permanently mount the two side pieces (b) and two top and bottom pieces (c), respectively, so they project outwardly 4-1/2 inches from the base. on the open front place a wood or metal plate (d), provided with a square opening (d), as in fig. 54, near its lower end. this plate is held to the box by screws (e). within is a magnet (f), screwed into the base (a), as shown in fig. 51; and pivoted to the bottom of the box is a vertical armature (g), which extends upwardly and contacts with the core of the magnet. the upper end of the armature has a shoulder (h), which is in such position that it serves as a rest for a v-shaped stirrup (i), which is hinged at j to the base (c). this stirrup carries the number plate (k), and when it is raised to its highest point it is held on the shoulder (h), unless the electro-magnet draws the armature out of range of the stirrup. a spring (l) bearing against the inner side of the armature keeps its upper end normally away from the magnet core. when the magnet draws the armature inwardly, the number plate drops and exposes the numeral through the opening in the front of the box. in order to return the number plate to its original position, as shown in fig. 51, a vertical trigger (m) passes up through the bottom, its upper end being within range of one of the limbs of the stirrup. this is easily made by the ingenious boy, and will be quite an acquisition to his stock of instruments. in practice, the annunciator may be located in any convenient place and wires run to that point. [illustration: _fig. 55._ alarm switch on window] [illustration: _fig. 56._ burglar alarm attachment to window] burglar alarm.--in order to make a burglar alarm connection with a bell, push buttons or switches may be put in circuit to connect with the windows and doors, and by means of the annunciators you may locate the door or window which has been opened. the simplest form of switch for a window is shown in the following figures: the base piece (a), which may be of hard rubber or fiber, is 1/4 inch thick and 1" ã� 1-1/2" in size. [illustration: _fig. 57._ burglar alarm contact] at one end is a brass plate (b), with a hole for a wood screw (c), this screw being designed to pass through the plate and also into the window-frame, so as to serve as a means of attaching one of the wires thereto. the inner end of the plate has a hole for a round-headed screw (c') that also goes through the base and into the window-frame. it also passes through the lower end of the heart-shaped metal switch-piece (d). the upper end of the base has a brass plate (e), also secured to the base and window by a screw (f) at its upper end. the heart-shaped switch is of such length and width at its upper end that when it is swung to the right with one of the lobes projecting past the edge of the window-frame, the other lobe will be out of contact with the plate (e). [illustration: _fig. 58._ neutral position of contact] the window sash (g) has a removable pin (h), which, when the sash moves upwardly, is in the path of the lobe of the heart-shaped switch, as shown in fig. 56, and in this manner the pin (h) moves the upper end of the switch (d) inwardly, so that the other lobe contacts with the plate (e), and establishes an electric circuit, as shown in fig. 57. during the daytime the pin (h) may be removed, and in order to protect the switch the heart-shaped piece (d) is swung inwardly, as shown in fig. 58, so that neither of the lobes is in contact with the plate (e). wire circuiting.--for the purpose of understanding fully the circuiting, diagrams will be shown of the simple electric bell with two push buttons; next in order, the circuiting with an annunciator and then the circuiting necessary for a series of windows and doors, with annunciator attachments. [illustration: _fig. 59._ circuiting for electric bell] circuiting system with a bell and two push buttons.--fig. 59 shows a simple circuiting system which has two push buttons, although any number may be used, so that the bell will ring when the circuit is closed by either button. the push buttons and the annunciator bells.--fig. 60 shows three push buttons and an annunciator for each button. these three circuits are indicated by a, b and c, so that when either button makes contact, a complete circuit is formed through the corresponding annunciator. [illustration: _fig. 60._ _annunciators_] [illustration: _fig. 61._ wiring system for a house] wiring up a house.--the system of wiring up a house so that all doors and windows will be connected to form a burglar alarm outfit, is shown in fig. 61. it will be understood that, in practice, the bell is mounted on or at the annunciator, and that, for convenience, the annunciator box has also a receptacle for the battery. the circuiting is shown diagramatically, as it is called, so as fully to explain how the lines are run. two windows and a door are connected up with an annunciator having three drops, or numbers 1, 2, 3. the circuit runs from one pole of the battery to the bell and then to one post of the annunciator. from the other post a wire runs to one terminal of the switch at the door or window. the other switch terminal has a wire running to the other pole of the battery. a, b, c represent the circuit wires from the terminals of the window and door switches, to the annunciators. it is entirely immaterial which side of the battery is connected up with the bell. from the foregoing it will readily be understood how to connect up any ordinary apparatus, remembering that in all cases the magnet must be brought into the electric circuit. chapter viii accumulators. storage or secondary batteries storing up electricity.--in the foregoing chapters we have seen that, originally, electricity was confined in a bottle, called the leyden jar, from which it was wholly discharged at a single impulse, as soon as it was connected up by external means. later the primary battery and the dynamo were invented to generate a constant current, and after these came the second form of storing electricity, called the storage or secondary battery, and later still recognized as accumulators. the accumulator.--the term _accumulator_ is, strictly speaking, the more nearly correct, as electricity is, in reality, "_stored_" in an accumulator. but when an accumulator is charged by a current of electricity, a chemical change is gradually produced in the active element of which the accumulator is made. this change or decomposition continues so long as the charging current is on. when the accumulator is disconnected from the charging battery or dynamo, and its terminals are connected up with a lighting system, or with a motor, for instance, a reverse process is set up, or the particles re-form themselves into their original compositions, which causes a current to flow in a direction opposite to that of the charging current. it is immaterial to the purposes of this chapter, as to the charging source, whether it be by batteries or dynamos; the same principles will apply in either case. [illustration: _fig. 62._ accumulator grids] accumulator plates.--the elements used for accumulator plates are red lead for the positive plates, and precipitated lead, or the well-known litharge, for the negative plates. experience has shown that the best way to hold this material is by means of lead grids. fig. 62 shows the typical form of one of these grids. it is made of lead, cast or molded in one piece, usually square, as at a, with a wing or projection (b), at one margin, extending upwardly and provided with a hole (c). the grid is about a quarter of an inch thick. the grid.--the open space, called the grid, proper, comprises cross bars, integral with the plate, made in a variety of shapes. fig. 62 shows three forms of constructing these bars or ribs, the object being to provide a form which will hold in the lead paste, which is pressed in so as to make a solid-looking plate when completed. the positive plate.--the positive plate is made in the following manner: make a stiff paste of red lead and sulphuric acid; using a solution, say, of one part of acid to two parts of water. the grid is laid on a flat surface and the paste forced into the perforations with a stiff knife or spatula. turn over the grid so as to get the paste in evenly on both sides. the grid is then stood on its edge, from 18 to 20 hours, to dry, and afterwards immersed in a concentrated solution of chloride of lime, so as to convert it into lead peroxide. when the action is complete it is thoroughly rinsed in cold water, and is ready to use. the negative plate.--the negative plate is filled, in like manner, with precipitated lead. this lead is made by putting a strip of zinc into a standard solution of acetate of lead, and crystals will then form on the zinc. these will be very thin, and will adhere together, firmly, forming a porous mass. this, when saturated and kept under water for a short time, may be put into the openings of the negative plate. [illustration: _fig. 63._ assemblage of accumulator plates] connecting up the plates.--the next step is to put these plates in position to form a battery. in fig. 63 is shown a collection of plates connected together. for simplicity in illustrating, the cell is made up of glass, porcelain, or hard rubber, with five plates (a), a, a representing the negative and b, b the positive plates. a base of grooved strips (c, c) is placed in the batteries of the cell to receive the lower ends of the plates. the positive plates are held apart by means of a short section of tubing (d), which is clamped and held within the plates by a bolt (e), this bolt also being designed to hold the terminal strip (f). in like manner, the negative plates are held apart by the two tubular sections (g), each of which is of the same length as the section d of the positives. the bolt (h) holds the negatives together as well as the terminal (i). the terminals should be lead strips, and it would be well, owing to the acid fumes which are formed, to coat all brass work, screws, etc., with paraffine wax. the electrolyte or acid used in the cell, for working purposes, is a pure sulphuric acid, which should be diluted with about four times its weight in water. remember, you should always add the strong acid to the water, and never pour the water into the acid, as the latter method causes a dangerous ebullition, and does not produce a good mixture. put enough of this solution into the cell to cover the tops of the plates, and the cell is ready. [illustration: _fig. 64._ connecting up storage battery in series] charging the cells.--the charge of the current must never be less than 2.5 volts. each cell has an output, in voltage, of about 2 volts, hence if we have, say, 10 cells, we must have at least 25 volts charging capacity. we may arrange these in one line, or in series, as it is called, so far as the connections are concerned, and charge them with a dynamo, or other electrical source, which shows a pressure of 25 volts, as illustrated in fig. 64, or, instead of this, we may put them into two parallel sets of 5 cells each, as shown in fig. 65, and use 12.5 volts to charge with. in this case it will take double the time because we are charging with only one-half the voltage used in the first case. the positive pole of the dynamo should be connected with the positive pole of the accumulator cell, and negative with negative. when this has been done run up the machine until it slightly exceeds the voltage of the cells. thus, if we have 50 cells in parallel, like in fig. 64, at least 125 volts will be required, and the excess necessary should bring up the voltage in the dynamo to 135 or 140 volts. [illustration: _fig. 65._ parallel series] [illustration: _fig. 66._ charging circuit] the initial charge.--it is usual initially to charge the battery from periods ranging from 36 to 40 hours, and to let it stand for 12 or 15 hours, after which to re-charge, until the positive plates have turned to a chocolate color, and the negative plates to a slate or gray color, and both plates give off large bubbles of gas. in charging, the temperature of the electrolyte should not exceed 100â° fahrenheit. when using the accumulators they should never be fully discharged. the charging circuit.--the diagram (fig. 66) shows how a charging circuit is formed. the lamps are connected up in parallel, as illustrated. each 16-candle-power 105-volt lamp will carry 1/2 ampere, so that, supposing we have a dynamo which gives 110 volts, and we want to charge a 4-volt accumulator, there will be 5-volt surplus to go to the accumulator. if, for instance, you want the cell to have a charge of 2 amperes, four of these lamps should be connected up in parallel. if 3 amperes are required, use 6 lamps, and so on. chapter ix the telegraph the telegraph is a very simple instrument. the key is nothing more or less than a switch which turns the current on and off alternately. the signals sent over the wires are simply the audible sounds made by the armature, as it moves to and from the magnets. mechanism in telegraph circuits.--a telegraph circuit requires three pieces of mechanism at each station, namely, a key used by the sender, a sounder for the receiver, and a battery. the sending key.--the base of the sending instrument is six inches long, four inches wide, and three-quarters of an inch thick, made of wood, or any suitable non-conducting material. the key (a) is a piece of brass three-eighths by one-half inch in thickness and six inches long. midway between its ends is a cross hole, to receive the pivot pin (b), which also passes through a pair of metal brackets (c, d), the bracket c having a screw to hold one of the line wires, and the other bracket having a metal switch (e) hinged thereto. this switch bar, like the brackets, is made of brass, one-half inch wide by one-sixteenth of an inch thick. below the forward end of the key (a) is a cross bar of brass (f), screwed to the base by a screw at one end, to receive the other line wire. directly below the key (a) is a screw (g), so that the key will strike it when moved downwardly. the other end of the bar (f) contacts with the forward end of the switch bar (e) when the latter is moved inwardly. [illustration: _fig. 67._ telegraph sending key] the forward end of the key (a) has a knob (h) for the fingers, and the rear end has an elastic (i) attached thereto which is secured to the end of the base, so that, normally, the rear end is held against the base and away from the screw head (g). the head (j) of a screw projects from the base at its rear end. key a contacts with it. when the key a contacts with the screw heads g, j, a click is produced, one when the key is pressed down and the other when the key is released. you will notice that the two plates c, f are connected up in circuit with the battery, so that, as the switch e is thrown, so as to be out of contact, the circuit is open, and may be closed either by the key a or the switch e. the use of the switch will be illustrated in connection with the sounder. [illustration: _fig. 68._ telegraph sounder] when the key a is depressed, the circuit of course goes through plate c, key a and plate f to the station signalled. the sounder.--the sounder is the instrument which carries the electro-magnet. in fig. 68 this is shown in perspective. the base is six inches long and four inches wide, being made, preferably, of wood. near the forward end is mounted a pair of electro-magnets (a, a), with their terminal wires connected up with plates b, b', to which the line wires are attached. midway between the magnets and the rear end of the base is a pair of upwardly projecting brackets (c). between these are pivoted a bar (d), the forward end of which rests between the magnets and carries, thereon, a cross bar (e) which is directly above the magnets, and serves as the armature. the rear end of the base has a screw (f) directly beneath the bar d of such height that when the rear end of the bar d is in contact therewith the armature e will be out of contact with the magnet cores (a, a). a spiral spring (g) secured to the rear ends of the arm and to the base, respectively, serves to keep the rear end of the key normally in contact with the screw f. connecting up the key and sounder.--having made these two instruments, we must next connect them up in the circuit, or circuits, formed for them, as there must be a battery, a key, and a sounder at each end of the line. in fig. 69 you will note two groups of those instruments. now observe how the wires connect them together. there are two line wires, one (a) which connects up the two batteries, the wire being attached so that one end connects with the positive terminal of the battery, and the other end with the negative terminal. [illustration: _fig. 69._ a telegraph circuit] the other line wire (b), between the two stations, has its opposite ends connected with the terminals of the electro-magnet c of the sounders. the other terminals of each electro-magnet are connected up with one terminal of each key by a wire (d), and to complete the circuit at each station, the other terminal of the key has a wire (e) to its own battery. two stations in circuit.--the illustration shows station 2 telegraphing to station 1. this is indicated by the fact that the switch f' of that instrument is open, and the switch f of station 1 closed. when, therefore, the key of station 2 is depressed, a complete circuit is formed which transmits the current through wire e' and battery, through line a, then through the battery of station 1, through wire e to the key, and from the key, through wire d, to the sounder, and finally from the sounder over line wire b back to the sounder of station 2, completing the circuit at the key through wire d'. when the operator at station 2 closes the switch f', and the operator at station 1 opens the switch f, the reverse operation takes place. in both cases, however, the sounder is in at both ends of the line, and only the circuit through the key is cut out by the switch f, or f'. the double click.--the importance of the double click of the sounder will be understood when it is realized that the receiving operator must have some means of determining if the sounder has transmitted a dot or a dash. whether he depresses the key for a dot or a dash, there must be one click when the key is pressed down on the screw head g (fig. 62), and also another click, of a different kind, when the key is raised up so that its rear end strikes the screw head j. this action of the key is instantly duplicated by the bar d (fig. 68) of the sounder, so that the sounder as well as the receiver knows the time between the first and the second click, and by that means he learns that a dot or a dash is made. illustrating the dot and the dash.--to illustrate: let us suppose, for convenience, that the downward movement of the lever in the key, and the bar in the sounder, make a sharp click, and the return of the lever and bar make a dull click. in this case the ear, after a little practice, can learn readily how to distinguish the number of downward impulses that have been given to the key. _the morse telegraph code_ a . n . & . ... b ... o .. 1 . . c .. . p ..... 2 .. .. d . . q .. . 3 ... . e . r . .. 4 .... f . . s ... 5 g . t 6 ...... h .... u .. 7 .. i .. v ... 8 .... j . . w . 9 .. k . x . .. 0 --------l - y .. .. m z ... . example in use.--let us take an example in the word "electrical." e l e c t r i c a l . - . .. . . .. .. .. . . -the operator first makes a dot, which means a sharp and a dull click close together; there is then a brief interval, then a lapse, after which there is a sharp click, followed, after a comparatively longer interval, with the dull click. now a dash by itself may be an l, a t, or the figure 0, dependent upon its length. the short dash is t, and the longest dash the figure 0. the operator will soon learn whether it is either of these or the letter l, which is intermediate in length. in time the sender as well as receiver will give a uniform length to the dash impulse, so that it may be readily distinguished. in the same way, we find that r, which is indicated by a dot, is followed, after a short interval, by two dots. this might readily be mistaken for the single dot for e and the two dots for i, were it not that the time element in r is not as long between the first and second dots, as it ordinarily is between the single dot of e when followed by the two dots of i. chapter x high tension apparatus, condensers, etc. induction.--one of the most remarkable things in electricity is the action of induction--that property of an electric current which enables it to pass from one conductor to another conductor through the air. another singular and interesting thing is that the current so transmitted across spaces changes its direction of flow, and, furthermore, the tension of such a current may be changed by transmitting it from one conductor to another. low and high tension.--in order to effect this latter change--that is, to convert it from a low tension to a high tension--coils are used, one coil being wound upon the other; one of these coils is called the primary and the other the secondary. the primary coil receives the current from the battery, or source of electrical power, and the secondary coil receives charges, and transmits the current. for an illustration of this examine fig. 70, in which you will note a coil of heavy wire (a), around which is wound a coil of fine wire (b). if, for instance, the primary coil has a low voltage, the secondary coil will have a high voltage, or tension. advantage is taken of this phase to use a few cells, as a primary battery, and then, by a set of _induction coils_, as they are called, to build up a high-tension electro-motive force, so that the spark will jump across a gap, as shown at c, for the purpose of igniting the charges of gas in a gasoline motor; or the current may be used for medical batteries, and for other purposes. [illustration: _fig. 70._ induction coil and circuit] the current passes, by induction, from the primary to the secondary coil. it passes from a large conductor to a small conductor, the small conductor having a much greater resistance than the large one. elastic property of electricity.--while electricity has no resiliency, like a spring, for instance, still it acts in the manner of a cushion under certain conditions. it may be likened to an oscillating spring acted upon by a bar. referring to fig. 71, we will assume that the bar a in falling down upon the spring b compresses the latter, so that at the time of greatest compression the bar goes down as far as the dotted line c. it is obvious that the spring b will throw the bar upwardly. now, electricity appears to have a kind of elasticity, which characteristic is taken advantage of in order to increase the efficiency of the induction in the coil. [illustration: _fig. 71._ illustrating elasticity] the condenser.--to make a condenser, prepare two pine boards like a, say, eight by ten inches and a half inch thick, and shellac thoroughly on all sides. then prepare sheets of tinfoil (b), six by eight inches in size, and also sheets of paraffined paper (c), seven by nine inches in dimensions. also cut out from the waste pieces of tinfoil strips (d), one inch by two inches. to build up the condenser, lay down a sheet of paraffined paper (c), then a sheet of tinfoil (b), and before putting on the next sheet of paraffined paper lay down one of the small strips (d) of tinfoil, as shown in the illustration, so that its end projects over one end of the board a; then on the second sheet of paraffine paper lay another sheet of tinfoil, and on this, at the opposite end, place one of the small strips (d), and so on, using from 50 to 100 of the tinfoil sheets. when the last paraffine sheet is laid on, the other board is placed on top, and the whole bound together, either by wrapping cords around the same or by clamping them together with bolts. [illustration: _fig. 72._ condenser] you may now make a hole through the projecting ends of the strips, and you will have two sets of tinfoil sheets, alternately connected together at opposite ends of the condenser. care should be exercised to leave the paraffine sheets perfect or without holes. you can make these sheets yourself by soaking them in melted paraffine wax. connecting up a condenser.--when completed, one end of the condenser is connected up with one terminal of the secondary coil, and the other end of the condenser with the other secondary terminal. [illustration: _fig. 73._ high-tension circuit] in fig. 73 a high-tension circuit is shown. two coils, side by side, are always used to show an induction coil, and a condenser is generally shown, as illustrated, by means of a pair of forks, one resting within the other. the interrupter.--one other piece of mechanism is necessary, and that is an _interrupter_, for the purpose of getting the effect of the pulsations given out by the secondary coil. a simple current interrupter is made as follows: prepare a wooden base (a), one inch thick, six inches wide, and twelve inches long. upon this mount a toothed wheel (b), six inches in diameter, of thin sheet metal, or a brass gear wheel will answer the purpose. the standard (c), which supports the wheel, may be of metal bent up to form two posts, between which the crankshaft (d) is journaled. the base of the posts has an extension plate (e), with a binding post for a wire. at the front end of the base is an l-shaped strip (f), with a binding post for a wire connection, and the upwardly projecting part of the strip contacts with the toothed wheel. when the wheel b is rotated the spring finger (f) snaps from one tooth to the next, so that, momentarily, the current is broken, and the frequency is dependent upon the speed imparted to the wheel. [illustration: _fig. 74._ current interrupter] uses of high-tension coils.--this high-tension coil is made use of, and is the essential apparatus in wireless telegraphy, as we shall see in the chapter treating upon that subject. chapter xi wireless telegraphy telegraphing without wires.--wireless telegraphy is an outgrowth of the ordinary telegraph system. when maxwell, and, later on, hertz, discovered that electricity, magnetism, and light were transmitted through the ether, and that they differed only in their wave lengths, they laid the foundations for wireless telegraphy. ether is a substance which is millions and millions of times lighter than air, and it pervades all space. it is so unstable that it is constantly in motion, and this phase led some one to suggest that if a proper electrical apparatus could be made, the ether would thereby be disturbed sufficiently so that its impulses would extend out a distance proportioned to the intensity of the electrical agitation thereby created. surging character of high-tension currents.--when a current of electricity is sent through a wire, hundreds of miles in length, the current surges back and forth on the wire many thousands of times a second. light comes to us from the sun, over 90,000,000 of miles, through the ether. it is as reasonable to suppose, or infer, that the ether can, therefore, convey an electrical impulse as readily as does a wire. it is on this principle that impulses are sent for thousands of miles, and no doubt they extend even farther, if the proper mechanism could be devised to detect movement of the waves so propagated. the coherer.--the instrument for detecting these impulses, or disturbances, in the ether is generally called a _coherer_, although detector is the term which is most satisfactory. the name coherer comes from the first practical instrument made for this purpose. [illustration: _fig. 75._ wireless telegraphy coherer] how made.--the coherer is simply a tube, say, of glass, within which is placed iron filings. when the oscillations surge through the secondary coil the pressure or potentiality of the current finally causes it to leap across the small space separating the filings and, as it were, it welds together their edges so that a current freely passes. the bringing together of the particles, under these conditions, is called cohering. fig. 75 shows the simplest form of coherer. the posts (a) are firmly affixed to the base (b), each post having an adjusting screw (c) in its upper end, and these screw downwardly against and serve to bind a pair of horizontal rods (d), the inner ends of which closely approach each other. these may be adjusted so as to be as near together or as far apart as desired. e is a glass tube in which the ends of the rods (d) rest, and between the separated ends of the rods (d) the iron filings (f) are placed. the decoherers.--for the purpose of causing the metal filings to fall apart, or decohere, the tube is tapped lightly, and this is done by a little object like the clapper of an electric bell. in practice, the coils and the parts directly connected with it are put together on one base. the sending apparatus.--fig. 76 shows a section of a coil with its connection in the sending station. the spark gap rods (a) may be swung so as to bring them closer together or farther apart, but they must not at any time contact with each other. the induction coil has one terminal of the primary coil connected up by a wire (b) with one post of a telegraph key, and the other post of the key has a wire connection (c), with one side of a storage battery. the other side of the battery has a wire (d) running to the other terminal of the primary. [illustration: _fig. 76._ wireless sending apparatus] the secondary coil has one of its terminals connected with a binding post (e). this binding post has an adjustable rod with a knob (f) on its end, and the other binding post (g), which is connected up with the other terminal of the secondary coil, carries a similar adjusting rod with a knob (h). from the post (e) is a wire (i), which extends upwardly, and is called the aerial wire, or wire for the antennã¦, and this wire also connects with one side of the condenser by a conductor (j). the ground wire (k) connects with the other binding post (g), and a branch wire (l) also connects the ground wire (k) with one end of the condenser. [illustration: _fig. 77._ wireless receiving apparatus] the receiving apparatus.--the receiving station, on the other hand, has neither condenser, induction coil, nor key. when the apparatus is in operation, the coherer switch is closed, and the instant a current passes through the coherer and operates the telegraph sounder, the galvanometer indicates the current. of course, when the coherer switch is closed, the battery operates the decoherer. how the circuits are formed.--by referring again to fig. 76, it will be seen that when the key is depressed, a circuit is formed from the battery through wire b to the primary coil, and back again to the battery through wire d. the secondary coil is thereby energized, and, when the full potential is reached, the current leaps across the gap formed between the two knobs (f, h), thereby setting up a disturbance in the ether which is transmitted through space in all directions. it is this impulse, or disturbance, which is received by the coherer at the receiving station, and which is indicated by the telegraph sounder. chapter xii the telephone vibrations.--every manifestation in nature is by way of vibration. the beating of the heart, the action of the legs in walking, the winking of the eyelid; the impulses from the sun, which we call light; sound, taste and color appeal to our senses by vibratory means, and, as we have hereinbefore stated, the manifestations of electricity and magnetism are merely vibrations of different wave lengths. the acoustic telephone.--that sound is merely a product of vibrations may be proven in many ways. one of the earliest forms of telephones was simply a "sound" telephone, called the _acoustic telephone_. the principle of this may be illustrated as follows: take two cups (a, b), as in fig. 78, punch a small hole through the bottom of each, and run a string or wire (c) from the hole of one cup to that of the other, and secure it at both ends so it may be drawn taut. now, by talking into the cup (a) the bottom of it will vibrate to and fro, as shown by the dotted lines and thereby cause the bottom of the other cup (b) to vibrate in like manner, and in so vibrating it will receive not only the same amplitude, but also the same character of vibrations as the cup (a) gave forth. [illustration: _fig. 78._ acoustic telephone] [illustration: _fig. 79._ illustrating vibrations] sound waves.--sound waves are long and short; the long waves giving sounds which are low in the musical scale, and the short waves high musical tones. you may easily determine this by the following experiment: stretch a wire, as at b (fig. 79), fairly tight, and then vibrate it. the amplitude of the vibration will be as indicated by dotted line a. now, stretch it very tight, as at c, so that the amplitude of vibration will be as shown at e. by putting your ear close to the string you will find that while a has a low pitch, c is very much higher. this is the principle on which stringed instruments are built. you will note that the wave length, which represents the distance between the dotted lines a is much greater than e. hearing electricity.--in electricity, mechanism has been made to enable man to note the action of the current. by means of the armature, vibrating in front of a magnet, we can see its manifestations. it is now but a step to devise some means whereby we may hear it. in this, as in everything else electrically, the magnet comes into play. [illustration: _fig. 80._ the magnetic field] in the chapter on magnetism, it was stated that the magnetic field extended out beyond the magnet, so that if we were able to see the magnetism, the end of a magnet would appear to us something like a moving field, represented by the dotted lines in fig. 80. the magnetic field is shown in fig. 80 at only one end, but its manifestations are alike at both ends. it will be seen that the magnetic field extends out to a considerable distance and has quite a radius of influence. the diaphragm in a magnetic field.--if, now, we put a diaphragm (a) in this magnetic field, close up to the end of the magnet, but not so close as to touch it, and then push it in and out, or talk into it so that the sound waves strike it, the movement or the vibration of the diaphragm (a) will disturb the magnetic field emanating from the magnet, and this disturbance of the magnetic field at one end of the magnet also affects the magnetic field at the other end in the same way, so that the disturbance there will be of the same amplitude. it will also display the same characteristics as did the magnetic field when the diaphragm (a) disturbed it. a simple telephone circuit.--from this simple fact grew the telephone. if two magnets are connected up in the same circuit, so that the magnetic fields of the two magnets have the same source of electric power, the disturbance of one diaphragm will affect the other similarly, just the same as the two magnetic fields of the single magnet are disturbed in unison. how to make a telephone.--for experimental and testing purposes two of these telephones should be made at the same time. the case or holder (a) may be made either of hard wood or hard rubber, so that it is of insulating material. the core (b) is of soft iron, 3/8 inch in diameter and 5 inches long, bored and threaded at one end to receive a screw (c) which passes through the end of the case (a). the enlarged end of the case should be, exteriorly, 2-1/4 inches in diameter, and the body of the case 1 inch in diameter. [illustration: _fig. 81._ section of telephone receiver] interiorly, the large end of the case is provided with a circular recess 1-3/4 inches in diameter and adapted to receive therein a spool which is, diametrically, a little smaller than the recess. the spool fits fairly tight upon the end of the core, and when in position rests against an annular shoulder in the recess. a hollow space (f) is thus provided behind the spool (d), so the two wires from the magnet may have room where they emerge from the spool. the spool is a little shorter than the distance between the shoulder (e) and the end of the casing, at g, and the core projects only a short distance beyond the end of the spool, so that when the diaphragm (h) is put upon the end of the case, and held there by screws (i) it will not touch the end of the core. a wooden or rubber mouthpiece (j) is then turned up to fit over the end of the case. [illustration: _fig. 82._ the magnet and receiver head] the spool (d) is made of hard rubber, and is wound with no. 24 silk-covered wire, the windings to be well insulated from each other. the two ends of the wire are brought out, and threaded through holes (k) drilled longitudinally through the walls of the case, and affixed to the end by means of screws (l), so that the two wires may be brought together and connected with a duplex wire (m). as the screw (c), which holds the core in place, has its head hidden within a recess, which can be closed up by wax, the two terminals of the wires are well separated so that short-circuiting cannot take place. telephone connections.--the simplest form of telephone connection is shown in fig. 83. this has merely the two telephones (a and b), with a single battery (c) to supply electricity for both. one line wire (d) connects the two telephones directly, while the other line (e) has the battery in its circuit. [illustration: _fig. 83._ simple telephone connection] complete installation.--to install a more complete system requires, at each end, a switch, a battery and an electro-magneto bell. you may use, for this purpose, a bell, made as shown in the chapter on bells. fig. 84 shows such a circuit. we now dispense with one of the line wires, because it has been found that the ground between the two stations serves as a conductor, so that only one line wire (a) is necessary to connect directly with the telephones of the two stations. the telephones (b, b', respectively) have wires (c, c') running to the pivots of double-throw switches (d, d'), one terminal of the switches having wires (e, e'), which go to electric bells (f, f'), and from the bells are other wires (g, g'), which go to the ground. the ground wires also have wires (h, h'), which go to the other terminals of the switch (d, d'). the double-throw switch (d, d'), in the two stations, is thrown over so the current, if any should pass through, will go through the bell to the ground, through the wires (e, g or e', g'). [illustration: _fig. 84._ telephone stations in circuit] now, supposing the switch (d'), in station 2, should be thrown over so it contacts with the wire (h'). it is obvious that the current will then flow from the battery (i') through wires (h', c') and line (a) to station 1; then through wire c, switch d, wire e to the bell f, to the ground through wire g. from wire g the current returns through the ground to station 2, where it flows up wire g' to the battery, thereby completing the circuit. [illustration: _fig. 85._ illustrating light contact points] the operator at station 2, having given the signal, again throws his switch (d') back to the position shown in fig. 84, and the operator at station 1 throws on his switch (d), so as to ring the bell in station 2, thereby answering the signal, which means that both switches are again to be thrown over so they contact with the battery wires (h and h'), respectively. when both are thus thrown over, the bells (g, g') are cut out of the circuit, and the batteries are both thrown in, so that the telephones are now ready for talking purposes. microphone.--originally this form of telephone system was generally employed, but it was found that for long distances a more sensitive instrument was necessary. light contact points.--in 1877 professor hughes discovered, accidentally, that a light contact point in an electric circuit augmented the sound in a telephone circuit. if, for instance, a light pin, or a nail (a, fig. 85) should be used to connect the severed ends of a wire (b), the sounds in the telephone not only would be louder, but they would be more distinct, and the first instrument made practically, to demonstrate this, is shown in fig. 86. [illustration: _fig. 86._ microphone] [illustration: _fig. 87._ transmitter] how to make a microphone.--this instrument has simply a base (a) of wood, and near one end is a perpendicular sounding-board (b) of wood, to one side of which is attached, by wax or otherwise, a pair of carbon blocks (c, d). the lower carbon block (c) has a cup-shaped depression in its upper side, and the upper block has a similar depression in its lower side. a carbon pencil (e) is lightly held within these cups, so that the lightest contact of the upper end of the pencil with the carbon block, makes the instrument so sensitive that a fly, walking upon the sounding-board, may be distinctly heard through the telephone which is in the circuit. microphone the father of the transmitter.--this instrument has been greatly modified, and is now used as a transmitter, the latter thereby taking the place of the pin (a), shown in fig. 85. automatic cut-outs for telephones.--in the operation of the telephone, the great drawback originally was in inducing users of the lines to replace or adjust their instruments carefully. when switches were used, they would forget to throw them back, and all sorts of trouble resulted. it was found necessary to provide an automatic means for throwing in and cutting out an instrument, this being done by hanging the telephone on the hook, so that the act merely of leaving the telephone made it necessary, in replacing the instrument, to cut out the apparatus. before describing the circuiting required for these improvements, we show, in fig. 87, a section of a transmitter. a cup-shaped case (a) is provided, made of some insulating material, which has a diaphragm (b) secured at its open side. this diaphragm carries the carbon pencil (c) on one side and from the blocks which support the carbon pencil the wires run to binding posts on the case. of course the carbon supporting posts must be insulated from each other, so the current will go through the carbon pencil (c). complete circuiting with transmitter.--in showing the circuiting (fig. 88) it will not be possible to illustrate the boxes, or casings, which receive the various instruments. for instance, the hook which carries the telephone or the receiver, is hinged within the transmitter box. the circuiting is all that it is intended to show. [illustration: _fig. 88._ complete telephonic circuit] the batteries of the two stations are connected up by a wire (a), unless a ground circuit is used. the other side of each battery has a wire connection (b, b') with one terminal of the transmitter, and the other terminal of the transmitter has a wire (c, c') which goes to the receiver. from the other terminal of the receiver is a wire (d, d') which leads to the upper stop contact (e, e') of the telephone hook. a wire (f, f') from the lower stop contact (g, g') of the hook goes to one terminal of the bell, and from the other terminal of the bell is a wire (h, h') which makes connection with the line wire (a). in order to make a complete circuit between the two stations, a line wire (i) is run from the pivot of the hook in station 1 to the pivot of the hook in station 2. in the diagram, it is assumed that the receivers are on the hooks, and that both hooks are, therefore, in circuit with the lower contacts (g, g'), so that the transmitter and receiver are both out of circuit with the batteries, and the bell in circuit; but the moment the receiver, for instance, in station 1 is taken off the hook, the latter springs up so that it contacts with the stop (e), thus establishing a circuit through the line wire (i) to the hook of station 2, and from the hook through line (f') to the bell. from the bell, the line (a) carries the current back to the battery of station (a), thence through the wire (b) to the transmitter wire (c) to receiver and wire (d) to the post (e), thereby completing the circuit. when, at station 2, the receiver is taken off the hook, and the latter contacts with the post (e'), the transmitter and receiver of both stations are in circuit with each other, but both bells are cut out. chapter xiii electrolysis, water purification, electroplating decomposing liquids.--during the earlier experiments in the field of electricity, after the battery or cell was discovered, it was noted that when a current was formed in the cell, the electrolyte was charged and gases evolved from it. a similar action takes place when a current of electricity passes through a liquid, with the result that the liquid is decomposed--that is, the liquid is broken up into its original compounds. thus, water is composed of two parts, by bulk, of hydrogen and of oxygen, so that if two electrodes are placed in water, and a current is sent through the electrodes in either direction, all the water will finally disappear in the form of hydrogen and oxygen gases. making hydrogen and oxygen.--during this electrical action, the hydrogen is set free at the negative pole and the oxygen at the positive pole. a simple apparatus, which any boy can make, to generate pure oxygen and pure hydrogen, is shown in fig. 89. it is constructed of a glass or earthen jar (a), preferably square, to which is fitted a wooden top (b), this top being provided with a packing ring (c), so as to make it air-tight. within is a vertical partition (d), the edges of which, below the cap, fit tightly against the inner walls of the jar. this partition extends down into the jar a sufficient distance so it will terminate below the water level. a pipe is fitted through the top on each side of the partition, and each pipe has a valve. an electrode, of any convenient metal, is secured at its upper end to the top of the cap, on each side of the partition. these electrodes extend down to the bottom of the jar, and an electric wire connects with each of them at the top. [illustration: _fig. 89._ device for making hydrogen and oxygen] if a current of electricity is passed through the wires and the electrodes, in the direction shown by the darts, hydrogen will form at the negative pole, and oxygen at the positive pole. these gases will escape upwardly, so that they will be trapped in their respective compartments, and may be drawn off by means of the pipes. purifying water.--advantage is taken of this electrolytic action, to purify water. oxygen is the most wonderful chemical in nature. it is called the acid-maker of the universe. the name is derived from two words, _oxy_ and _gen_; one denoting oxydation, and the other that it generates. in other words, it is the _generator of oxides_. it is the element which, when united with any other element, produces an acid, an alkali or a neutral compound. rust.--for instance, iron is largely composed of ferric acid. when oxygen, in a free or gaseous state, comes into contact with iron, it produces ferrous oxide, which is recognized as rust. oxygen as a purifier.--but oxygen is also a purifier. all low forms of animal life, like bacteria or germs in water, succumb to free oxygen. by _free oxygen_ is meant oxygen in the form of gas. composition of water.--now, water, in which harmful germs live, is one-third oxygen. nevertheless, the germs thrive in water, because the oxygen is in a compound state, and, therefore, not an active agent. but if oxygen, in the form of gas, can be forced through water, it will attack the germs, and destroy them. common air not a good purifier.--water may be purified, to a certain extent, by forcing common air through it, and the foulest water, if run over rocks, will be purified, in a measure, because air is intermingled with it. but common air is composed of four-fifths nitrogen, and only one-fifth oxygen, and, as nitrogen is the staple article of food for bacteria, the purifying method by air is not effectual. pure oxygen.--when, however, oxygen is generated from water, by means of electrolysis, it is pure; hence is more active and is not tainted by a life-giving substance for germs, such as nitrogen. the mechanism usually employed for purifying water is shown in fig. 90. a water purifier.--the case (a, fig. 90) may be made of metal or of an insulating material. if made of metal it must be insulated within with slate, glass, marble or hard rubber, as shown at b. the case is provided with exterior flanges (c, d), with upper and lower ends, and it is mounted upon a base plate (e) and affixed thereto by bolts. the upper end has a conically-formed cap (f) bolted to the flanges (c), and this has an outlet to which a pipe (g) is attached. the water inlet pipe (h) passes through the lower end of the case (a). the electrodes (i, j) are secured, vertically, within the case, separated from each other equidistant, each alternate electrode being connected up with one wire (k), and the alternate electrodes with a wire (l). [illustration: _fig. 90._ electric water purifier] when the water passes upwardly, the decomposed or gaseous oxygen percolates through the water and thus attacks the germs and destroys them. the use of hydrogen in purification.--on the other hand, the hydrogen also plays an important part in purifying the water. this depends upon the material of which the electrodes are made. aluminum is by far the best material, as it is one of nature's most active purifiers. all clay contains aluminum, in what is known as the sulphate form, and water passing through the clay of the earth thereby becomes purified, because of this element. aluminum electrodes.--when this material is used as the electrodes in water, hydrate of aluminum is formed, or a compound of hydrogen and oxygen with aluminum. the product of decomposition is a flocculent matter which moves upwardly through the water, giving it a milky appearance. this substance is like gelatine, so that it entangles or enmeshes the germ life and prevents it from passing through a filter. if no filter is used, this flocculent matter, as soon as it has given off the gases, will settle to the bottom and carry with it all decomposed matter, such as germs and other organic matter attacked by the oxygen, which has become entangled in the aluminum hydrate. electric hand purifier.--an interesting and serviceable little purifier may be made by any boy with the simplest tools, by cutting out three pieces of sheet aluminum. hard rolled is best for the purpose. it is better to have one of the sheets (a), the middle one, thicker than the two outer plates (b). [illustration: _fig. 91._ portable electric purifier] let each sheet be 1-1/2 inches wide and 5-1/2 inches thick. one-half inch from the upper ends of the two outside plates (b, b) bore bolt holes (c), each of these holes being a quarter of an inch from the edge of the plate. the inside plate (a) has two large holes (d) corresponding with the small holes (c) in the outside plates. at the upper end of this plate form a wing (e), 1/2 inch wide and 1/2 inch long, provided with a small hole for a bolt. next cut out two hard-rubber blocks (f), each 1-1/2 inches long, 1 inch wide and 3/8 inch thick, and then bore a hole (g) through each, corresponding with the small holes (c) in the plates (b). the machine is now ready to be assembled. if the inner plate is 1/8 inch thick and the outer plates each 1/16 inch thick, use two small eighth-inch bolts 1-1/4 inches long, and clamp together the three plates with these bolts. one of the bolts may be used to attach thereto one of the electric wires (h), and the other wire (i) is attached by a bolt to the wing (e). [illustration: _figs. 92-95._ details of portable purifier] such a device will answer for a 110-volt circuit, in ordinary water. now fill a glass nearly full of water, and stand the purifier in the glass. within a few minutes the action of electrolysis will be apparent by the formation of numerous bubbles on the plates, followed by the decomposition of the organic matter in the water. at first the flocculent decomposed matter will rise to the surface of the water, but before many minutes it will settle to the bottom of the glass and leave clear water above. purification and separation of metals.--this electrolytic action is utilized in metallurgy for the purpose of producing pure metals, but it is more largely used to separate copper from its base. in order to utilize a current for this purpose, a high ampere flow and low voltage are required. the sheets of copper, containing all of its impurities, are placed within a tank, parallel with a thin copper sheet. the impure sheet is connected with the positive pole of an electroplating dynamo, and the thin sheet of copper is connected with the negative pole. the electrolyte in the tank is a solution of sulphate of copper. the action of the current will cause the pure copper in the impure sheet to disintegrate and it is then carried over and deposited upon the thin sheet, this action continuing until the impure sheet is entirely eaten away. all the impurities which were in the sheet fall to the bottom of the tank. other metals are treated in the same way, and this treatment has a very wide range of usefulness. electroplating.--the next feature to be considered in electrolysis is a most interesting and useful one, because a cheap or inferior metal may be coated by a more expensive metal. silver and nickel plating are brought about by this action of a current passing through metals, which are immersed in an electrolyte. plating iron with copper.--we have room in this chapter for only one concrete example of this work, which, with suitable modifications, is an example of the art as practiced commercially. iron, to a considerable extent, is now being coated with copper to preserve it from rust. to carry out this work, however, an electroplating dynamo, of large amperage, is required, the amperage, of course, depending upon the surface to be treated at one time. the pressure should not exceed 5 volts. the iron surface to be treated should first be thoroughly cleansed, and then immediately put into a tank containing a cyanide of copper solution. two forms of copper solution are used, namely, the cyanide, which is a salt solution of copper, and the sulphate, which is an acid solution of copper. cyanide is first used because it does not attack the iron, as would be the case if the sulphate solution should first come into contact with the iron. a sheet of copper, termed the anode, is then placed within the tank, parallel with the surface to be plated, known as the cathode, and so mounted that it may be adjusted to or from the iron surface, or cathode. a direct current of electricity is then caused to flow through the copper plate and into the iron plate or surface, and the plating proceeded with until the iron surface has a thin film of copper deposited thereon. this is a slow process with the cyanide solution, so it is discontinued as soon as possible, after the iron surface has been completely covered with copper. this copper surface is thoroughly cleaned off to remove therefrom the saline or alkaline solution, and it is then immersed within a bath, containing a solution of sulphate of copper. the current is then thrown on and allowed so to remain until it has deposited the proper thickness of copper. direction of current.--if a copper and an iron plate are put into a copper solution and connected up in circuit with each other, a primary battery is thereby formed, which will generate electricity. in this case, the iron will be positive and the copper negative, so that the current within such a cell would flow from the iron (in this instance, the anode) to the negative, or cathode. the action of electroplating reverses this process and causes the current to flow from the copper to the iron (in this instance, the cathode). chapter xiv electric heating, thermo electricity generating heat in a wire.--when a current of electricity passes through a conductor, like a wire, more or less heat is developed in the conductor. this heat may be so small that it cannot be measured, but it is, nevertheless, present in a greater or less degree. conductors offer a resistance to the passage of a current, just the same as water finds a resistance in pipes through which it passes. this resistance is measured in ohms, as explained in a preceding chapter, and it is this resistance which is utilized for electric heating. resistance of substances.--silver offers less resistance to the passage of a current than any other metal, the next in order is copper, while iron is, comparatively, a poor conductor. the following is a partial list of metals, showing their relative conductivity: silver 1. copper 1.04 to 1.09 gold 1.38 to 1.41 aluminum 1.64 zinc 3.79 nickel 4.69 iron 6.56 tin 8.9 lead 13.2 german silver 12.2 to 15 from this table it will be seen that, for instance, iron offers six and a half times the resistance of silver, and that german silver has fifteen times the resistance of silver. this table is made up of strands of the different metals of the same diameters and lengths, so as to obtain their relative values. sizes of conductors.--another thing, however, must be understood. if two conductors of the same metal, having different diameters, receive the same current of electricity, the small conductor will offer a greater resistance than the large conductor, hence will generate more heat. this can be offset by increasing the diameter of the conductor. the metal used is, therefore, of importance, on account of the cost involved. comparison of metals.--a conductor of aluminum, say, 10 feet long and of the same weight as copper, has a diameter two and a quarter times greater than copper; but as the resistance of aluminum is 50 per cent. more than that of silver, it will be seen that, weight for weight, copper is the cheaper, particularly as aluminum costs fully three times as much as copper. [illustration: _fig. 96._ simple electric heater] the table shows that german silver has the highest resistance. of course, there are other metals, like antimony, platinum and the like, which have still higher resistance. german silver, however, is most commonly used, although there are various alloys of metal made which have high resistance and are cheaper. the principle of all electric heaters is the same, namely, the resistance of a conductor to the passage of a current, and an illustration of a water heater will show the elementary principles in all of these devices. a simple electric heater.--in fig. 96 the illustration shows a cup or holder (a) for the wire, made of hard rubber. this may be of such diameter as to fit upon and form the cover for a glass (b). the rubber should be 1/2 inch thick. two holes are bored through the rubber cup, and through them are screwed two round-headed screws (c, d), each screw being 1-1/2 inches long, so they will project an inch below the cap. each screw should have a small hole in its lower end to receive a pin (e) which will prevent the resistance wire from slipping off. the resistance wire (f) is coiled for a suitable length, dependent upon the current used, one end being fastened by wrapping it around the screw (c). the other end of the wire is then brought upwardly through the interior of the coil and secured in like manner to the other screw (d). caution must be used to prevent the different coils or turns from touching each other. when completed, the coil may be immersed in water, the current turned on, and left so until the water is sufficiently heated. [illustration: _figs. 97-98._ resistance device] how to arrange for quantity of current used.--it is difficult to determine just the proper length the coil should be, or the sizes of the wire, unless you know what kind of current you have. you may, however, rig up your own apparatus for the purpose of making it fit your heater, by preparing a base of wood (a) 8 inches long, 3 inches wide and 1 inch thick. on this mount four electric lamp sockets (b). then connect the inlet wire (c) by means of short pieces of wire (d) with all the sockets on one side. the outlet wire (e) should then be connected up with the other sides of the sockets by the short wires (f). if, now, we have one 16-candlepower lamp in one of the sockets, there is a half ampere going through the wires (c, f). if there are two lamps on the board you will have 1 ampere, and so on. by this means you may readily determine how much current you are using and it will also afford you a means of finding out whether you have too much or too little wire in your coil to do the work. [illustration: _fig. 99._ plan view of electric iron] an electric iron.--an electric iron is made in the same way. the upper side of a flatiron has a circular or oval depression (a) cast therein, and a spool of slate (b) is made so it will fit into the depression and the high resistance wire (c) is wound around this spool, and insulating material, such as asbestos, must be used to pack around it. centrally, the slate spool has an upwardly projecting circular extension (d) which passes through the cap or cover (e) of the iron. the wires of the resistance coil are then brought through this circular extension and are connected up with the source of electrical supply. wires are now sold for this purpose, which are adapted to withstand an intense heat. [illustration: _fig. 100._ section of electric iron] the foregoing example of the use of the current, through resistance wires, has a very wide application, and any boy, with these examples before him, can readily make these devices. thermo electricity.--it has long been the dream of scientists to convert heat directly into electricity. the present practice is to use a boiler to generate steam, an engine to provide the motion, and a dynamo to convert that motion into electricity. the result is that there is loss in the process of converting the fuel heat into steam; loss to change the steam into motion, and loss to make electricity out of the motion of the engine. by using water-power there is less actual loss; but water-power is not available everywhere. converting heat directly into electricity.--heat may be converted directly into electricity without using a boiler, an engine or a dynamo, but it has not been successful from a commercial standpoint. it is interesting, however, to know and understand the subject, and for that reason it is explained herein. metals; electric positive-negative.--to understand the principle, it may be stated that all metals are electrically positive-negative to each other. you will remember that it has hereinbefore been stated that if, for instance, iron and copper are put into an acid solution, a current will be created or generated thereby. so with zinc and copper, the usual primary battery elements. in all such cases an electrolyte is used. thermo-electricity dispenses with the electrolyte, and nothing is used but the metallic elements and heat. the word thermo means heat. if, now, we can select two strips of different metals, and place them as far apart as possible--that is, in their positive-negative relations with each other, and unite the end of one with one end of other by means of a rivet, and then heat the riveted ends, a current will be generated in the strips. if, for instance, we use an iron in conjunction with a copper strip, the current will flow from the copper to the iron, because copper is positive to iron, and iron negative to copper. it is from this that the term positive-negative is taken. the two metals most available, which are thus farthest apart in the scale of positive-negative relation, are bismuth and antimony. [illustration: _fig. 101._ thermo-electric couple] in fig. 101 is shown a thermo-electric couple (a, b) riveted together, with thin outer ends connected by means of a wire (c) to form a circuit. a galvanometer (d) or other current-testing means is placed in this circuit. a lamp is placed below the joined ends. thermo-electric couples.--any number of these couples may be put together and joined at each end to a common wire and a fairly large flow of current obtained thereby. one thing must be observed: a current will be generated only so long as there exists a difference in temperature between the inner and the outer ends of the bars (a, b). this may be accomplished by water, or any other cooling means which may suggest itself. chapter xv alternating currents, choking coils, transformers, converters and rectifiers direct current.--when a current of electricity is generated by a cell, it is assumed to move along the wire in one direction, in a steady, continuous flow, and is called a _direct_ current. this direct current is a natural one if generated by a cell. alternating current.--on the other hand, the natural current generated by a dynamo is alternating in its character--that is, it is not a direct, steady flow in one direction, but, instead, it flows for an instant in one direction, then in the other direction, and so on. a direct-current dynamo such as we have shown in chapter iv, is much easier to explain, hence it is illustrated to show the third method used in generating an electric current. it is a difficult matter to explain the principle and operation of alternating current machines, without becoming, in a measure, too technical for the purposes of this book, but it is important to know the fundamentals involved, so that the operation and uses of certain apparatus, like the choking coil, transformers, rectifiers and converters, may be explained. the magnetic field.--it has been stated that when a wire passes through the magnetic field of a magnet, so as to cut the lines of force flowing out from the end of a magnet, the wire will receive a charge of electricity. [illustration: _fig. 102._ cutting a magnetic field] to explain this, study fig. 102, in which is a bar magnet (a). if we take a metal wire (b) and bend it in the form of a loop, as shown, and mount the ends on journal-bearing blocks, the wire may be rotated so that the loop will pass through the magnetic field. when this takes place, the wire receives a charge of electricity, which moves, say, in the direction of the darts, and will make a complete circuit if the ends of the looped wire are joined, as shown by the conductor (d). action of the magnetized wire.--you will remember, also that we have pointed out how, when a current passes over a wire, it has a magnetic field extending out around it at all points, so that while it is passing through the magnetic field of the magnet (a), it becomes, in a measure, a magnet of its own and tries to set up in business for itself as a generator of electricity. but when the loop leaves the magnetic field, the magnetic or electrical impulse in the wire also leaves it. the movement of a current in a charged wire.--your attention is directed, also, to another statement, heretofore made, namely, that when a current from a charged wire passes by induction to a wire across space, so as to charge it with an electric current, it moves along the charged wire in a direction opposite to that of the current in the charging wire. now, the darts show the direction in which the current moves while it is approaching and passing through the magnetic field. but the moment the loop is about to pass out of the magnetic field, the current in the loop surges back in the opposite direction, and when the loop has made a revolution and is again entering the magnetic field, it must again change the direction of flow in the current, and thus produce alternations in the flow thereof. let us illustrate this by showing the four positions of the revolving loop. in fig. 103 the loop (b) is in the middle of the magnetic field, moving upwardly in the direction of the curved dart (a), and while in that position the voltage, or the electrical impulse, is the most intense. the current used flows in the direction of the darts (c) or to the left. in fig. 104, the loop (a) has gone beyond the influence of the magnetic field, and now the current in the loop tries to return, or reverse itself, as shown by the dart (d). it is a reaction that causes the current to die out, so that when the loop has reached the point farthest from the magnet, as shown in fig. 105, there is no current in the loop, or, if there is any, it moves faintly in the direction of the dart (e). [illustration: _figs. 103-106._ illustrating alternations] current reversing itself.--when the loop reaches its lowest point (fig. 106) it again comes within the magnetic field and the current commences to flow back to its original direction, as shown by darts (c). self-induction.--this tendency of a current to reverse itself, under the conditions cited, is called self-induction, or inductance, and it would be well to keep this in mind in pursuing the study of alternating currents. you will see from the foregoing, that the alternations, or the change of direction of the current, depends upon the speed of rotation of the loop past the end of the magnet. [illustration: _figs. 107-108._ form for increasing alternations] instead, therefore, of using a single loop, we may make four loops (fig. 107), which at the same speed as we had in the case of the single loop, will give four alternations, instead of one, and still further, to increase the periods of alternation, we may use the four loops and two magnets, as in fig. 108. by having a sufficient number of loops and of magnets, there may be 40, 50, 60, 80, 100 or 120 such alternating periods in each second. time, therefore, is an element in the operation of alternating currents. let us now illustrate the manner of connecting up and building the dynamo, so as to derive the current from it. in fig. 109, the loop (a) shows, for convenience, a pair of bearings (b). a contact finger (c) rests on each, and to these the circuit wire (d) is attached. do not confuse these contact fingers with the commutator brushes, shown in the direct-current motor, as they are there merely for the purpose of making contact between the revolving loop (a) and stationary wire (d). [illustration: _fig. 109._ connection of alternating dynamo armature] brushes in a direct-current dynamo.--the object of the brushes in the direct-current dynamo, in connection with a commutator, is to convert this _inductance_ of the wire, or this effort to reverse itself into a current which will go in one direction all the time, and not in both directions alternately. to explain this more fully attention is directed to figs. 110 and 111. let a represent the armature, with a pair of grooves (b) for the wires. the commutator is made of a split tube, the parts so divided being insulated from each other, and in fig. 110, the upper one, we shall call and designate the positive (+) and the lower one the negative (-). the armature wire (c) has one end attached to the positive commutator terminal and the other end of this wire is attached to the negative terminal. [illustration: _fig. 110._ direct current dynamo] one brush (d) contacts with the positive terminal of the commutator and the other brush (e) with the negative terminal. let us assume that the current impulse imparted to the wire (c) is in the direction of the dart (f, fig. 110). the current will then flow through the positive (+) terminal of the commutator to the brush (d), and from the brush (d) through the wire (g) to the brush (e), which contacts with the negative (-) terminal of the commutator. this will continue to be the case, while the wire (c) is passing the magnetic field, and while the brush (d) is in contact with the positive (+) terminal. but when the armature makes a half turn, or when it reaches that point where the brush (d) contacts with the negative (-) terminal, and the brush (e) contacts with the positive (+) terminal, a change in the direction of the current through the wire (g) takes place, unless something has happened to change it before it has reached the brushes (d, e). [illustration: _fig. 111._ circuit wires in direct current dynamo] now, this change is just exactly what has happened in the wire (c), as we have explained. the current attempts to reverse itself and start out on business of its own, so to speak, with the result that when the brushes (d and e) contact with the negative and positive terminals, respectively, the surging current in the wire (c) is going in the direction of the dart (h)--that is, while, in fig. 110, the current flows from the wire (c) into the positive terminal, and out of the negative terminal into the wire (c), the conditions are exactly reversed in fig. 111. here the current in wire c flows _into_ the negative (-) terminal, and _from_ the positive (+) terminal into the wire c, so that in either case the current will flow out of the brush d and into the brush e, through the external circuit (g). it will be seen, therefore, that in the direct-current motor, advantage is taken of the surging, or back-and-forth movement, of the current to pass it along in one direction, whereas in the alternating current no such change in direction is attempted. alternating positive and negative poles.--the alternating current, owing to this surging movement, makes the poles alternately positive and negative. to express this more clearly, supposing we take a line (a, fig. 112), which is called the zero line, or line of no electricity. the current may be represented by the zigzag line (b). the lines (b) above zero (a) may be designated as positive, and those below the line as negative. the polarity reverses at the line a, goes up to d, which is the maximum intensity or voltage above zero, and, when the current falls and crosses the line a, it goes in the opposite direction to e, which is its maximum voltage in the other direction. in point of time, if it takes one second for the current to go from c to f, on the down line, then it takes only a half second to go from c to g, so that the line a represents the time, and the line h the intensity, a complete cycle being formed from c, d, f, then through f, e, c, and so on. [illustration: _fig. 112._ alternating polarity lines] how an alternating dynamo is made.--it is now necessary to apply these principles in the construction of an alternating-current machine. fig. 113 is a diagram representing the various elements, and the circuiting. [illustration: _fig. 113._ alternating current dynamo] let a represent the ring or frame containing the inwardly projecting field magnet cores (b). c is the shaft on which the armature revolves, and this carries the wheel (d), which has as many radially disposed magnet cores (e) as there are of the field magnet cores (b). the shaft (c) also carries two pulleys with rings thereon. one of these rings (f) is for one end of the armature winding, and the other ring (g) for the other end of the armature wire. the windings.--the winding is as follows: one wire, as at h, is first coiled around one magnet core, the turnings being to the right. the outlet terminal of this wire is then carried to the next magnet core and wound around that, in the opposite direction, and so on, so that the terminal of the wire is brought out, as at i, all of these wires being connected to binding posts (j, j'), to which, also, the working circuits are attached. the armature wires.--the armature wires, in like manner, run from the ring (g) to one armature core, being wound from right to left, then to the next core, which is wound to the right, afterward to the next core, which is wound to the left, and so on, the final end of the wire being connected up with the other ring (f). the north (n) and the south (s) poles are indicated in the diagram. choking coil.--the self-induction in a current of this kind is utilized in transmitting electricity to great distances. wires offer resistance, or they impede the flow of a current, as hereinbefore stated, so that it is not economical to transmit a direct current over long distances. this can be done more efficiently by means of the alternating current, which is subject to far less loss than is the case with the direct current. it affords a means whereby the flow of a current may be checked or reduced without depending upon the resistance offered by the wire over which it is transmitted. this is done by means of what is called a choking coil. it is merely a coil of wire, wound upon an iron core, and the current to be choked passes through the coil. to illustrate this, let us take an arc lamp designed to use a 50-volt current. if a current is supplied to it carrying 100 volts, it is obvious that there are 50 volts more than are needed. we must take care of this excess of 50 volts without losing it, as would happen were we to locate a resistance of some kind in the circuit. this result we accomplish by the introduction of the choking coil, which has the effect of absorbing the excessive 50 volts, the action being due to its quality of self-induction, referred to in the foregoing. [illustration: _fig. 114._ choking coil] in fig. 114, a is the choking coil and b an arc lamp, connected up, in series, with the choking coil. the transformer.--it is more economical to transmit 10,000 volts a long distance than 1,000 volts, because the lower the pressure, or the voltage, the larger must be the conductor to avoid loss. it is for this reason that 500 volts, or more, are used on electric railways. for electric light purposes, where the current goes into dwellings, even this is too high, so a transformer is used to take a high-voltage current from the main line and transform it into a low voltage. this is done by means of two distinct coils of wire, wound upon an iron core. [illustration: _fig. 115._ a transformer] in fig. 115 the core is o-shaped, so that a primary winding (a), from the electrical source, can be wound upon one limb, and the secondary winding (b) wound around the other limb. the wires, to supply the lamps, run from the secondary coil. there is no electrical connection between the two coils, but the action from the primary to the secondary coil is solely by induction. when a current passes through the primary coil, the surging movement, heretofore explained, is transmitted to the iron core, and the iron core, in turn, transmits this electrical energy to the secondary coil. how the voltage is determined.--the voltage produced by the secondary coil will depend upon several things, namely, the strength of the magnetism transmitted to it; the rapidity, or periodicity of the current, and the number of turns of wire around the coil. the voltage is dependent upon the length of the winding. but the voltage may also be increased, as well as decreased. if the primary has, we will say, 100 turns of wire, and has 200 volts, and the secondary has 50 turns of wire, the secondary will give forth only one-half as much as the primary, or 100 volts. if, on the other hand, 400 volts would be required, the secondary should have 200 turns in the winding. voltage and amperage in transformers.--it must not be understood that, by increasing the voltage in this way, we are getting that much more electricity. if the primary coil, with 100 turns, produces a current of 200 volts and 50 amperes, which would be 200 ã� 50 = 10,000 watts, and the secondary coil has 50 turns, we shall have 100 volts and 100 amperes: 100 (v.) ã� 100 (a.) = 10,000 watts. or, if, on the other hand, our secondary winding is composed of 200 turns, we shall have 400 volts and 25 amperes, 400 (volts) ã� 25 (amperes) also gives 10,000 watts. necessarily, there will be some loss, but the foregoing is offered as the theoretical basis of calculation. chapter xvi electric lighting the most important step in the electric field, after the dynamo had been brought to a fairly workable condition, was its utilization to make light. it was long known prior to the discovery of practical electric dynamos, that the electric current would produce an intense heat. ordinary fuels under certain favorable conditions will produce a temperature of 4,500 degrees of heat; but by means of the electric arc, as high as six, eight and ten thousand degrees are available. the fact that when a conductor, in an electric current, is severed, a spark will follow the drawing part of the broken ends, led many scientists to believe, even before the dynamo was in a practical shape, that electricity, sooner or later, would be employed as the great lighting agent. when the dynamo finally reached a stage in development where its operation could be depended on, and was made reversible, the first active steps were taken to not only produce, but to maintain an arc between two electrodes. it would be difficult and tedious to follow out the first experiments in detail, and it might, also, be useless, as information, in view of the present knowledge of the science. a few steps in the course of the development are, however, necessary to a complete understanding of the subject. reference has been made in a previous chapter to what is called the _electric arc_, produced by slightly separated conductors, across which the electric current jumps, producing the brilliantly lighted area. this light is produced by the combustion of the carbon of which the electrodes are composed. thus, the illumination is the result of directly burning a fuel. the current, in passing from one electrode to the other, through the gap, produces such an intense heat that the fuel through which the current passes is consumed. carbon in a comparatively pure state is difficult to ignite, owing to its great resistance to heat. at about 7,000 degrees it will fuse, and pass into a vapor which causes the intense illumination. the earliest form of electric lighting was by means of the arc, in which the light is maintained so long as the electrodes were kept a certain distance apart. to do this requires delicate mechanism, for the reason that when contact is made, and the current flows through the two electrodes, which are connected up directly with the coils of a magnet, the cores, or armatures, will be magnetized. the result is that the electrode, connected with the armature of the magnet, is drawn away from the other electrode, and the arc is formed, between the separated ends. as the current also passes through a resistance coil, the moment the ends of the electrodes are separated too great a distance, the resistance prevents a flow of the normal amount of current, and the armature is compelled to reduce its pull. the effect is to cause the two electrodes to again approach each other, and in doing so the arc becomes brighter. it will be seen, therefore, that there is a constant fight between the resistance coil and the magnet, the combined action of the two being such, that, if properly arranged, and with powers in correct relation to each other, the light may be maintained without undue flickering. such devices are now universally used, and they afford a steady and reliable means of illumination. many improvements are made in this direction, as well as in the ingredients of the electrodes. a very novel device for assuring a perfect separation at all times between the electrodes, is by means of a pair of parallel carbons, held apart by a non-conductor such as clay, or some mixture of earth, a form of which is shown in fig. 116. the drawing shows two electrodes, separated by a non-conducting material, which is of such a character that it will break down and crumble away, as the ends of the electrodes burn away. [illustration: _fig. 116. parallel carbons._] this device is admirable where the alternating current is used, because the current moves back and forth, and the two electrodes are thus burned away at the same rate of speed. in the direct or continuous current the movement is in one direction only, and as a result the positive electrode is eaten away twice as fast as the negative. this is the arc form of lamp universally used for lighting large spaces or areas, such as streets, railway stations, and the like. it is important also as the means for utilizing searchlight illumination, and frequently for locomotive headlights. arc lights are produced by what is called the _series current_. this means that the lamps are all connected in a single line. this is illustrated by reference to fig. 117, in which a represents the wire from the dynamo, and b, c the two electrodes, showing the current passing through from one lamp to the next. [illustration: _fig. 117. arc-lighting circuit._] a high voltage is necessary in order to cause the current to leap across the gap made by the separation of the electrodes. the incandescent system.--this method is entirely different from the arc system. it has been stated that certain metals conduct electricity with greater facility than others, and some have higher resistance than others. if a certain amount of electricity is forced through some metals, they will become heated. this is true, also, if metals, which, ordinarily, will conduct a current freely, are made up into such small conductors that it is difficult for the current to pass. [illustration: _fig 118. interrupted conductor._] in the arc method high voltage is essential; in the incandescent plan, current is the important consideration. in the arc, the light is produced by virtue of the break in the line of the conductor; in the incandescent, the system is closed at all times. supposing we have a wire a, a quarter of an inch in diameter, carrying a current of, say, 500 amperes, and at any point in the circuit the wire is made very small, as shown at b, in fig. 118, it is obvious that the small wire would not be large enough to carry the current. the result would be that the small connection b would heat up, and, finally, be fused. while the large part of the wire would carry 500 amperes, the small wire could not possibly carry more than, say, 10 amperes. now these little wires are the filaments in an electric bulb, and originally the attempt was made to have them so connected up that they could be illuminated by a single wire, as with the arc system above explained, one following the other as shown in fig. 117. [illustration: _fig. 119. incandescent circuit._] it was discovered, however, that the addition of each successive lamp, so wired, would not give light in proportion to the addition, but at only about one-fourth the illumination, and such a course would, therefore, make electric lighting enormously expensive. this knowledge resulted in an entirely new system of wiring up the lamps in a circuit. this is explained in fig. 119. in this figure a represents the dynamo, b, b the brushes, c, d the two line wires, e the lamps, and f the short-circuiting wires between the two main conductors c, d. it will be observed that the wires c, d are larger than the cross wires f. the object is to show that the main wires might carry a very heavy amperage, while the small cross wires f require only a few amperes. this is called the _multiple_ circuit, and it is obvious that the entire amperage produced by the dynamo will not be required to pass through each lamp, but, on the other hand, each lamp takes only enough necessary to render the filament incandescent. this invention at once solved the problem of the incandescent system and was called the subdivision of the electric light. by this means the cost was materially reduced, and the wiring up and installation of lights materially simplified. but the divisibility of the light did not, by any means, solve the great problem that has occupied the attention of electricians and experimenters ever since. the great question was and is to preserve the little filament which is heated to incandescence, and from which we get the light. the effort of the current to pass through the small filament meets with such a great resistance that the substance is heated up. if it is made of metal there is a point at which it will fuse, and thus the lamp is destroyed. it was found that carbon, properly treated, would heat to a brilliant white heat without fusing, or melting, so that this material was employed. but now followed another difficulty. as this intense heat consumed the particles of carbon, owing to the presence of oxygen, means were sought to exclude the air. this was finally accomplished by making a bulb of glass, from which the air was exhausted, and as such a globe had no air to support combustion, the filaments were finally made so that they would last a long time before being finally disintegrated. the quest now is, and has been, to find some material of a purely metallic character, which will have a very high fusing point, and which will, therefore, dispense with the cost of the exhausted bulb. some metals, as for instance, osmium, tantalum, thorium, and others, have been used, and others, also, with great success, so that the march of improvements is now going forward with rapid strides. vapor lamps.--one of the directions in which considerable energy has been directed in the past, was to produce light from vapors. the cooper hewitt mercury vapor lamp is a tube filled with the vapor of mercury, and a current is sent through the vapor which produces a greenish light, and owing to that peculiar color, has not met with much success. it is merely cited to show that there are other directions than the use of metallic conductors and filaments which will produce light, and the day is no doubt close at hand when we may expect some important developments in the production of light by means of the hertzian waves. directions for improvements.--electricity, however, is not a cheap method of illumination. the enormous heat developed is largely wasted. the quest of the inventor is to find a means whereby light can be produced without the generation of the immense heat necessary. man has not yet found a means whereby he can make a heat without increasing the temperature, as nature does it in the glow worm, or in the firefly. a certain electric energy will produce both light and heat, but it is found that much more of this energy is used in the heat than in the light. what wonderful possibilities are in store for the inventor who can make a heatless light! it is a direction for the exercise of ingenuity that will well repay any efforts. _curious superstitions concerning electricity_ electricity, as exhibited in light, has been the great marvel of all times. the word electricity itself comes from the thunderbolt of the ancient god zeus, which is known to be synonymous with the thunderbolt and the lightning. magnetism, which we know to be only another form of electricity, was not regarded the same as electricity by the ancients. iron which had the property to attract, was first found near the town of magnesia, in lydia, and for that reason was called magnetism. later on, a glimmer of the truth seemed to dawn on the early scientists, when they saw the resemblance between the actions of the amber and the loadstone, as both attracted particles. and here another curious thing resulted. amber will attract particles other than metals. the magnet did not; and from this imperfect observation and understanding, grew a belief that electricity, or magnetism would attract all substances, even human flesh, and many devices were made from magnets, and used as cures for the gout, and to affect the brain, or to remove pain. even as early as 2,500 years before the birth of christ the chinese knew of the properties of the magnet, and also discovered that a bar of the permanent magnet would arrange itself north and south, like the mariners' compass. there is no evidence, however, that it was used as a mariner's compass until centuries afterwards. but the matter connected with light, as an electrical development, which interests us, is its manifestations to the ancients in the form of lightning. the electricity of the earth concentrates itself on the tops of mountains, or in sharp peaks, and accounts for the magnificent electrical displays always found in mountainous regions. some years ago, a noted scientist, dr. siemens, while standing on the top of the great pyramid of cheops, in egypt, during a storm, noted that an electrical discharge flowed from his hand when extended toward the heavens. the current manifested itself in such a manner that the hissing noise was plainly perceptible. the literature of all ages and of all countries shows that this manifestation of electrical discharges was noted, and became the subject of discussions among learned men. all these displays were regarded as the bolts of an angry god, and historians give many accounts of instances where, in his anger, he sent down the lightning to destroy. among the romans jupiter thus hurled forth his wrath; and among many ancient people, even down to the time of charlemagne, any space struck by lightning was considered sacred, and made consecrated ground. from this grew the belief that it was sacrilegious to attempt to imitate the lightning of the sky--that deity would visit dire punishment on any man who attempted to produce an electric light. virgil relates accounts where certain princes attempted to imitate the lightning, and were struck by thunderbolts as punishments. less than a century ago benjamin franklin devised the lightning rod, in order to prevent lightning from striking objects. the literature of that day abounds with instances of protests made, on the part of those who were as superstitions as the people in ancient times, who urged that it was impious to attempt to ward off heaven's lightnings. it was argued that the lightning was one way in which the creator manifested his displeasure, and exercised his power to strike the wicked. when such writers as pliny will gravely set forth an explanation of the causes of lightning, as follows in the paragraph below, we can understand why it inculcated superstitious fears in the people of ancient times. he says: "most men are ignorant of that secret, which, by close observation of the heavens, deep scholars and principal men of learning have found out, namely, that they are the fires of the uppermost planets, which, falling to the earth, are called lightning; but those especially which are seated in the middle, that is about jupiter, perhaps because participating in the excessive cold and moisture from the upper circle of saturn, and the immoderate heat of mars, that is next beneath, by this means he discharges his superfluity, and therefore it is commonly said, 'that jupiter shooteth and darteth lightning.' therefore, like as out of a burning piece of wood a coal flieth forth with a crack, even so from a star is spit out, as it were, and voided forth this celestial fire, carrying with it presages of future things; so that the heavens showeth divine operations, even in these parcels and portions which are rejected and cast away as superfluous." chapter xvii power, and various other electrical manifestations it would be difficult to mention any direction in human activity where electricity does not serve as an agent in some form or manner. man has learned that the creator gave this great power into the hands of man to use, and not to curse. when the dynamo was first developed it did not appear possible that it could generate electricity, and then use that electricity in order to turn the dynamo in the opposite direction. it all seems so very natural to us now, that such a thing should practically follow; but man had to learn this. let us try to make the statement plain by a few simple illustrations. by carefully going over the chapter on the making of the dynamo, it will be evident that the basis of the generation of the current depends on the changing of the direction of the flow of an electric current. look at the simple horse-shoe magnet. if two of them are gradually moved toward each other, so that the north pole of one approaches the north pole of the other, there is a sensible attempt for them to push away from each other. if, however, one of them is turned, so that the north pole of one is opposite the south pole of the other, they will draw together. in this we have the foundation physical action of the dynamo and the motor. when power is applied to an armature, and it moves through a magnetic field, the action is just the same as in the case of the hand drawing the north and the south pole of the two approaching magnets from each other. the influence of the electrical disturbance produced by that act permeated the entire winding of the field and armature, and extended out on the whole line with which the dynamo was connected. in this way a current was established and transmitted, and with proper wires was sent in the form of circuits and distributed so as to do work. but an electric current, without suitable mechanism, is of no value. it must have mechanism to use it, as well as to make it. in the case of light, we have explained how the arc and the incandescent lamps utilize it for that purpose. but now, attempting to get something from it in the way of power, means another piece of mechanism. this is done by the motor, and this motor is simply a converter, or a device for reversing the action of the electricity. attention is called to figs. 120 and 121. let us assume that the field magnets a, a are the positives, and the magnets b, b the negatives. the revolving armature has also four magnet coils, two of them, c, c, being positive, and the other two, d, d, negative, each of these magnet coils being so connected up that they will reverse the polarities of the magnets. [illustration: _figs. 120-121._ action of magnets in a dynamo] now in the particular position of the revolving armature, in fig. 120, the magnets of the armature have just passed the respective poles of the field magnets, and the belt e is compelled to turn the armature past the pole pieces by force in the direction of the arrow f. after the armature magnets have gone to the positions in fig. 121, the positives a try to draw back the negatives d of the armature, and at the same time the negatives b repel the negatives d, because they are of the same polarities. this repulsion of the negatives a, b continues until the armature poles c, d have slightly passed them, when the polarities of the magnets c, d are changed; so that it will be seen, by reference to fig. 122, that d is now retreating from b, and c is going away from a--that is, being forced away contrary to their natural attractive influences, and in fig. 123, when the complete cycle is nearly finished, the positives are again approaching each other and the negatives moving together. [illustration: _figs. 122-123._ cycle action in dynamo] in this manner, at every point, the sets of magnets are compelled to move against their magnetic pull. this explains the dynamo. now take up the cycle of the motor, and note in fig. 124 that the negative magnets d of the armature are closely approaching the positive and negative magnets, on one side; and the positive magnets c are nearing the positive and negatives on the other side. the positives a, therefore, attract the negatives d, and the negative b exert a pull on the positives c at the same time. the result is that the armature is caused to revolve, as shown by the dart g, in a direction opposite to the dart in fig. 120. [illustration: _figs. 124-125._ action of magnets in motor] when the pole pieces of the magnets c, d are about to pass magnets a, b, as shown in fig. 125, it is necessary to change the polarities of the armature magnets c, d; so that by reference to fig. 126, it will be seen that they are now indicated as c-, and d+, respectively, and have moved to a point midway between the poles a, b (as in fig. 125), where the pull on one side, and the push on the other are again the same, and the last figure 127 shows the cycle nearly completed. the shaft of the motor armature is now the element which turns the mechanism which is to be operated. to convert electrical impulses into power, as thus shown, results in great loss. the first step is to take the steam boiler, which is the first stage in that source which is the most common and universal, and by means of fuel, converting water into steam. the second is to use the pressure of this steam to drive an engine; the third is to drive the dynamo which generates the electrical impulse; and the fourth is the conversion from the dynamo into a motor shaft. loss is met with at each step, and the great problem is to eliminate this waste. [illustration: _figs. 126-127._ positions of magnets in motor] the great advantage of electrical power is not in utilizing it for consumption at close ranges, but where it is desired to transmit it for long distances. such illustrations may be found in electric railways, and where water power can be obtained as the primal source of energy, the cost is not excessive. it is found, however, that even with the most improved forms of mechanism, in electrical construction, the internal combustion engines are far more economical. _transmission of energy_ one of the great problems has been the transmission of the current to great distances. by using a high voltage it may be sent hundreds of miles, but to use a current of that character in the cars, or shops, or homes, would be exceedingly dangerous. to meet this requirement transformers have been devised, which will take a current of very high voltage, and deliver a current of low tension, and capable of being used anywhere with the ordinary motors. the transformer.--this is an electrical device made up of a core or cores of thin sheet metal, around which is wound sets of insulated wires, one set being designed to receive the high voltage, and the other set to put out the low voltage, as described in a former chapter. these may be made where the original output is a very high voltage, so that they will be stepped down, first from one voltage to a lower, and then from that to the next lower stage. this is called the "step down" transformer, and is now used over the entire world, where large voltages are generated. electric furnaces.--the most important development of electricity in the direction of heat is its use in furnaces. as before stated, an intense heat is capable of being generated by the electric current, so that it becomes the great agent to use for the treatment of refractory material. in furnaces of this kind the electric arc is the mechanical form used to produce the great heat, the only difference being in the size of the apparatus. the electric furnace is simply an immense form of arc light, capable of taking a high voltage, and such an arc is enclosed within a suitable oven of refractory material, which still further conserves the heat. welding by electricity.--the next step is to use the high heat thus capable of being produced, to fuse metals so that they may be welded together. it is a difficult matter to unite two large pieces of metal by the forging method, because the highest heat is required, owing to their bulk, and in addition immense hammers, weighing tons, must be employed. electric welding offers a simple and easy method of accomplishing the result, and in the doing of which it avoids the oxidizing action of the forging heat. instead of heating the pieces to be welded in a forge, as is now done, the ends to be united are simply brought into contact, and the current is sent through the ends until they are in a soft condition, after which the parts are pressed together and united by the simple merging of the plastic condition in which they are reduced by the high electric heat. this form of welding makes the most perfect joint, and requires no hammering, as the mass of the metal flows from one part or end to the other; the unity is a perfect one, and the advantage is that the metals can be kept in a semi-fluid state for a considerable time, thus assuring a perfect admixture of the two parts. with the ordinary form of welding it is necessary to drive the heated parts together without any delay, and at the least cooling must be reheated, or the joint will not be perfect. the smallest kinds of electric heating apparatus are now being made, so that small articles, sheet metal, small rods, and like parts can be united with the greatest facility. chapter xviii x-ray, radium, and the like the camera sees things invisible to the human eye. its most effective work is done with beams which are beyond human perception. the photographer uses the _actinic_ rays. ordinary light is composed of the seven primary colors, of which the lowest in the scale is the red, and the highest to violet. those below the red are called the infra-red, and they are the hertzian waves, or those used in wireless telegraphy. those above the violet are called ultra-violet, and these are employed for x-ray work. the former are produced by the high tension electric apparatus, which we have described in the chapter relating to wireless telegraphy; and the latter, called also the roentgen rays, are generated by the crookes' tube. this is a tube from which all the atmosphere has been extracted so that it is a practical vacuum. within this are placed electrodes so as to divert the action of the electrical discharge in a particular direction, and this light, when discharged, is of such a peculiar character that its discovery made a sensation in the scientific world. the reason for this great wonder was not in the fact that it projected a light, but because of its character. ordinary light, as we see it with the eye, is capable of being reflected, as when we look into a mirror at an angle. the x-ray will not reflect, but instead, pass directly through the glass. then, ordinary light is capable of refraction. this is shown by a ray of light bending as it passes through a glass of water, which is noticed when the light is at an angle to the surface. the x-ray will pass through the water without being changed from a straight line. the foregoing being the case, it was but a simple step to conclude that if it were possible to find a means whereby the human eye could see within the ultra-violet beam, it would be possible to see through opaque substances. from the discovery so important and far reaching it was not long until it was found that if the ultra-violet rays, thus propagated, were transmitted through certain substances, their rates of vibration would be brought down to the speeds which send forth the visible rays, and now the eye is able to see, in a measure at least, what the actinic rays show. this discovery was but the forerunner of a still more important development, namely, the discovery of _radium_. the actual finding of the metal was preceded by the knowledge that certain minerals, and water, as well, possessed the property of radio-activity. radio-activity is a word used to express that quality in metals or other material by means of which obscure rays are emitted, that have the capacity of discharging electrified bodies, and the power to ionize gases, as well as to actually affect photograph plates. certain metals had this property to a remarkable degree, particularly uranium, thorium, polonium, actinium, and others, and in 1898 the curies, husband and wife, french chemists, isolated an element, very ductile in its character, which was a white metal, and had a most brilliant luster. pitchblende, the base metal from which this was extracted, was discovered to be highly radio-active, and on making tests of the product taken from it, they were surprised to find that it emitted a form of energy that far exceeded in calculations any computations made on the basis of radio-activity in the metals hitherto examined. but this was not the most remarkable part of the developments. the energy, whatever it was, had the power to change many other substances if brought into close proximity. it darkens the color of diamonds, quartz, mica, and glass. it changes some of the latter in color, some kinds being turned to brown and others into violet or purple tinges. radium has the capacity to redden the skin, and affect the flesh of persons, even at some considerable distance, and it is a most powerful germicide, destroying bacteria, and has been found also to produce some remarkable cures in diseases of a cancerous nature. the remarkable similarity of the rays propagated by this substance, with the x-rays, lead many to believe that they are electrical in their character, and the whole scientific world is now striving to use this substance, as well as the more familiar light waves of the roentgen tube, in the healing of diseases. it is not at all remarkable that this use of it should first be considered, as it has been the history of the electrical developments, from the earliest times, that each successive stage should find advocates who would urge its virtues to heal the sick. it was so when the dynamo was invented, when the high tension current was produced; and electrical therapeutics became a leading theme when transmission by induction became recognized as a scientific fact. it is not many years since the x-rays were discovered, and the first announcement was concerning its wonderful healing powers. this was particularly true in the case of radium, but for some reason, after the first tests, all experimenters were thwarted in their theories, because the science, like all others, required infinite patience and experience. it was discovered, in the case of the x-ray, that it must be used in a modified form, and accordingly, various modifications of the waves were introduced, called the _m_ and the _n_ rays, as well as many others, each having some peculiar qualification. in time, no doubt, the investigators will find the right quality for each disease, and learn how to apply it. thus, electricity, that most alluring thing which, in itself, cannot be seen, and is of such a character that it cannot even be defined in terms which will suit the exact scientific mind, is daily bringing new wonders for our investigation and use. it is, indeed, a study which is so broad that it has no limitations, and a field which never will be exhausted. the end glossary of words used in text of this volume acid. accumulator material is sulphuric acid, diluted with water. active that part of the material in accumulator plates material. which is acted upon by the electric current. accumulator. a cell, generally known as a storage battery, which while it initially receives a charge of electricity, is nevertheless, of such a character, owing to the active material of which it is made, that it accumulates, or, as it were, generates electricity. aerial wire, the wire which, in wireless telegraphy, is carried or conductor. up into the air to connect the antennã¦ with the receiving and sending apparatus. alarm, burglar. a circulating system in a building, connected up with a bell or other signaling means. alloy. a mixture of two or more metals; as copper and zinc to make brass; nickel and zinc to form german silver. alternating current. a current which goes back and forth in opposite directions, unlike a direct current which flows continuously in one direction over a wire. alternation. the term applied to a change in the direction of an alternating current, the frequency of the alternations ranging up to 20,000 or more vibrations per second. amber. a resin, yellow in color, which when rubbed with a cloth, becomes excited and gives forth negative electricity. ammeter. an instrument for measuring the quantity or flow of electricity. ampere. the unit of current; the term in which strength of the current is measured. an ampere is an electromotive force of one volt through a resistance of one ohm. annunciator. a device which indicates or signals a call given from some distant point. anode. the positive terminal in a conducting circuit, like the terminal of the carbon plate in a battery. it is a plate in an electroplating bath from which the current goes over to the cathode or negative plate or terminal. arc. a term employed to designate the gap, or the current which flows across between the conductors, like the space between the two carbons of an arc lamp, which gives the light. armature. a body of iron, or other suitable metal, which is in the magnetic field of a magnet. armature bar. the piece which holds the armature. also one of a series of bars which form the conductors in armature windings. armature coil. the winding around an armature, or around the core of an armature. armature core. the part in a dynamo or motor which revolves, and on which the wire coils are wound. astatic (galvanometer). that which has no magnetic action to direct or divert anything exterior to it. atom. the ultimate particle of an elementary substance. attraction. that property of matter which causes particles to adhere, or cohere, to each other. it is known under a variety of terms, such as gravitation, chemical affinity, electro-magnetism and dynamic attraction. automatic cut-out. a device which acts through the operation of the mechanism with which it is connected. it is usually applied to a device which cuts out a current when it overcharges or overloads the wire. bath. in electroplating, the vessel or tank which holds the electroplating solution. battery. a combination of two or more cells. battery, dry. a primary battery in which the electrolyte is made in a solid form. battery, galvanic. a battery which is better known by the name of the voltaic pile, made up of zinc and copper plates which alternate, and with a layer of acidulated paper between each pair of plates. battery, storage. a battery which accumulates electricity generated by a primary battery or a generator. brush. a term applied to the conducting medium that bears against the cylindrical surface of a commutator. buzzer. an electric call produced by a rapidly moving armature of an electro-magnet. cable. a number of wires or conductors assembled in one strand. candle-power. the amount of light given by the legal-standard candle. this standard is a sperm candle, which burns two grains a minute. capacity. the carrying power of a wire or circuit, without heating. when heated there is an overload, or the _capacity_ of the wire is overtaxed. capacity, storage. the quantity of electricity in a secondary battery when fully charged, usually reckoned in ampere hours. carbon. a material, like coke, ground or crushed, and formed into sticks or plates by molding or compression. it requires a high heat to melt or burn, and is used as electrodes for arc lamps and for battery elements. it has poor conductivity, and for arc lamps is coated with copper to increase its conductivity. cell, electrolytic. a vessel containing an electrolyte for electroplating purposes. charge. the quantity of electricity on the surface of a body or conductor. chemical change. when a current passes through electrodes in a solution, a change takes place which is chemical in its character. adding sulphuric acid to water produces heat. if electrodes of opposite polarity are placed in such an acid solution, a chemical change is produced, which is transformed into electricity. choking coil. an instrument in a circuit which by a form of resistance regulates the flow of the current, or returns part of it to the source of its generation. counter-electromotive force. cells which are inserted in opposition to a battery to reduce high voltage. circuit, astatic. a circuit in an instrument so wound that the earth's magnetism will not affect it. circuit breaker. any instrument in a circuit which cuts out or interrupts the flow of a current. circuit, external. a current flows through a wire or conductor, and also along the air outside of the conductor, the latter being the _external circuit._ circuit indicator. an instrument, like a galvanometer, that shows the direction in which a current is flowing through a conductor. circuit, return. usually the ground return, or the negative wire from a battery. circuit, short. any connection between the mains or parallel lines of a circuit which does not go through the apparatus for which the circuit is intended. coherer. a tube, or other structure, containing normally high resistance particles which form a path or bridge between the opposite terminals of a circuit. coil. a wire, usually insulated, wound around a spool. coil, induction. one of a pair of coils designed to change the voltage of a current of electricity, from a higher to a lower, or from a lower to a higher electro-motive force. coil, resistance. a coil so wound that it will offer a resistance to a steady current, or reduce the flow of electricity. commutator. a cylinder on the end of the armature of a dynamo or motor and provided with a pair of contact plates for each particular coil in the armature, in order to change the direction of the current. compass. an apparatus which indicates the direction or flow of the earth's magnetism. condenser. a device for storing up electro-static charges. conductance. that quality of a conductor to carry a current of electricity, dependent on its shape for the best results. conduction. the transmission of a current through a rod, wire or conductor. conductivity. that quality which has reference to the capacity to conduct a current. conductor. any body, such as a bar, rod, wire, or machine, which will carry a current. connector. a binding post, clamp, screw, or other means to hold the end of a wire, or electric conductor. contact. to unite any parts in an electric circuit. controller. the handle of a switchboard, or other contact making and breaking means in a circuit. converter. an induction coil in an alternating circuit for changing potential difference, such as high alternating voltage into low direct current voltage. convolution. to wind like a clock spring. core. the inner portion of an electro-magnet. the inside part of an armature wound with wire. core, laminated. when the core is built up of a number of separate pieces of the same material, but not insulated from each other. coulomb. the unit of electrical quantity. it is the quantity passed by a current of one ampere intensity in one second of time. couple, electric. two or more electrodes in a liquid to produce an electric force. current, alternating. a natural current produced by the action of electro-magnets. it is a succession of short impulses in opposite directions. current, constant. a current which is uniformly maintained in a steady stream. current, induced. a current produced by electro-dynamic induction. current meter. an apparatus for indicating the strength of a current. an ammeter. current, oscillating. a current which periodically alternates. current, periodic. a periodically varying current strength. current, undulating. a current which has a constant direction, but has a continuously varying strength. decomposition. the separation of a liquid, such as an electrolyte, into its prime elements, either electrically or otherwise. deflection. the change of movement of a magnetic needle out of its regular direction of movement. demagnetization. when a current passes through a coil wound on an iron core, the core becomes magnetized. when the current ceases the core is no longer a magnet. it is then said to be _demagnetized_. it also has reference to the process for making a watch non-magnetic so that it will not be affected when in a magnetic field. density. the quantity of an electric charge in a conductor or substance. depolarization. the removal of magnetism from a permanent magnet, or a horse-shoe magnet, for instance. it is generally accomplished by applying heat. deposition, the act of carrying metal from one pole of a cell to electrolysis. another pole, as in electroplating. detector. mechanism for indicating the presence of a current in a circuit. diaphragm. a plate in a telephone, which, in the receiver, is in the magnetic field of a magnet, and in a transmitter carries the light contact points. dielectric. a non-conductor for an electric current, but through which electro-static induction will take place. for example: glass and rubber are dielectrics. discharge. the current flowing from an accumulator. disintegration. the breaking up of the plate or active material. disruptive. a static discharge passing through a dielectric. duplex wire. a pair of wires usually twisted together and insulated from each other to form the conducting circuit of a system. dynamic electricity. the term applied to a current flowing through a wire. dynamo. an apparatus, consisting of core and field magnets, which, when the core is turned, will develop a current of electricity. earth returns. instead of using two wires to carry a circuit, the earth is used for what is called the _return_ circuit. efficiency. the total electrical energy produced, in which that wasted, as well as that used, is calculated. elasticity. that property of any matter which, after a stress, will cause the substance to return to its original form or condition. electricity has elasticity, which is utilized in condensers, as an instance. electricity, lightning, and, in short, any current or electrical atmospheric. impulse, like wireless telegraphic waves, is called _atmospheric_. electricity, electricity with a low potentiality and large current voltaic. density. electrification. the process of imparting a charge of electricity to any body. electro-chemistry. the study of which treats of electric and chemical forces, such as electric plating, electric fusing, electrolysis, and the like. electrode. the terminals of a battery, or of any circuit; as, for instance, an arc light. electrolyte. any material which is capable of being decomposed by an electric current. electro-magnetism. magnetism which is created by an electric current. electrometer. an instrument for measuring static electricity, differing from a galvanometer, which measures a current in a wire that acts on the magnetic needle of the galvanometer. electro-motive voltage, which is the measure or unit of e. m. f. force. (e. m. f.) electroscope. a device for indicating not only the presence of electricity, but whether it is positive or negative. electro-static surfaces separated by a dielectric for opposite accumulator. charging of the surface. element. in electricity a form of matter, as, for instance, gold, or silver, that has no other matter or compound. original elements cannot be separated, because they are not made up of two or more elements, like brass, for instance. excessive charge. a storage battery charged at too high a rate. excessive discharge. a storage battery discharged at too high a rate. excessive overcharge. charging for too long a time. exciter. a generator, either a dynamo or a battery, for exciting the field of a dynamo. exhaustive discharge. an excessive over-discharge of an accumulator. f. the sign used to indicate the heat term fahrenheit. fall of voltage. the difference between the initial and the final voltage in a current. field. the space or region near a magnet or charged wire. also the electro-magnets in a dynamo or motor. flow. the volume of a current going through a conductor. force, electro-magnetic. the pull developed by an electro-magnet. frictional a current produced by rubbing dissimilar electricity. substances together. full load. the greatest load a battery, accumulator or dynamo will sustain. galvanic. pertaining to the electro-chemical relations of metals toward each other. galvanizing. the art of coating one metal with another, such, for instance, as immersing iron in molten zinc. galvanometry. an instrument having a permanently magnetized needle, which is influenced by a coil or a wire in close proximity to it. galvanoscope. an instrument, like a galvanometer, which determines whether or not a current is present in a tested wire. generator. a term used to generally indicate any device which originates a current. german silver. an alloy of copper, nickel and zinc. graphite. one form of carbon. it is made artificially by the electric current. grid. the metallic frame of a plate used to hold the active material of an accumulator. gravity. the attraction of mass for mass. weight. the accelerating tendency of material to move toward the earth. gutta percha. caoutchouc, which has been treated with sulphur, to harden it. it is produced from the sap of tropical trees, and is a good insulator. harmonic receiver. a vibrating reed acted on by an electro-magnet, when tuned to its pitch. high e. m. f. a term to indicate currents which have a high voltage, and usually low amperage. igniter. mechanism composed of a battery, induction coil and a vibrator, for making a jump spark, to ignite gas, powder, etc. i. h. p. abbreviation, which means indicated horse power. impulse. a sudden motion of one body acting against another. an electro-magnetic wave magnetizing soft iron, and this iron attracting another piece of iron, as an example. incandescence, a conductor heated up by a current so it will electric. glow. induced current. a current of electricity which sets up lines of force at right angles to the body of the wire through which the current is transmitted. induction, magnetic. a body within a magnetic field which is excited by the magnetism. installation. everything belonging to an equipment of a building, or a circuiting system to do a certain thing. insulation. a material or substance which resists the passage of a current placed around a conductor. intensity. the strength of a magnetic field, or of a current flowing over a wire. internal resistance. the current strength of electricity of a wire to resist the passage. interrupter. a device in a wire or circuit for checking a current. it also refers to the vibrator of an induction coil. joint. the place where two or more conductors are united. joint resistance. the combined resistance offered by two or more substances or conductors. jump spark. a spark, disruptive in its character, between two conducting points. initial charge. the charge required to start a battery. kathode, or cathode. the negative plate or side of a battery. the plate on which the electro deposit is made. key. the arm of a telegraph sounder. a bar with a finger piece, which is hinged and so arranged that it will make and break contacts in an electric circuit. keyboard. a switch-board; a board on which is mounted a number of switches. kilowatt. a unit, representing 1,000 watts. an electric current measure, usually expressed thus: k.w. kilowatt hour. the computation of work equal to the exertion of one kilowatt in one hour. knife switch. a bar of a blade-like form, adapted to move down between two fingers, and thus establish metallic connections. laminated. made up of thin plates of the same material, laid together, but not insulated from each other. lamp arc. a voltaic arc lamp, using carbon electrodes, with mechanism for feeding the electrodes regularly. lamp, incandescent. a lamp with a filament heated up to a glow by the action of an electric current. the filament is within a vacuum in a glass globe. leak. loss of electrical energy through a fault in wiring, or in using bare wires. load. the ampere current delivered by a dynamo under certain conditions. low frequency. a current in which the vibrations are of few alternations per second. magnet. a metallic substance which has power to attract iron and steel. magnet bar. a straight piece of metal. magnet coil. a coil of wire, insulated, surrounding a core of iron, to receive a current of electricity. magnet core. a bar of iron adapted to receive a winding of wire. magnet, field. a magnet in a dynamo. a motor to produce electric energy. magnet, permanent. a short steel form, to hold magnetism for a long time. magnetic adherence. the adherence of particles to the poles of a magnet. magnetic that quality of a metal which draws metals. also attraction and the pulling action of unlike poles for each repulsion. other, and pushing away of like poles when brought together. magnetic force. the action exercised by a magnet of attracting or repelling. magnetic pole. the earth has north and south magnetic poles. the south pole of a magnetic needle is attracted so it points to the north magnetic pole; and the north pole of the needle is attracted to point to the south magnetic pole. magneto-generator. a permanent magnet and a revolving armature for generating a current. maximum voltage. the final voltage after charging. molecule. invisible particles made up of two or more atoms of different matter. an atom is a particle of one substance only. morse sounder. an electric instrument designed to make a clicking sound, when the armature is drawn down by a magnet. motor-dynamo. a motor and a dynamo having their armatures connected together, whereby the motor is driven by the dynamo, so as to change the current into a different voltage and amperage. motor-transformer. a motor which delivers the current like a generator. needle. a bar magnet horizontally poised on a vertical pivot point, like the needle of a mariner's compass. negative amber, when rubbed, produces negative electricity. electricity. a battery has positive as well as negative electricity. negative element. that plate in the solution of a battery cell which is not disintegrated. normal. the usual, or ordinary. the average. in a current the regular force required to do the work. north pole, the term applied to the force located near electric. the north pole of the globe, to which a permanent magnet will point if allowed to swing freely. o. abbreviation for ohm. ohm. the unit of resistance. equal to the resistance of a column of mercury one square millimeter in cross section, and 106.24 centimeters in length. ohm's law. it is expressed as follows: 1. the current strength is equal to the electro-motive force divided by its resistance. 2. the electro-motive force is equal to the current strength multiplied by the resistance. 3. the resistance is equal to the electro-motive force divided by the current strength. overload. in a motor an excess of mechanical work which causes the armature to turn too slowly and produces heat. phase. one complete oscillation. the special form of a wave at any instant, or at any interval of time. plate, condenser. in a static machine it is usually a plate of glass and revoluble. plate, negative. the plate in a battery, such as carbon, copper or platinum, which is not attacked by the solution. plating, electro-. the method of coating one metal with another by electrolysis. polarity. the peculiarity, in a body, of arranging itself with reference to magnetic influence. parallel. when a number of cells are coupled so that their similar poles are grouped together. that is to say, as the carbon plates, for instance, are connected with one terminal, and all the zinc plates with the other terminal. polarization. when the cell is deprived of its electro-motive force, or any part of it, polarization is the result. it is usually caused by coating of the plates. porosity. having small interstices or holes. positive current. one which deflects a needle to the left. positive any current flowing from the active element, electricity. such as zinc, in a battery. the negative electricity flows from the carbon to the zinc. potential, electric. the power which performs work in a circuit. potential energy. that form of force, which, when liberated, does or performs work. power unit. the volt-amperes or watt. primary. the induction coil in induction machines, or in a transformer. push button. a thumb piece which serves as a switch to close a circuit while being pressed inwardly. quantity. such arrangement of electrical connections which give off the largest amount of current. receiver. an instrument in telephony and telegraphy which receives or takes in the sound or impulses. relay. the device which opens or closes a circuit so as to admit a new current which is sent to a more distant point. repulsion, that tendency in bodies to repel each other when electric. similarly charged. resilience. the springing back to its former condition or position. electricity has resilience. resistance. the quality in all conductors to oppose the passage of a current. resistance coil. a coil made up of wire which prevents the passage of a current to a greater or less degree. resistance, the counter force in an electrolyte which seeks electrolytic. to prevent a decomposing current to pass through it. resistance: internal, the opposing force to the movement of a current external. which is in the cell or generator. this is called the _internal_. that opposite action outside of the cell or generator is the _external_. resonator, an open-circuited conductor for electrically electric. resounding or giving back a vibration, usually exhibited by means of a spark. rheostat. a device which has an adjustable resistance, so arranged that while adjusting the same the circuit will not be open. safety fuse. a piece of fusible metal of such resistance that it breaks down at a certain current strength. saturated. when a liquid has taken up a soluble material to the fullest extent it is then completely saturated. secondary. one of the two coils in a transformer, or induction coil. secondary plates. the brown or deep red plates in a storage battery when charged. self-excited. producing electricity by its own current. series. arranged in regular order. from one to the other directly. if lamps, for instance, should be arranged in circuit on a single wire, they would be in series. series, multiple. when lamps are grouped in sets in parallel, and these sets are then connected up in series. series windings. a generator or motor wound in such a manner that one of the commutator brush connections is joined to the field magnet winding, and the other end of the magnet winding joined to the outer circuit. shunt. going around. shunt winding. a dynamo in which the field winding is parallel with the winding of the armature. snap switch. a switch so arranged that it will quickly make a break. sounder. the apparatus at one end of a line actuated by a key at the other end of the line. spark coil. a coil, to make a spark from a low electro-motive force. spark, electric. the flash caused by drawing apart the ends of a conductor. specific gravity. the weight or density of a body. static electricity. generated by friction. also lightning. any current generated by a high electro-motive force. strength of current. the quantity of electricity in a circuit. synchronize. operating together; acting in unison. terminal. the end of any electric circuit or of a body or machine which has a current passing through it. thermostat, electric. an electric thermometer. usually made with a metal coil which expands through the action of the electricity passing through it, and, in expanding, it makes a contact and closes a circuit. transformer. the induction coil with a high initial e. m. f. changes into a low electro-motive force. unit. a standard of light, heat, electricity, or of other phenomena. vacuum. a space from which all matter has been exhausted. vibrator. mechanism for making and breaking circuits in induction coils or other apparatus. volt. the unit of electro-motive force. voltage. electro-motive force which is expressed in volts. voltaic. a term applied to electric currents and devices. volt-meter. an apparatus for showing the difference of potential, or e. m. f. in the term of volts. watt. the unit of electrical activity. the product of amperes multiplied by volts. watt hour. one watt maintained through one hour of time. waves, electric waves in the ether caused by electro-magnetic magnetic. disturbances. x-rays. the radiation of invisible rays of light, which penetrate or pass through opaque substances. yoke, or bar. a soft iron body across the ends of a horseshoe magnet, to enable the magnet to retain its magnetism an indefinite time. zinc battery. a battery which uses zinc for one of its elements. index a accumulated, 31. accumulation, 29. accumulator cell, 87. accumulators, 82, 88, 89. accumulators, plates, 83. acid, 34, 37, 125. acid maker, 125. acid, sulphuric, 31, 84. acidulated, 55. acidulated water, 34. acoustics, 110. actinic rays, 184, 185. actinium, 186. active element, 82. adjustable rod, 107. adjusting screw, 70, 71, 72, 73, 106. aerial wire, 108. agents, 13, 32. alarms, burglar, 11, 76, 80. alkali, 125. alkaline, 37. alternate, 127. alternating, 38, 149, 150, 153, 154, 155, 156. alternating current, 145. alternating periods, 149. alternations, 147. aluminum, 128, 129, 135, 137. aluminum hydrate, 129. amber, 5, 171. ammeter, 7, 88. amperage, 38, 61, 62, 132, 159, 160, 168. ampere, 7, 37, 60, 63, 139, 140, 167. amplitude, 111. annunciator, 65, 74, 76, 79, 80, 81. annunciator bells, 11. anode, 35, 133, 134. antennã¦, 108. antimony 137, 143. anvil, 13, 14. apparatus, 11, 57, 106, 139, 145. arc, 163, 182. arc lighting, 38, 165. arc system, 166. armature, 18, 25, 38, 40, 42, 43, 45, 46, 47, 48, 53, 55, 70, 72, 73, 74, 90, 93, 112, 151, 152, 155, 163, 176, 177, 178, 179, 180. armature brush, 48. armature post, 71. armature, vertical, 75. armature winding, 42, 43, 156. asbestos, 140. astatic galvanometer, 108. atmosphere, 184. attract, 30. attracted, 72. attraction, 21, 25. attractive, 178. automatic, 120. auxiliary, 44. awls, 14. b bacteria, 126, 187. bar, cross, 66. bar, horizontal, 46. bar, parallel switch, 67. bar, switch, 65, 68. base block, 66. batteries, 11, 93, 122. battery, 29, 30, 32, 35, 36, 46, 47, 80, 81, 82, 83, 85, 86, 88, 92, 94, 95, 107, 108, 116, 117, 118, 121, 134, 142. battery charging, 82. bearings, 45, 46. bells, 65, 73, 76, 122. bells, electric, 70. bench, 13, 15, 17. binding post, 52, 70, 71, 72, 103, 107, 108, 121. binding screw, 65, 66. bismuth, 18, 143. bit, 13. blue vitriol, 57. brass plate, 77, 78. brazing, 17, 65. bridge, 52. brush holder, 46. brushes, 48, 150, 151, 153, 167. burglar, 11. burglar alarm, 76, 80. buttons, contact, 80. buttons, push, 65, 68, 69, 70, 76, 79. c calorimeter, 56. cancerous, 187. candle power, 89, 139. cap, removable, 73. cap screws, 42. carbon, 35, 119, 121, 162, 163, 169. carbon block, 120. carbon pencil, 119. cathode, 35, 36, 133, 134. cell, 29, 33. cell, accumulator, 87. cell, charging, 87. channel, 43. channel, concave, 40. charged, 120. charged battery, 82. charging circuit, 82, 89. charging source, 83. charged wire, 147. chemical, 57. chisels, 13. chloride of lime, 84. choked, 157. choking coils, 145, 146, 156, 158. circuit, 33, 69, 73, 76, 78, 80, 81, 90, 92, 93, 109, 113, 116, 121, 122, 131, 134, 143, 156. circuit, primary, 99. circuit, secondary, 99. circuiting, 81, 155. circuiting system, 79. clapper arm, 70. closed rings, 26. coherer, 105, 108, 109. cohering, 106. coils, 18, 26, 52, 55, 74, 160. coils, choking, 145, 146, 156, 158. coils, induction, 99, 102. coils, primary, 109. coils, secondary, 102, 109. coincide, 42. cold, 14. collecting surfaces, 30. collector, 31. column, 61. combustion, 169. commutator, 44, 46, 151, 152. commutator brushes, 46. commutator plates, 45. compass, 22, 24, 172. composition, 83, 124. compound wound, 47. concave channel, 40. condenser, 98, 100, 101, 102, 108. conduct, 6, 108. conduction, 135, 136, 138, 166, 170. conduction current, 27. conductor, 21, 31, 33, 63, 98, 116, 161, 162. conduit, 72. conically formed, 126. conjunction, 143. connecting wire, 58. connection, 72, 76. construction, magnet, 39. consumption, 180. contact, 122, 123, 152, 162. contact finger, 150. contact plate, 67, 68, 79. contact screws, 93. contact surface, 66. continuous, 145. converter, 176. converting, 142, 145, 146. copper, 18, 34, 36, 65, 66, 132, 133, 134, 135, 136, 137, 142, 143. copper cyanide, 133. copper plate, 33, 35, 58, 67. copper sulphate, 57. copper voltameter, 55, 57. core, 27, 28, 36, 39, 40, 115. core, magnet, 75, 93. counter, clock-wise, 51. coupled, 36. crank, 30. crookes' tube, 184. cross bar, 52, 66. crown of cups, 32. crystal, 85. current, 6, 7, 13, 18, 26, 27, 28, 35, 36, 37, 38, 47, 50, 51, 52, 55, 56, 57, 58, 59, 62, 63, 70, 72, 73, 90, 95, 98, 105, 108, 116, 133, 134, 135, 136, 138, 139, 140, 141, 142, 143, 147, 148, 149, 150, 152, 153, 157, 160, 161, 163, 165, 166, 170. current, alternating, 150. current changing, 82. current conduction, 27. current, continuous, 164. current, direct, 145, 150. current direction, 50. current, exterior, 50, 150. current, reversing, 148. current strength, 7, 57. current testing, 143. cut-out, 120. cutter, 14. cutting, lines of force, 38. cylinder, 44. cylindrical, 43. d dash, 95, 97. decoherer, 106, 108. decomposed, 57, 128. decomposes, 55. decomposing, 123. decomposition, 12, 35, 82. deflected, 54. degree, 135, 162. demagnetized, 24, 72. deposited, 58, 133. depression, 15, 140. detecting current, 49. detector, 49, 52, 54, 105. devices, measuring, 27. diagrams, 46, 48, 79, 89. diagrammatically, 81. diamagnetic, 19. diametrically, 114. diaphragm, 112, 113, 116, 120, 122. diamonds, 186. diluted, 86. direct current, 38, 140. direction of current, 50. direction of flow, 98. discharge, 172. disintegrate, 132. disk, 43. dissimilar, 37. disturbance, 176. dividers, 14. divisibility, 168. dot, 96, 97. dot and dash, 96. double click, 95. double line, 65. double-pole switch, 65. double-throw switch, 117. drawing, 20. drill, ratchet, 13. drops, 81. ductile, 186. duplex wire, 115. dynamo, 7, 27, 38, 42, 46, 48, 62, 82, 83, 87, 89, 132, 141, 142, 145, 150, 155, 161, 165, 167, 175, 176, 180, 187. dynamo fields, 40, 41. e earth, 22. elasticity, 100, 142. electric, 6, 31, 49, 50, 76, 78, 81, 131, 142, 158, 162, 173, 176. electric arc, 63, 163. electric bell, 19, 69, 70, 71, 72, 106, 117. electric bulbs, 167. electric circuit, 118. electric fan, 55. electric field, 76. electric hand purifier, 129. electric heating, 135, 137, 161. electric iron, 130, 141. electric lamp socket, 139. electric light, 56, 66. electric lighting, 161. electric power, 113. electric welding, 183. electrical, 8, 11, 65, 96, 98, 104, 141, 159, 180, 184, 187. electrical impulses, 105, 147, 148. electrical manifestations, 175. electrically, 32, 70. electricity, 5, 6, 7, 8, 9, 12, 13, 18, 21, 26, 27, 28, 29, 38, 49, 54, 60, 61, 62, 82, 97, 98, 100, 104, 110, 112, 116, 123, 124, 133, 134, 136, 138, 145, 146, 147, 154, 156, 160, 166, 170, 171, 172, 175, 182, 187. electricity measuring, 49. electricity, thermo-, 142. electrified, 37, 186. electro-chemical, 55. electrode, 35, 124, 127, 128, 161, 162, 163, 164, 165, 184. electrolysis, 7, 123, 126, 132. electrolyte, 33, 35, 36, 57, 86, 88, 123, 132, 142. electrolytic, 55, 123, 125. electro-magnet, 59, 78. electro-magnetic, 7, 24, 25, 29, 37, 55, 92, 93, 94. electro-magnetic force, 7. electro-magnetic rotation, 7. electro-magnetic switch, 116. electro-meter, 7. electro-motive force, 37, 63, 99. electroplate, 12, 38, 48, 123, 132, 134. electro-positive-negative, 142, 143. elements, 36, 83. engine energy, 170, 180. equidistant, 127. ether, 104. example, 61. excited, 47. extension plate, 103. exterior, 3. exterior magnetic, 27. external, 37. external circuit, 153. external current, 50. external resistance, 37. f factor, 61. ferrous oxide, 125. field, 46, 47. field, dynamo, 40, 41. field magnet cores, 155. field, magnetic, 38. field of force, 33. field wire, 48. filament, 168, 169, 170. filter, 128. flat iron, 140. flocculent, 128. force, 50. formulated, 19. friction, 32. frictional, 6, 7, 29. fuse, 169. g galvani, 7. galvanic, 7, 23, 30. galvanometer, 7, 49, 108, 143. galvanoscope, 55, 58, 59. gaseous, 128. gasoline, 99. gas stove, 17. gelatine, 128. generate, 29, 38, 134, 136, 145. generated, 55. generating, 32, 134. generation, 170. generator, 32, 125, 147. german silver, 136, 137. germicide, 187. gimlets, 17. glass, 30, 86, 126, 186. gold, 135. grid, 84. ground circuit, 121. gunpowder, 6. h hack-saw, 14. hammer, 13. heart-shaped switch, 77. heater, 136. heating, 13, 38. hertzian rays, 170. hertzian wave, 184. high tension, 38, 102, 184. high tension apparatus, 98. high tension coils, 103. high voltage, 158. horizontal bar, 46. horseshoe magnet, 22, 24, 175. hydrate of aluminum, 129. hydrogen, 35, 123, 125, 128. i igniting, 99. illumination, 162, 163, 165, 167, 170. immersed, 133. impulses, 60, 62, 96, 104, 109, 152, 179. incandescent, 166, 168. induced, 28. inductance, 149, 150. induction, 27, 37, 98, 147. induction coils, 99, 102, 106. influences, 178. initial charge, 88. insulated, 27, 28, 40, 43, 52, 55, 73, 115, 151, 180. insulating, 66, 69, 120, 140, 164. insulating material, 114. insulation, 40, 116. instruments, 49, 94, 112, 118, 120. instruments, measuring, 8. intensity, 55, 60, 104, 154. interior, magnetic, 23. internal resistance, 37. interruption, 102, 103. installation, 168. ionize, 186. iron, 19, 132, 133, 136, 142, 171. isolated, 186. j jar, 29, 31, 32. journal, 46. journal block, 16, 146. jump spark, 99. k key, 90, 91, 95. key, sending, 90. knob, 32. knob, terminal, 31. l laboratory, 9. lead, 31, 136. lead, precipitated, 83, 85. lead, red, 83, 84. lever switching, 67. light, 104. light method, 56. lighting, 9, 38. lighting circuit, 48. lighting system, 82. lightning, 6, 171, 172, 173. lightning rod, 173. lime, chloride of, 84. line of force, 146. line wire, 122. line, magnetic, 22, 23. liquid, 32. litharge, 83. loadstone, 17. locomotives, 165. low tension, 38, 98, 102, 179. m magnet bar, 20. magnet core, 16, 75, 93. magnet, electro, 59, 78. magnet, horseshoe, 22, 25, 175. magnet lines, 22, 23. magnet, permanent, 25, 38, 46, 50, 172. magnet, reversed, 20. magnet, steel, 53. magnet, swinging, 53. magnetic, 7, 19, 20, 21, 22, 25, 113, 178. magnetic construction, 39. magnetic exterior, 27. magnetic field, 22, 24, 27, 38, 50, 112, 146, 148, 155. magnetic interior, 23. magnetic pull, 59. magnetic radiator, 37. magnetism, 19, 54, 104, 110, 159, 171. magnetized, 18, 25, 27, 50. magnetized wire, 146. magnets, 13, 14, 18, 19, 20, 21, 22, 23, 24, 25, 39, 51, 53, 54, 70, 71, 73, 75, 81, 90, 93, 112, 113, 115, 147, 150, 163, 176, 177, 178. main conductor, 31. mandrel, 15, 16. manganese, 19. manifestations, 19. mariner, 172. material, non-conducting, 90. maximum, 154. measure, 55, 56, 60, 62. measurement, 62. measuring devices, 27. measuring instruments, 8. mechanism, 47, 180. medical batteries, 99. mercury, 63, 169. metal base, 73. mica, 186. microphone, 118, 119, 120. millimeter, 63. minus, 34. minus sign, 21. morse code, 76. motor, 7, 21, 27, 46, 47, 62, 82, 99, 150, 176, 180. mouthpiece, 115. mouthpiece rays, 188. moving field, 117. multiple, 168. musical scale, 111. n negative, 21, 35, 36, 68, 83, 86, 87, 94, 125, 151, 152, 154, 165, 177, 178, 179. neutral, 125. neutral plate, 84. nickel, 136. nickel plating, 132. nitrate of silver, 62. nitrogen, 126. non-conducting material, 90. non-conductor, 164. non-magnetic, 19. north pole, 20, 21, 22, 23, 25, 50, 54, 156. number plate, 75. n-ray, 188. o ohms, 60, 63. ohms, international, 63. ohms law, 7. operator, 95, 118. oscillating, 99, 105. osmium, 169. oxides, 125. oxidizing, 183. oxygen, 35, 123, 125, 126, 128, 129, 169. p packing ring, 124. paraffine, 56, 100, 101, 102. paraffine wax, 86. parallel, 87, 88, 89. parallel switch bar, 67. parallel wires, 28, 49. partition, 124. peon, 13. percolate, 128. periodicity, 159. periods of alternations, 149. permanent, 18, 19, 50. permanent magnet, 25, 38, 46, 50, 172. phase, 19. phenomenon, 27, 65. photograph, 186. physical, 21. pile, voltaic, 33. pipe, 61. pitchblende, 186. pivot pin, 53. pivotal, 22. plane, 13. plate, 57, 93. plate, contact, 67, 68, 79. plate, copper, 33, 35, 58, 67. plate, negative, 84. plate, number, 75. plate, positive, 84, 88. plate, zinc, 33. platinum, 13, 57, 137. pliers, 14. plus sign, 21, 24. pointer, 53. polarity, 154, 177, 178, 179. polarization, 35. pole, north, 20, 21, 22, 23, 25, 50, 54, 156. pole piece, 40, 42. pole, south, 20, 21, 22, 25, 50, 54, 156. poles, 177, 179. polonium, 186. porcelain, 86. porous, 85. positive, 4, 21, 25, 36, 40, 68, 83, 86, 87, 94, 123, 125, 151, 152, 153, 155, 165. post, binding, 52, 71. potentiality, 105, 109. power, 38, 186. power, candle, 89, 139. precipitate of lead, 83, 85. precision, 7. pressure, 87. primary, 35, 62, 98, 134, 142, 159, 184. primary battery, 7, 99. primary circuit, 99. primary coil, 106, 109. prime conductor, 6. projected, 185. propagated, 105, 185. properties, 55. purification, 123, 128. purifier, 126, 131. push button, 65, 68, 69, 70, 76, 79. q quantity, 55, 60, 61, 138. quartz, 186. r radio-activity, 186. radium, 184, 185, 187, 188. ratchet drill, 13. reaction, 148. receiver, 12, 90, 97, 121, 122. receiving station, 109. rectangular, 69. rectifiers, 146. red lead, 83, 84. reel, 13. reflected, 185. refraction, 185. refractory, 182. register, 57. removable, 54. removable cap, 73. repel, 20. repulsion, 21, 128. reservoir, 61, 62. resiliency, 99. resistance, 7, 36, 37, 60, 63, 99, 135, 136, 137, 138, 140, 141, 156, 157, 163, 166, 168. resistance bridge, 7. resistance, external, 37. resistance, internal, 37. rheostat, 7. reversed, 20, 50, 153. reversible, 163. reversing, 176. reversing switch, 67. revolubly, 46. revolve, 179. revolving, 177. roentgen rays, 184. roentgen tube, 187. rotation, 149. rubber, 40, 46, 77, 115, 126, 130, 138. s sad-irons, 13. saline, 133. sanitation, 12. saturated, 85. screw, 15. screw, binding, 65, 66. screw-driver, 14. screw, set, 72. sealing wax, 53. secondary, 62, 98, 105, 158, 159, 160. secondary circuit, 99. secondary coil, 107, 108. self-induction, 149, 156. sender, 90, 97. sending apparatus, 106. sending key, 90. separately excited, 46. series-wound, 47. severed magnet, 20. sewage, 12. shaft, 30. shears, 14, 17. shellac, 40. shunt-wound, 47. signal, 118. silver, 19, 63, 125. silver nitrate, 62. socket, 54, 139. soldering, 14. soldering iron, 17. solution, 55, 57, 62, 63, 84, 86, 133, 134, 142. sounder, 90, 92, 95, 96. sounding board, 119. source, charging, 83. south pole, 20, 21, 22, 25, 50, 54, 156. spark gap, 102, 106. spark jump, 99. spring finger, 69. square, 14, 17. standard, 62, 63. station, 94, 95, 117, 122. steel, 18, 19. steel magnet, 53. sterilized, 12. stirrup, 75. stock bit, 13. stock contact, 121. storage, 82. storage battery, 107. storing, 82. substances, 135. sulphate, 55, 128, 133. sulphur, 19. sulphuric acid, 31, 84. sulphuric acid voltameter, 55, 57. superstition, 171, 173. surging, 153, 154. swinging magnet, 53. swinging switch blade, 67. switch blades, 66. switches, 65, 66, 70, 77, 78, 90, 117. switches, bar, 65, 68, 90, 91. switches, bar, parallel, 67. switches, heart-shaped, 78. switches, piece, 77. switches, reversing, 67. switches, sliding, 67, 80. switches, terminal, 8. switches, two-pole, 65. system, circuiting, 79. t tail-piece, 16. tantalum, 169. telegraph, 11, 90, 96. telegraph key, 106. telegraph sounder, 108, 109. telegraphing, 94. telephone, 12, 110, 113, 117, 118, 119, 120. telephone circuit, 118. telephone connections, 116. telephone hook, 122. temperature, 56, 88, 134, 161, 170. tension, high, 38, 102, 184. tension, low, 38, 98, 102, 179. terminal, 31, 34, 35, 40, 48, 82, 86, 93, 95, 107, 116, 121, 122, 151, 152, 153, 154, 156. terminal knob, 31. terminal, secondary, 102. terminal switch, 81. theoretical, 160. therapeutics, 187. thermo-electric couples, 146. thermo-electricity, 135. thermometer, 56. thorium, 169, 186. thunderbolt, 171, 173. tin, 136. tinfoil, 31, 101. tools, 11, 13, 17. torch, brazing, 17. transformer, 145, 146, 158, 159, 180, 182. transformer, step-down, 182. transmission, 38, 187. transmit, 63, 95, 157. transmitter, 12, 120, 121, 122, 123. transverse, 16, 52. transversely, 43. trigger, 75. tripod, 31. tubular, 44, 45. two-pole switch, 65. u ultra-violet, 185. uranium, 186. v vacuum, 184. vapor lamps, 169. velocity, 60, 73. vertical armature, 75. vibration, 110, 111, 113. vibratory, 110. vise, 13. voltage, 37, 38, 60, 61, 62, 63, 87, 88, 99, 147, 154, 165, 180, 182. voltage, high, 158. voltaic, 29, 32. voltaic pile, 33. voltameter, 7, 58, 88. voltameter, sulphuric, acid, 55, 57. volts, 60, 62, 87, 89, 132, 158, 159. w water, 123, 138, 144. water power, 142. watts, 60, 61, 160. wave lengths, 104, 110. weight, 49. welding, 13, 182. winding, 18, 40, 47, 58, 159, 196. winding reel, 14. window connection, 76. window frame, 78. wire, 6, 18, 21, 26, 28, 156. wire, circuiting, 79. wire coil, 40. wire lead, 70. wire, parallel, 28, 49. wireless, 12. wireless telegraphy, 103, 104, 184. wiring, 80. wiring, window, 77. workshop, 11, 17. wound, compound, 48. wound-series, 47. wound-shunt, 47. x x-ray, 184, 185, 187, 188. z zinc, 17, 34, 35, 85, 135. zinc plates, 33. the "how-to-do-it" books carpentry for boys a book which treats, in a most practical and fascinating manner all subjects pertaining to the "king of trades"; showing the care and use of tools; drawing; designing, and the laying out of work; the principles involved in the building of various kinds of structures, and the rudiments of architecture. it contains over two hundred and fifty illustrations made especially for this work, and includes also a complete glossary of the technical terms used in the art. the most comprehensive volume on this subject ever published for boys. electricity for boys the author has adopted the unique plan of setting forth the fundamental principles in each phase of the science, and practically applying the work in the successive stages. it shows how the knowledge has been developed, and the reasons for the various phenomena, without using technical words so as to bring it within the compass of every boy. it has a complete glossary of terms, and is illustrated with two hundred original drawings. practical mechanics for boys this book takes the beginner through a comprehensive series of practical shop work, in which the uses of tools, and the structure and handling of shop machinery are set forth; how they are utilized to perform the work, and the manner in which all dimensional work is carried out. every subject is illustrated, and model building explained. it contains a glossary which comprises a new system of cross references, a feature that will prove a welcome departure in explaining subjects. fully illustrated. _price 60 cents per volume_ the new york book company 147 fourth avenue new york +-----------------------------------------------------------------+ | transcriber's note. | | | | every effort has been made to replicate this text as faithfully | | as possible, including obsolete and variant spellings and other | | inconsistencies. | | | | minor punctuation and printing errors have been corrected. | | | | the first page of the original book is an advertisement. the | | page was moved to the end of the text. | | | | some hyphenation inconsistencies in the text were retained: | | 16-candle-power and 16-candlepower, | | electromotive and electro-motive, | | electro-meter and electrometer, | | horseshoe and horse-shoe, | | switchboard and switch-board, | | | | two occurrences of 'colorimeter' for 'calorimeter' repaired. | +-----------------------------------------------------------------+ transcriber's note: in transcribing this book, the proofreaders found and corrected several minor typographical errors which did not affect the sense of the text. in the caption to figure 541, the equation for the voltage of a weston cell at different temperatures was missing a digit "1" and this has been corrected. there is a reference to a figure 619 but no such figure exists in the original text. there are references to a figure 119 and a figure 443; these presumably exist in one of the preceding volumes of the series. throughout, the use of italic type is indicated by _underscores_. in equations, superscripts are indicated by a caret and braces, as in x^{2} for "x squared". subscripts are indicated by underscore and braces, as in e_{t} to mean "e sub t". the thought is in the question the information is in the answer hawkins electrical guide number three questions answers & illustrations a progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications a practical treatise by hawkins and staff theo. audel & co. 72 fifth ave. new york copyrighted, 1914, by theo. audel & co., new york. printed in the united states. table of contents guide no. 3. galvanometers 431 to 464 action of compass needle--simple galvanometer--difference between galvanoscope and galvanometer--sensibility--action of short and long coil galvanometers--classes of galvanometer--astatic galvanometer--tangent galvanometer--graduation of tangent galvanometer scale--table of galvanometer constants--mechanical explanation of tangent law--sine galvanometer--table of natural sines and tangents--comparison of sine and tangent galvanometers--differential galvanometer--ballistic galvanometer--kick--damping effect--use of mirrors in galvanometers--lamp and scale--damping--d'arsonval galvanometer: construction, operation; uses--galvanometer constant or figure of merit--shunts. testing and testing apparatus 465 to 536 pressure measurement--clark cell--weston cadmium cell--pressure measurement error with ordinary voltmeter--international volt--hydraulic analogy of amperes--coulombs--current measurement--international ampere--voltameters--ohm's law and the ohm--international ohm--ohm table--practical standards of resistance--various methods of resistance measurement--direct deflection method--method of substitution--resistance box--fall of potential method--differential galvanometer method--drop method--voltmeter method--wheatstone bridge--usual arrangement of resistances of wheatstone bridge--ratio coils of wheatstone bridge--the decade plan--two plug arrangement--"plug out" and "plug in" type of resistance box--testing sets--direct deflection method with queen acme set--ohmmeter--fall of potential method with queen acme set--apparatus for measuring low resistances--how to check a voltmeter--kelvin wire bridge--internal resistance measurement--evershed portable ohmmeter set--l and n fault finder--ammeter test--diagram of queen standard potentiometer--diagrams illustrating loop testing--the murray loop--the varley loop--special loop--the potentiometer--location of opens--to pick out faulty wires in a cable--voltage of cell measurement with potentiometer--care of potentiometer--location of faults where the loop is composed of cables of different cross sections. ammeters, voltmeters, and wattmeters 537 to 572 definition of ammeter--classification of ammeter and voltmeters--moving iron type instrument--keystone voltmeter--winding in ammeters and volts--connections for series and shunt ammeters--voltmeter connections--westinghouse ammeter shunts--various types of instrument--plunger type instrument--magnetic vane instrument--inclined coil instrument--whitney hot wire instruments--principle of electrostatic instruments--multipliers--portable shunts--siemens electro-dynamometer--station instruments--thompson watt hour meter--how to read a meter--installation of wattmeters--westinghouse watt hour meter--thompson prepayment watt hour meter--how to test a meter--sangamo watt hour meter--columbia watt hour meter--duncan watt hour meter. operation of dynamos 573 to 596 before starting a dynamo--adjusting the brushes--brush position--how to set the brushes--method of soldering cable to carbon brush--brush contact pressure--direction of rotation--method of winding cables with marlin--method of assembling core discs--starting a dynamo--tinning block for electric soldering tool--shunt dynamos in parallel--shunt dynamos on three wire system--how to start a series machine--the term "build up"--how to start a shunt or compound machine--"picking up"--indication of reversed connections--how to correct reversed polarity--finding the reversed coil--loss of residual magnetism--remedy for reversed dynamo--attention while running--lead of brushes--method of taking temperature--lubrication--oils--allowable degree of heating--attention to brushes and brush gear. coupling of dynamos 597 to 610 series and parallel connections--coupling series dynamos in series; in parallel--equalizer--shunt dynamos in series; in parallel--switching dynamo into and out of parallel--to cut out a machine--dividing the lead--compound dynamos in series; in parallel--equalizer connection--switching a compound dynamo into and out of parallel--equalizing the load--shunt and compound dynamos in parallel. dynamo fails to excite 611 to 622 various causes--brushes not properly adjusted--defective contacts--incorrect adjustment of regulators--speed too low--testing for break--insufficient residual magnetism; remedy--open circuits--test for field circuit breakers--probable location of breaks--watson armature discs--fort wayne commutator truing device--short circuits--watson armature--wrong connections--reversed field magnetism. armature troubles 623 to 634 causes--how avoided--various faults--short circuit in individual coils--location of faulty coil--test for break in armature lead--bar to bar test for open or short circuit in coil or between segments--short circuits between adjacent coils--alternate bar test for short circuits between sections--short circuits between sections through frame or core of armature; between sections through binding wires--partial short circuits in armatures--method of testing for breaks--burning of armature coils--watson field coils--grounds in armatures--method of locating grounded armature coil--magneto test for grounded armatures--method of binding armature winding--breaks in armature circuit. care of the commutator and brushes 635 to 652 conditions for satisfactory operation--oil for commutator--attention to brushes--bissell brush gear--two kinds of sparking--commutator clamp--causes of sparking--bad adjustment of brushes--rocking--bad condition of brushes--brushes making bad contact--bad condition of commutator--detection of untrue commutator--high segments--"flats"--causes of flats; remedy--method of repairing broken joint between commutator segment and lug--segments loose or knocked in--how to re-turn a commutator--bissell commutators--overload of dynamo--method of repairing large hole burned in two adjacent bars of a commutator--operating dynamos with metal brushes--indication of excessive voltage--method of smoothing commutator with a stone--causes of excessive voltage--loose connections, terminals, etc.,--breaks in armature circuit--sandpaper holder for commutator--short circuits, in armature circuits; in field--breaks in field--sandpaper block--short circuits in commutator. heating 653 to 662 various causes--how detected--procedure--heating, of connections; of brushes, commutator and armature--excessive heating--ventilated commutator--self-oiling bearing--some causes of hot bearing--effect of hot bearings--points relating to hot bearings--operation above rated voltage and below normal speed--forced system of lubrication--heating of field magnets--causes of eddy currents in pole pieces--detection of moisture in field coils--indication of short circuits in field coils. operation of motors 663 to 696 before starting a motor--starting a motor--various starting resistances--starting boxes--speed regulators--cutler hammer starter--time required to start motor--how to start--sliding contact starters--series motors on battery circuits--starting a shunt motor--multiple switch starters--effect of reverse voltage--rheostat with no voltage and overload release--failure to start--starting panel--cutler hammer starting rheostats--allen bradley automatic starter--monitor starter with relay for push button control--a remote control of shunt motors--regulation of motor speed; various methods--monitor printing press controller--speed regulation of series motor, by short circuiting sections of the field winding--varying the speed of shunt and compound motors--cutler hammer multiple switch starter--regulation by armature resistance--compound starter--regulation by shunt field resistance--holzer cabot instructions for shunt wound motor--reliance adjustable speed motor--cutler hammer reversible starter--combined armature and shunt field control--selection of starters and regulators--watson commutators--organ blower speed regulator--general electric controller--speed regulation of traction motors--controller of the rauch and lang electric vehicles--two motor regulation--controller connection diagrams--stopping a motor. chapter xxvi galvanometers if a compass needle be allowed to come to rest in its natural position, and a current of electricity be passed through a wire just over it from north to south, the north seeking end of the needle will be deflected toward the east. if the wire be placed under the needle and the current continued from north to south the needle will be deflected toward the west. again, if the current be passed from north to south over the needle, and back from south to north under the needle, as shown in fig. 504, the magnetic effect will be doubled, and the needle deflected proportionately. upon these phenomena depend the working of galvanometers. [illustration: fig. 503.--effect of neighboring current upon a magnetic needle. above the needle and parallel to it is a conductor carrying an electric current, the current flowing in the direction indicated by the arrow. this causes the north pole of the needle to turn toward the east. if the conductor be held _below_ the needle, its north pole will turn in the opposite direction or toward the west. these movements are easily determined by ampere's rule as follows: _if a man could swim in the conductor with the current, and turn to face the needle, then the north pole of the needle will be deflected toward his left hand_.] ques. describe a simple galvanometer. ans. it consists essentially of a magnetic needle suspended within a coil of wire, and free to swing over the face of a graduated dial. ques. what is a galvanoscope and how does it differ from a galvanometer? ans. a galvanoscope, as shown in fig. 504, serves merely to indicate the presence of an electric current without measuring its strength. it is an indicator of currents where the movement of the needle shows the direction of the current, and indicates whether it is a strong or a weak one. when the value of the readings has been determined by experiment or calculation any galvanoscope becomes a galvanometer. [illustration: fig. 504.--effect upon a magnetic needle of a neighboring current in a loop. in this arrangement the same conductor is simply carried back _beneath_ the needle and hence both the upper and lower portions tend to turn it in the same direction, while the side branch or vertical section is ineffective. in accordance with ampere's swimming rule, the _upper_ wire causes the n pole of the needle to turn to the left, while if a man can imagine himself swimming in the lower wire in the direction of the current, and facing the needle (that is, swimming on his back), the n pole of the needle will turn to his left--that is to the east. the effect of the loop then has double the effect of the single wire in fig. 503.] ques. for what use are galvanometers employed? ans. they are used for detecting the presence of an electric current, and for determining its direction and strength. ques. how is the direction and strength of the current indicated? ans. when a galvanometer is connected in a circuit, the direction of the current is indicated by the side towards which the north pole of the needle moves, and the current strength by the extent of the needle's deflection. [illustration: fig. 505.--effect upon a magnetic needle of a neighboring current in a coil. the coil as shown, is equivalent to several loops, that is, the force tending to deflect the needle is equal to that of a single loop multiplied by the number of turns. hence, by using a coil with a large number of turns, a galvanometer may be made very sensitive so that the needle will be perceptibly deflected by very feeble currents. an instrument, as shown in the figure is called a _galvanoscope_. when it is accurately constructed, and supplied with a scale showing how many degrees the needle is deflected it is then called a galvanometer.] ques. how should a galvanometer be set up before using? ans. when no current is flowing, the coil should be parallel to the magnetic needle when at rest. ques. what is a "sensitive" galvanometer? ans. one which requires a very small current or pressure to produce a stated deflection. it does not follow that a galvanometer which is sensitive for current measurement will also be sensitive for pressure measurement. [illustration: fig. 506.--bunnell simple detector galvanometer. it has middle clamps and scale divided into degrees.] ques. define the term "sensibility." ans. with reference to mirror reflecting galvanometers it may be defined in three ways. first, in _megohms_, the sensibility being the number of _megohms_ through which one volt will produce a deflection of one millimeter with the scale at one meter distance. second, in _micro-volts_, the sensibility being the number of micro-volts which applied directly to the terminals of the galvanometer will produce a deflection of one millimeter on a scale one meter from mirror. the sensibility is best stated in megohms for high resistance galvanometers and in micro-volts for low resistance galvanometers, and is frequently given both for galvanometers for intermediate resistance. third, in micro-amperes, the sensibility being the number of micro-amperes that will give one millimeter deflection with scale at a distance of one meter. ques. upon what does the sensibility depend? ans. 1, upon the number of times the current circulates around the coil, 2, the distance of the needle from the coil, 3, the weight of the needle, 4, the current strength, and 5, the amount of friction produced by its movement. [illustration: fig. 507.--breguet upright galvanometer with glass shade.] [illustration: fig. 508.--bunnell horizontal galvanometer. it has two coils, one of which is of zero resistance and one of fifty ohms resistance adapting it to a variety of test.] the needle is usually quite small, and often a compound one. in very sensitive galvanometers, the coils are wound with thousands of turns of very fine wire, and shunts are generally used in connection with them. note.--strong currents must not be passed through very sensitive galvanometers, for even if they be not ruined, the deflections of the needle will be too large to give accurate measurements. in such cases the galvanometer is used with a shunt, or coil of wire arranged so that the greater part of the current will flow through it, and only a small portion through the galvanometer. ques. what two kinds of coil are used? ans. the short coil and the long coil. ques. what is the difference between a short coil and a long coil galvanometer? ans. a short coil galvanometer has a coil consisting of a few turns of heavy wire; a long coil galvanometer is wound with a large number of turns of fine wire. [illustration: fig. 509.--bunnell galvanometer for measurements of instruments, lines, batteries, wires and any object from 1/100 to 10,000 ohms or more.] ques. what is the action of short and long coil galvanometers? ans. with a given current, the total magnetizing force which deflects the needle is the same, but with a short coil, it is produced by a large current circulating around a few turns, instead of a small current circulating around thousands of turns as in the long coil. the short coil being of low resistance is used to measure the current, and the long coil with high resistance, is suitable for measuring the pressure. hence, a short coil instrument with its scale directly graduated in amperes is an _ammeter_, and the long coil type with graduation in volts is a _voltmeter_. classes of galvanometer.--there are numerous kinds of galvanometer designed to meet the varied requirements. according to construction, galvanometers may be divided into two classes, as those having: 1. movable magnet and stationary coil; 2. stationary magnet and movable coil. [illustration: fig. 510.--astatic needles. two magnetic needles of equal moment are mounted in opposition on a light support. the whole system is suspended by a delicate fibre, and when placed in a uniform magnetic field such as that of the earth, there will be no tendency to assume any fixed direction, the only restraining influence on the needles being that due to torsion in the suspension fibre.] either type may be constructed with short or long coil, and there are several ways in which the deflections are indicated. the principal forms of galvanometer are as follows: 1. astatic; 2. tangent; 3. sine; 4. differential; 5. ballistic; 6. d'arsonval. astatic galvanometer.--it has been pointed out how a compass needle is affected when a wire carrying a current is held over or under it, the needle being turned in one direction in the first instance, and in the opposite direction for the second position of the wire. [illustration: fig. 511.--connections of single coil astatic needles. the coil surrounds the lower needle and the direction of the current between the two needles tends to turn them the same way.] the earth's magnetism naturally holds the compass needle north and south. the magnetic field encircling the wire, being at right angles to the needle (when the wire itself is parallel therewith), operates to turn it from its normal position, north and south, so as to set it partially east and west. however, on account of the fact that the earth's magnetism does exert some force tending to hold the needle north and south, it is evident that no matter how strong the current, the latter can never succeed in turning the needle entirely east and west. the accomplishment of this is further prevented by the reason of the points of the needle, where the magnetic effect is greatest, quickly passing out of the reach of the magnetic field, where it is now practically operated on only in a slight degree. thus it would take quite a powerful current to hold the needle deflected any appreciable distance. the use of a shorter needle is, therefore, more desirable. it is evident in this style of instrument that the effect of the current cannot be accurately measured, because it acts in opposition to the earth's magnetism, and as this is constantly varying, some method must be employed which will either destroy the earth's magnetism or else neutralize it. in the astatic galvanometer, the earth's magnetism is neutralized by means of _astatic needles_. these consist of a combination of two magnetic needles of equal size and strength, connected rigidly together with their poles pointing in opposite and parallel directions, as shown in fig. 510. as the north pole of the earth attracts the south pole of one of the needles, it repels with equal strength the north pole of the other needle, hence, the combination is independent of the earth's magnetism and will remain at rest in any position. [illustration: fig. 512.--connections of double coil astatic needles. with this arrangement, the direction of current in both coils will tend to turn the system in the same direction, making the needles more sensitive than with a single coil as in fig. 511.] if one of the needles be surrounded by a coil, as shown in fig. 511, the magnetic effect of the current will be correctly indicated by the deflection of the needle. sometimes each needle is surrounded by a coil, as in fig. 512, the coils being so connected that the direction of current in each will tend to deflect the needles in the same direction. ques. for what use is the astatic galvanometer adapted? ans. for the detection of small currents. it is used in the "nil" or zero methods, in which the current between the points to which the galvanometer is connected is reduced to zero. [illustration: fig. 513.--queen reflecting astatic galvanometer. it is mounted on a mahogany base with levelling screws. a plain mirror is attached above the upper needle. the entire combination of mirror and needles is suspended by unspun silk from the interior of a brass tube, which also carries a weak controlling magnet. a dial 4 inches in diameter and graduated in degrees, enables the deflections of the needle to be accurately read. the mirror can be used with a reading telescope and scale, or by means of a lantern, the image of a slit may be reflected from the mirror to a screen. resistance, .5 to 1,000 ohms.] ques. upon what does the movement of the needles depend? ans. upon the combined effect of the magnetic attraction of the current which tends to deflect the needles, and the torsion in the suspension fibre which tends to keep the needle at the zero position. ques. does the astatic galvanometer give correct readings for different values of the current? ans. when the deflections are _small_ (that is, less than 10â° or 15â°), they are very nearly proportional to the strength of the currents that produce them. thus, if a current produce a deflection of 6â° it is known to be approximately three times as strong as a current which only turns the needle through 2â°. but this approximate proportion ceases to be true if the deflection be more than 15â° or 20â°. [illustration: fig. 514.--central scientific co. tangent galvanometer. a 9 inch brass ring is mounted on a mahogany base which rotates on a tripod provided with levelling screws. the needle has an aluminum pointer and jewelled bearing. the winding consists of 300 turns of magnet wire so connected to the plugs in front that 20, 40, 80, or 160 turns or any combination of these numbers may be used. for heavy currents a band of copper is used by connecting to the extra pair of binding posts in the rear of the instrument.] ques. why does the instrument not give accurate readings for large deflections? ans. the needles are not so advantageously acted upon by the current, since the poles are no longer within the coils, but protrude at the side. moreover, the needles being oblique to the force acting on them, part only of the force is turning them against the directive force of the fibre; the other part is uselessly pulling or pushing them along their length. [illustration: fig. 515.--bunnell tangent galvanometer. this instrument is mounted on a circular hard rubber base, 7-3/8 inches diameter, provided with levelling screws and anchoring points. the galvanometer consists of a magnetized needle 7/8 inch in length, suspended at the center of a rubber ring six inches in diameter, containing the coils. there are five coils of 0, 1, 10, 50 and 150 ohms resistance. the first is a stout copper band of inappreciable resistance; the others are of different sized copper wires, carefully insulated. five terminals are provided, marked, respectively, 0, 1, 10, 50 and 150. the ends of the coils are so arranged that the plug inserted at the terminal marked 50 puts in circuit all the coils; marked at the terminal 50--all except the 150 ohm coil; and so on, till at the zero terminal only the copper band is in circuit. fixed to the needle, which is balanced on jewel and point, is an aluminum pointer at right angles, extending across a five inch dial immediately beneath. one side of the dial is divided into degrees; on the other side, the graduations correspond to the tangent of the angles of deflection.] ques. how may correct readings be obtained? ans. the instrument may be calibrated, that is, it may be ascertained by special measurements, or by comparison with a standard instrument, the amounts of deflection corresponding to particular current strengths. thus, if it be once known that a deflection of 32â° on a particular galvanometer is produced by a current of 1/100 of an ampere, then a current of that strength will _always_ produce on that instrument the same deflection, unless from any accident the torsion force or the intensity of the magnetic field be altered. [illustration: fig. 516.--tangent galvanometer. it consists of a short magnetic needle suspended at the center of a coil of large diameter and small cross section. in practice, the diameter of the coil is about 17 times the length of the needle. if the instrument be so placed that, when there is no current in the coil, the suspended magnet lies in the plane of the coil, that is, if the plane of the coil be set in the magnetic meridian, then _the current passing through the coil is proportional to the tangent of the angle by which the magnet is deflected from the plane of the coil_, or zero position--hence the name: "tangent galvanometer."] the tangent galvanometer.--it is not possible to construct a galvanometer in which the _angle_ (as measured in degrees of arc) through which the needle is deflected is proportional throughout its whole range to the strength of the current. but it is possible to construct a very simple galvanometer in which the _tangent of the angle of deflection_ shall be accurately proportional to the strength of the current. [illustration: fig. 517.--horizontal section through middle of tangent galvanometer, showing magnetic whirls around the coil and corresponding deflection of needle.] [illustration: fig. 518.--diagram of forces acting on the needle of a tangent galvanometer.] a simple form of tangent galvanometer is shown in fig. 516. the coil of this instrument consists of a simple circle of stout copper wire from ten to fifteen inches in diameter. at the center is delicately suspended a magnetized steel needle not exceeding one inch in length, and usually furnished with a light index of aluminum. when the galvanometer is in use, the plane of the ring must be vertical and in the magnetic meridian. a horizontal section through the middle of the instrument is shown in fig. 517. for simplicity, the coil is supposed to have but a single turn of wire, the circles surrounding the wire representing the magnetic lines of force. by extending the lines of force until they reach the needle, it will be seen that with a short needle, the deflecting force acts in an east and west direction when the galvanometer is placed with its coil in the magnetic meridian. if, in fig. 518, _ab_ represent the deflecting force acting on the n end of the needle, the component of this force that acts at a right angle to the needle will be _ab_ cos _x_ in which, _x_ is the angle of the deflection. the controlling force is _ad_ = h and when the needle is in equilibrium, the component _ae_ = h sin _x_ is equal and opposite to _ac_, hence _ab_ cos _x_ = h sin _x_ from which _ab_ = h(sin _x_ / cos _x_) = h tan _x_ since _ab_ is proportional to the current, _ab_ = _k_ c = h tan _x_ in which _k_ is a constant depending upon the instrument. for any other current c', _k_ c' = h tan _x'_ hence c: c' = tan _x_ : tan _x'_ this means that the currents passing through the coil of a tangent galvanometer are proportional, not to the angle of deflection, but to the tangent of that angle. [illustration: fig. 519.--diagram illustrating the tangent law. this is the law of the combined action of two magnetic fields upon a magnetic needle. if two magnetic fields be at right angles in direction as indicated in the figure, the resultant field is obtained by the parallelogram of forces and it makes an angle [theta] with one of the component fields such that tan [theta] = m + h where m and h are the strengths of the component fields. in the tangent galvanometer this principle is employed in the measurement of currents. a magnetic needle is pivoted in a field of known strength. the current to be measured is passed round a coil (or coils) which generates a field at right angles to the original field. the needle then lies along the direction of the resultant field, and by finding the tangent of its angle of deflection, and knowing the field strength produced by unit current in the coil, the current strength can be found.] [illustration: fig. 520.--graduation of tangent galvanometer scale with divisions representing tangent values. in the figure let a tangent ot be drawn to the circle, and along this line let any number of equal divisions be set off, beginning at o. from these points draw lines back to the center. the circle will thus be divided into a number of spaces, of which those near o are nearly equal, but which get smaller and smaller as they recede from o. these unequal spaces correspond to equal increments of the tangent. if the scale were divided thus, the readings would be proportional to the tangents.] ques. upon what does the sensibility of a tangent galvanometer depend? ans. it is directly proportional to the number of turns of the coil and inversely proportional to the diameter of the coil. ques. how may the tangent galvanometer be used as an ammeter? ans. the strength of the current may be calculated in amperes by the formula given below when the dimensions of the instrument are known. the needle is supposed to be subject to only the earth's magnetism and to move in a horizontal plane. the current is calculated as follows: (1) amperes = ((h ã� _r_)/n) tan _x_ in which h = constant from table below; r = radius of coil; n = number of turns of coil; x = angle of deflection of needle. the constant h, given in the following table represents the horizontal force of the earth's magnetism for the place where the galvanometer is used. each value has been multiplied by (2[pi])/10 so that the formula (1) for amperes is correct as given. table of galvanometer constants.--values of h. boston | .699| chicago | .759| denver | .919| jacksonville | 1.094| london | .745| minneapolis | .681| new york | .744| new haven | .731| philadelphia | .783| portland, me. | .674| san francisco | 1.021| st. louis | .871| washington | .810| [illustration: fig. 521.--mechanical explanation of the tangent law. construct an apparatus as shown in the figure. the short wooden block, ns, represents the magnetic needle. this piece of wood turns around its center, c, which may be an ordinary nail. it will now be seen that two different forces act upon n; namely, the weight, g (one or two ounces), and the changeable weights which are placed in the scoop, w (made of cardboard). the height of the roll, or wheel, r, is such that the cord, rn, runs horizontally, when ns stands vertically, i.e., when there is no weight in the little scoop. if the wheel, r, be placed sufficiently far from ns, the string rn, will always remain almost horizontal, even if ns be deviated. the thin hand on ns moves over a vertical scale, which is divided into equal parts, as shown. this scale may be made of cardboard. if the hand point to division 1 when one ounce is placed in the scoop, it will point to 2 for two ounces, to 3 for three ounces, etc. at 45â° the needle is deviated at its greatest angle, and this is, therefore, the sensitivity angle of the tangent galvanometer. the deviating values are, therefore, proportionate to the scale parts 01, 02, and 03, and so on; and, inasmuch as these themselves are tangents, the tangent law will hold good.] ques. how is the tangent galvanometer constructed to give direct readings? ans. to obviate reference to a table, the circular scale of the instrument is sometimes graduated into tangent values, as in fig. 520, instead of being divided into equal degrees. [illustration: fig. 522.--queen tangent and sine galvanometer. this instrument properly adjusted can be used as a standard instrument for laboratory work. the brass ring is 12 inches in diameter, and the grooves in which the wire is wound are carefully turned so as to be of true rectangular cross section, thus allowing the constant of the instrument to be accurately calculated and compared with the constant as obtained by other methods. the compass box is 5 inches in diameter and is so held in position that it may be raised or lowered, rotated on its vertical axis, shifted out of the plane of the coil, etc., thus enabling the operator to acquire proficiency with the instrument and to meet all cases of derangement possible. the dial is graduated to single degrees, and the needle is suspended by a very light cocoon fibre. the whole instrument can be turned about its vertical axis, and a quadrant graduated in degrees upon the base allows the amount of rotation to be accurately measured, and the laws of the sine galvanometer investigated. the instrument is wound to measure .25 ampere to 8 amperes.] ques. what is the objection to the scale with tangent values? ans. it is more difficult to divide an arc into tangent lines with accuracy than into equal degrees. ques. what disadvantage has the tangent galvanometer? ans. the coil being much larger than the needle, and hence far away from it, reduces the sensitiveness of the instrument. the sine galvanometer.--this type of instrument has a vertical coil which may be rotated around a vertical axis, so that it can be made to follow the magnetic needle in its deflections. in the sine galvanometer, the coil is moved so as to follow the needle until it is parallel with the coil. under these circumstances, the strength of the deflecting current is proportional to sine of angle of deflection. [illustration: fig. 523.--central scientific co. universal tangent galvanometer. this instrument may be used as a tangent, gaugain, helmholtz-gaugain, sine, cosine, wiedemann or detector galvanometer. the coils, which slide on a beam parallel to the one carrying the needle box, are wound on brass rings 12 inches in diameter. on each ring are wound two coils of 48 turns each, connected to separate binding posts, and double wound so as to be of equal resistance. the coils and needle box are each provided with an indicator for reading their position on the scale. the needle box is swivelled and removable and one coil may be rotated about its vertical axis and its position read on a disc graduated in degrees. currents may be measured ranging from .000002 ampere to 100 amperes.] ques. describe the construction of a sine galvanometer. ans. a form of sine galvanometer is shown in fig. 524. the vertical wire coil is seen at m. a needle of any length less than the diameter of the coil m, moves over the graduated circle n. the coil m, and graduated circle n may be rotated on a vertical axis, and the amount of angular movement necessary to bring the needle to zero, measured on the graduated circle h. ques. how is the current strength measured? ans. it is proportional to the _sine_ of the angle measured on the horizontal circle h, through which it is necessary to turn the coil m, from the plane of the earth's magnetic meridian to the plane of the needle when it is not further deflected by the current. [illustration: fig. 524.--sine galvanometer. it differs from the tangent galvanometer in that the vertical coil and magnetic needle are mounted upon a standard free to revolve around a vertical axis, with provision for determining the angular position of the coil. the needle may be of any length shorter than the diameter of the coil. in the figure the parts are: m, coil; n, graduated dial of magnetic needle; h, graduated dial by which the amount of rotation necessary to bring the needle to zero is measured; e, terminals of the coil; o, upright standard carrying coil and graduated dial of magnetic needle; c, base with levelling screws.] ques. how is the sine galvanometer operated? ans. in using the instrument, after the needle has been set to zero, the current is sent through the coil, producing a deflection of the needle. the coil is then rotated to follow the motion of the needle, the current being kept constant, the rotation being continued until the zero on the upper dial again registers with the needle. the current then is proportional to the sine of the angle through which the coil has been turned, as determined by the lower dial. ques. has the sine galvanometer a large range? ans. for a given controlling field, it does not admit of a very large range of current measurement, since, for large deflection, on rotating the coil the position of instability is soon reached. table of natural sines and tangents angle| sin.| tan.| 0â°| .0000| .0000| 1 | .0175| .0175| 2 | .0349| .0349| 3 | .0523| .0524| 4 | .0698| .0699| 5 | .0871| .0875| 6 | .1045| .1051| 7 | .1219| .1228| 8 | .1392| .1405| 9 | .1564| .1564| 10â°| .1736| .1763| 11 | .1908| .1944| 12 | .2079| .2126| 13 | .2250| .2309| 14 | .2419| .2493| 15 | .2588| .2679| 16 | .2756| .2867| 17 | .2924| .3057| 18 | .3090| .3249| 19 | .3256| .3443| 20â°| .3420| .3640| 21 | .3584| .3839| 22 | .3746| .4040| 23 | .3907| .4245| 24 | .4067| .4452| 25 | .4226| .4663| 26 | .4384| .4877| 27 | .4540| .5095| 28 | .4695| .5317| 29 | .4848| .5543| 30â°| .5000| .5774| 31 | .5150| .6009| 32 | .5299| .6249| 33 | .5446| .6494| 34 | .5592| .6745| 35 | .5736| .7002| 36 | .5878| .7265| 37 | .6018| .7536| 38 | .6157| .7813| 39 | .6293| .8098| 40â°| .6428| .8391| 41 | .6561| .8693| 42 | .6691| .9004| 43 | .6820| .9325| 44 | .6947| .9657| 45 | .7071| 1.0000| 46 | .7193| 1.0355| 47 | .7314| 1.0724| 48 | .7431| 1.1106| 49 | .7547| 1.1504| 50â°| .7660| 1.1918| 51 | .7771| 1.2349| 52 | .7880| 1.2799| 53 | .7986| 1.3270| 54 | .8090| 1.3764| 55 | .8192| 1.4281| 56 | .8290| 1.4826| 57 | .8387| 1.5399| 58 | .8480| 1.6003| 59 | .8572| 1.6643| 60â°| .8660| 1.7321| 61 | .8746| 1.8040| 62 | .8829| 1.8807| 63 | .8910| 1.9626| 64 | .8988| 2.0503| 65 | .9063| 2.1445| 66 | .9135| 2.2460| 67 | .9205| 2.3559| 68 | .9272| 2.4751| 69 | .9339| 2.6051| 70â°| .9397| 2.7475| 71 | .9455| 2.9042| 72 | .9511| 3.0772| 73 | .9563| 3.2709| 74 | .9613| 3.4874| 75 | .9659| 3.7321| 76 | .9703| 4.0108| 77 | .9744| 4.3315| 78 | .9781| 4.7046| 79 | .9816| 5.1446| 80â°| .9848| 5.6713| 81 | .9877| 6.3138| 82 | .9903| 7.1154| 83 | .9925| 8.1443| 84 | .9945| 9.5144| 85 | .9962| 11.43| 86 | .9976| 14.30| 87 | .9986| 19.08| 88 | .9994| 28.64| 89 | .9998| 57.29| ques. what is the position of instability? ans. the position of the needle beyond which the rotation of the coil will cause it to turn all the way round. ques. how may the range be increased? ans. by an adjustable controlling field or a shunt. ques. what advantage has the sine galvanometer over the tangent instrument? ans. its advantage is in the case where the relative values of two or more currents are required to be measured, or where the constant of the instrument is obtained by comparison with a standard measuring instrument and not calculated from the dimensions of the coil, because all galvanometers thus used follow the sine law independently of the shape of the coil, while only circular coils will follow the sine law. [illustration: fig. 525.--differential galvanometer. it consists of two coils of wire, so wound as to have opposite magnetic effects on a magnetic needle suspended centrally between them. the needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions, since, under these conditions, each coil neutralizes the effect of the other. sometimes the current is so sent through the two coils, that each coil deflects the needle in the same direction. in this case the instrument is no longer differential in action. if, when this condition obtains, the magnetic needle be suspended at the exact center of the line which joins the centers of the coils, the advantage is gained by obtaining a field of more nearly uniform intensity around the needle. when the needle is suspended by a silk fibre, a final and most delicate adjustment can be obtained by raising or lowering one of the levelling screws slightly, so as to tilt the needle nearer to or farther from one of the coils.] the differential galvanometer.--this is a form of galvanometer in which a magnetic needle is suspended between two coils of equal resistance so wound as to tend to deflect the needle in opposite directions. the needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions, since under these conditions, each coil neutralizes the other's effects. such instruments may be used in comparing resistances, although the _wheatstone bridge_, in most cases, affords a preferable method. ques. what is the special use of the differential galvanometer? ans. it is used for comparing two currents. ques. what is the method of comparing currents? ans. if two equal currents be sent in opposite directions through the coils of the galvanometer, the needle will not move; if the currents be unequal, the needle will be deflected by the stronger of them with an intensity corresponding to the difference of the strengths of the two currents. ques. how are the coils adjusted? ans. this is done by coupling them in series in such a way that they tend to turn the needle in opposite directions, and when a current is passing through them, they are moved nearer to the needle or farther from it until the needle stands at zero with any current. if the coils be not movable, a turn or more can be unwound from the coil giving the greatest magnetic effect until a balance is obtained, the wire so unwound can then be coiled in the base of the instrument. ballistic galvanometer.--this type of galvanometer is designed to measure the strength of momentary currents, such for instance, as the discharge of a condenser. in construction the magnetic system is given considerable weight, and arranged to give the least possible _damping effect_. the term "damping effect" means the offering of a retarding force to control swinging vibrations, such as the movements of a galvanometer needle, and to bring them quickly to rest. if a momentary current be passed through a ballistic galvanometer, the impulse given to the needle does not cause appreciable movement to the magnetic system until the current ceases, owing to the inertia of the heavy moving parts, the result being a slow swing of the needle. [illustration: fig. 526.--queen dead beat and ballistic reflecting galvanometer. as illustrated, the coils are easily removable and enclose a heavy block of copper fixed in a central fork. in a cylindrical hole bored in this block hangs the bell magnet which with its mirror is suspended by a long cocoon fibre, and the eddy currents induced in the copper bring the system quickly to rest after a deflection. by lifting the copper block out of the frame the instrument is made ballistic. the instrument is made with coils of any desired resistance up to 1,000 ohms.] ques. what name is given to the swing of a ballistic galvanometer needle? ans. it is called the _kick_. ques. how is the current measured? ans. as the needle swings slowly around it adds up, as it were, the varying impulses received during the passage of the momentary current, and _the quantity of electricity that has passed is proportional to the sine of half the angle of the first swing or kick_. if a reflecting method be used with a straight scale, the observed deflection depends upon the tangent of twice the angle of movement of the needle. for small deflections, however, the change of flux can be taken as directly proportional to the observed deflection. [illustration: fig. 527.--thompson galvanometer with mirror reflecting system for reading the deflections of a galvanometer needle by the movements of a spot of light reflected from a mirror attached to the needle or movable magnetic system.] use of mirrors in galvanometers.--in order that small currents may be measured accurately, some means must be provided to easily read a small deflection of the needle. accordingly, it is desirable that the pointer be very long so that a large number of scale divisions may correspond to small deflections. in construction, since sensitive galvanometers must be made with the moving parts of little weight, it would not do to use a long needle, hence a ray of light is used instead, which is reflected on a distant scale by a small mirror attached to the moving part. in the thompson mirror reflecting galvanometer, as shown in fig. 528, a small vertical slit is cut in the lamp screen below the scale, and the ray of light from the lamp, passing through the slit, strikes the mirror which is about three feet distant, and which reflects the beam back to the scale. it should be noted that the angle between the original ray of light and the reflected ray is twice the angle of the deflection of the mirror; the deflections of the ray of light on the scale, however, are practically proportional to the strength of currents through the instrument. the mirror arrangement as shown in fig. 528, requires a darkened room for its operation, but such is not necessary when a telescope is used as in fig. 529. here the scale readings are reflected in the mirror and their value observed by the telescope without artificial light. [illustration: fig. 528.--telescope method of reading galvanometer deflections by reflection of scale reading in mirror. here two mirrors are used, but in most cases the telescope is pointed directly toward the mirror on galvanometer shown in fig. 527, because the two mirror system, as illustrated in the figure, is used on portable galvanometers since it is the more compact.] damping.--this relates to the checking or reduction of oscillations. thus, a galvanometer is said to be damped when so constructed that any oscillations of the pointer which may be started, rapidly die away. galvanometers are frequently provided with damping devices for the purpose of annulling these oscillations, thus causing the moving part to assume its final position as quickly as possible. sometimes the instrument is fitted with a damping coil, or closed coil so arranged with respect to the moving system that the oscillations of the latter give rise to electric currents in the closed coil, whereby energy is dissipated. again, air vanes are employed, but anything in the nature of solid friction cannot be used. [illustration: figs. 529 and 530.--galvanometer lamp and scale for individual use. the scale is etched on a ground glass strip 6 centimeters wide by 60 centimeters long with long centimeter divisions and short millimeter divisions the entire length, reading both ways from zero in the center. it is mounted in an adjustable wooden frame. a straight filament lamp (110 volts) is enclosed in a metal hood japanned black to cut out all reflected light. this form of filament makes a single brilliant line on the scale, enabling closer readings than the "spot of light" arrangement. the lamp hood is adjustable to any desired height on the support rod.] d'arsonval galvanometer.--this instrument has a movable coil in place of a needle, and its operation depends upon the principle that if a flat coil of wire be suspended with its axis perpendicular to a strong magnetic field, it will be deflected whenever a current of electricity passes through it. ques. describe the construction of a d'arsonval galvanometer. ans. the essential features are shown in figs. 532 and 533. the coil, which is rectangular in section is wound upon a copper form, and suspended between a permanent magnet by fine wires to the points a and b. the magnet has its poles at n and s. it is a soft iron cylinder fixed between the poles in order to intensify the magnetic field across the air gaps in which the coil moves. [illustration: fig. 531.--queen reading telescope. this arrangement is utilized to measure the deflections of a galvanometer having suspended mirror moving system. it consists of a reading telescope mounted as illustrated with a millimeter scale, having a length of 50 centimeters. in use, the image of the scale is seen in the galvanometer mirror through the telescope. the eye piece of the telescope has a cross hair which acts as a reference line so that by noting the particular division on the scale when the galvanometer is at rest, the amount of deflection can be readily observed when the galvanometer is deflected. the instrument has all the necessary adjustments to set it up quickly and for bringing the cross hair and scale in focus. it is generally placed at a distance of one meter from the galvanometer mirror.] ques. explain its operation. ans. an enlarged horizontal cross section of the galvanometer on line xy is shown in fig. 533. the current is flowing in the coil as in fig. 532, up on the left side and down on the right. the position of the coil when no current is flowing is indicated by _n' s'_. by applying the law of mutual action between magnetic poles, it is seen that when the current is applied, the poles developed at _n' s'_ will move into the position _n'' s''_. see fig. 119. ques. how is the coil affected by a change in the direction of the current? ans. the polarity of the coil is reversed and consequently the direction of the deflection. [illustration: figs. 532 and 533.--diagrams showing essential features of construction and principle of operation of d'arsonval galvanometer.] ques. upon what does the sensitiveness of the instrument depend? ans. upon the strength of the field of the permanent magnet, the number of turns in the suspended coil, and the torsion of the wires by which it is suspended. ques. when is this galvanometer called "dead beat"? ans. when the construction is such that the moving part comes quickly to rest without a series of diminishing vibrations. [illustration: figs. 534 to 536.--queen horizontal magnet d'arsonval galvanometer with telescope and scale. it is very sensitive and is used in many electrical measurements, including commercial testing, such as measuring insulation of cables, fault location, etc. it is not affected by surrounding magnetic disturbances, and may, therefore, be used in proximity to dynamos and switchboards. the instrument has a pair of binding post terminals, one of which connects to a bottom spiral of the system and the other forms a junction with the top of the tube holding the system, forming a complete circuit through the coil. the tube containing the system may be readily removed from the magnet and another tube having a different system inserted as is required for various kinds of electrical measurement. the entire system with its suspension may be inspected by the removal of a thumb screw. to inspect interior of tube first be sure that the screw b is turned so that the coil is clamped. entirely remove screw c, and, holding the outside tube near the window, press firmly with the finger on the extreme top of the suspension support. the inside rib, with complete suspension, will draw from the tube, and the working parts can be fully inspected. carefully return same to its original position in tube, setting tight the screw c. the galvanometer is designed so that the coil is clamped in position when the galvanometer must be transported. the insulation of the galvanometer terminals and binding posts is such as to guard against any possible leakage. as a further protection, each levelling screw is provided with a hard rubber insulator. this feature is essential since, in making insulation measurements, the operator wishes to be assured that the deflection being obtained is the result of leakage upon the cable or wire being measured and not leakage between the galvanometer terminals. the galvanometer is provided with an attached telescope and scale for noting the deflections. the deflections produced by this galvanometer are proportional to the current. to facilitate quickly setting up the instrument, two way levels are provided.] ques. what causes this? ans. the instrument is made dead beat by winding the coil on a copper or aluminum frame, so that when in operation, currents are induced in the frame by the motion of the coil in the magnetic field; these currents oppose the motion of the coil. ques. for what service is the d'arsonval galvanometer adapted? ans. it is desirable for general use as it is not much affected by changes in the magnetic field. it may be made with high enough period and sensibility to be satisfactory as a ballistic instrument, but for extreme sensibility an instrument of the astatic type is more generally used. galvanometer "constant" or "figure of merit."--in order that a galvanometer shall be of value as a measuring instrument, the relation between the current and the deflection produced by it must be known. this may be obtained experimentally by determining the value of the current required to produce one scale division. the galvanometer constant then may be defined as _the resistance through which the galvanometer will give a deflection of one scale division when the current applied is at a pressure of one volt_. accordingly, the deflection as indicated on the scale must be multiplied by its constant or figure of merit, in order to obtain the correct reading. if the scale readings be not directly proportional to the quantity to be measured, the law of the instrument must also be considered. thus in a tangent galvanometer as previously explained i = k tan [phi] where i = current, [phi] the deflection or scale reading, and k the galvanometer constant. [illustration: fig. 537.--diagram showing method of connecting galvanometer shunt. by the use of a shunt the range of measurement of a galvanometer can be greatly increased.] [illustration: fig. 538.--diagram of a form of universal shunt box for use with galvanometers of widely different resistances. the galvanometer, as indicated at g, is connected across the ends of a series of resistances ab. the main wires are connected, one to end a of the series and the other to a travelling point whose position is varied by means of plugs or by a dial switch.] galvanometer shunts.--the sensitiveness of a galvanometer used for measuring current may be reduced to any desired extent by connecting a resistance of known value in parallel with it. thus, if it be desired to measure a current greater than can be measured directly by the galvanometer, a part of the current can be sent through the resistance or shunt, and the total value of the current calculated. a galvanometer shunt bears a definite ratio to the resistance of the galvanometer, being usually adjusted so that only .1, .01, or .001 part of the current passes through the galvanometer. the degree in which a shunt increases the range of deflection of a galvanometer is called its "multiplying power." [illustration: fig. 539.--ayrton-mather universal shunt. this shunt may be used with any galvanometer. the total resistance is 10,000 ohms, with shunt powers of 1, 5, 10, 50, 100, 500, and 1,000. it is also fitted with positions in which the galvanometer is shorted and off. the coils are of constantan wire.] if .1 of the current flowing, passed through the galvanometer and .9 through the shunt, then the current in the circuit would be ten times that through the galvanometer. accordingly the current in the galvanometer must be multiplied by the multiplying power of the shunt to obtain the true value of the current in the circuit. in order to determine the resistance necessary to be used with a certain galvanometer, the resistance of the latter _is to be divided by the multiplying power desired less one_. example.--what must be the resistance of a shunt for a galvanometer of 2,000 ohms resistance where only one fifth of the current is to pass through the galvanometer? the multiplying power less one is 5 1 = 4 and the required resistance is 2,000 ã· 4 = 500 ohms. when it is essential that the total resistance of the circuit should not be altered by an alternation of the galvanometer shunt, a compensating box should be used which automatically inserts a resistance for each shunt in series with the shunted galvanometer to bring the total resistance up equal to the unshunted value. thus the current in the main circuit is not altered. chapter xxvii testing and testing apparatus the practical electrician frequently has to make tests of various kinds which require the rapid and accurate measurement of voltage, current and resistance. it is therefore essential that he understand the methods employed in testing and the operation of the instruments used. most tests are made with a galvanometer, and the devices, such as resistances, switches, etc., which are used in connection with the galvanometer may be obtained put up in a neat and substantial box together with the galvanometer, the combination being called a "testing set." numerous forms of testing set are illustrated in this chapter. the construction, use, and operation of the various types of galvanometer have been explained in chapter twenty-six. ammeters and voltmeters, which are simply special forms of galvanometer, and which are largely used are fully described in the preceding chapter. pressure measurement.--an electromotive force has been defined as that which causes or tends to cause a current; it is analogous to water pressure; potential difference corresponds to difference of level. the _total_ electromotive force of a circuit is independent of resistance or current, and cannot be limited to mean the fall of pressure between any two points, as for instance the terminals of a battery. if the pressure of a battery be two volts when measured on open circuit by a static voltmeter, there will still be two volts on closed circuit, but there will now be a loss of pressure through the internal resistance of the battery and the voltage across the terminals will be less than the _total_ voltage. the static voltmeter, never closing the circuit, actually measures the total voltage. [illustration: fig. 540.--clark cell (kahle's modification of the rayleigh h form), the standard for the international volt. the cell has for its positive electrode, mercury, and for its negative electrode, amalgamated zinc. the electrolyte consists of a saturated solution of zinc sulphate and mercurous sulphate. the pressure is 1.434 volts at 15â°c., and between 10â°c. and 25â°c. the pressure decreases .00115 of a volt for each increase of 1â°c. the containing glass vessel consists of two limbs, closed at bottom and joined above to a common neck fitted with a ground glass stopper. the diameter of the limbs should be at least 2 cms., and their length at least 3 cms. the neck should be not less than 1.5 cms. in diameter. at the bottom of each limb a platinum wire of about .4 mm. in diameter is sealed through the glass. to set up the cell, place mercury in one limb, and in the other hot liquid amalgam, containing 90 parts mercury and 10 parts zinc. the platinum wires at the bottom must be completely covered by the mercury and the amalgam, respectively. on the mercury, place a layer 1 cm. thick of the zinc and mercurous sulphate paste. both this paste and the zinc amalgam must be covered with a layer of the neutral zinc sulphate crystals 1 cm. thick. the whole vessel must then be filled with the saturated zinc sulphate solution, and the stopper inserted so that it shall just touch it, leaving, however, a small bubble to guard against breakage when the temperature rises. before finally inserting the glass stopper a strong alcoholic solution of shellac is applied to the upper edge, after which the stopper is pressed firmly in place.] ques. what error is introduced in measuring the pressure of a battery with an ordinary voltmeter? ans. since the measurement is made on _closed circuit_ the reading does not give the total pressure of the battery. the error is very slight because the resistance of the voltmeter is very high and the current so small that the loss of pressure in the battery can be neglected. [illustration: fig. 541.--weston cadmium cell. it is made in two forms; one known as the weston normal cell, in which the solution of cadmium sulphate is saturated at all temperatures at which the cell may be used. the other, known as the weston standard cell, in which the cadmium sulphate solution is unsaturated at all temperatures above 4â°c. the weston normal cell, or saturated form is slightly affected by changes in temperature, but, on account of the fact that it can be accurately reproduced, it was adopted by the london conference in 1908, as a convenient voltage standard. the value of its voltage suggested by the committee of the london conference on electrical units and standards, and adopted by the bureau of standards at washington, jan. 1st, 1911, is 1.0183 international volts at 20â°c. at any other temperature its voltage is: e_{t} = e_{20} 0.0000406(t-20) 0.00000095(t-20)^{2} + 0.00000001(t-20)^{3} the weston standard cell, or unsaturated form is practically unaffected by changes in temperature and is the form most commonly used for laboratory work and general testing. the average pressure of this form is 1.0187 int. volts.] ques. define the international volt. ans. _it is the electromotive force that, steadily applied to a conductor whose resistance is one international ohm, will produce a current of one international ampere, and which is represented sufficiently well for practical use by 1,000/1,434 of the voltage between the poles of the clark cell at a temperature of 15â° c., when prepared as in fig. 540._ the relation between the units volt, ampere and ohm, are shown graphically in figs. 542 and 543. [illustration: figs. 542 and 543.--diagrams showing hydraulic analogy illustrating the difference between amperes and coulombs. the _rate_ of current flow of one ampere in fig. 543 may be compared to the rate of discharge of a pump as in fig. 542. assuming the pump to be of such size that it discharges a gallon per stroke and is making 60 strokes per minute, the quantity of water discharged per hour (coulombs in fig. 543) is 1 ã� 60 ã� 60 = 3,600 gallons. following the analogy further (in fig. 543), the pressure of one volt is required to force the electricity through the resistance of one ohm between the terminals a and b. in fig. 542, the boiler must furnish steam pressure on the pump piston to overcome the friction (resistance) offered by the pipe and raise the water from the lower level a' to the higher level b'. the difference of pressure between a and b in the electric circuit corresponds to the difference of pressure between a' and b'. the cell furnishes the energy to move the current by maintaining a difference of pressure at its terminals c and d; similarly, the boiler furnishes energy to raise the water by maintaining a difference of pressure between the steam pipe c and exhaust pipe d'.] [illustration: fig. 543. if the current strength in fig. 543 be one ampere, the quantity of electricity passing any point in the circuit per hour is 1 ã� 60 ã� 60 = 3,600 coulombs.] current measurement.--it is necessary to adopt some arbitrary standard in order to compare currents of different strengths. the term _strength of a current_, or current strength means the _rate of flow_ past any point in the circuit in a given unit of time. the unit of current, called the _ampere_, is defined as _the unvarying current which, when passed through a solution of nitrate of silver in water (15 per cent. by weight of the nitrate) deposits silver at the rate of .001118 gramme per second_. [illustration: fig. 544.--queen weight voltameter for determining the strength of current by the weight of metal deposited in a given time. the two outside plates form the anode and are joined together and to one binding post, while the cathode is placed between them and connected to the other binding post. the cathode thus receives a deposit on both sides. an adjustable arm serves to lower the plates into the electrolyte. to calculate the strength of an unknown current which has passed through a weight voltameter, _divide the gain in weight by the number of seconds the current flows through the instrument and by the weight deposited by one ampere in one second_. that is, current strength in amperes = gain in weight ã· (time in seconds ã� .0003286).] ques. how much copper or zinc will one ampere deposit in one second? ans. .0003286 gramme of copper in a copper voltameter, or .0003386 gramme of zinc in a zinc voltameter. ques. what is the difference between an ampere and a coulomb? ans. an ampere is the unit _rate of flow_ of the current, and a coulomb is the unit _quantity_ of electricity, that is, the ampere is the rate of current flow that will deposit .0003286 grammes of copper in one second and a coulomb is the _quantity_ of electricity that passes a given point in one second when the current strength is one ampere. in other words a coulomb is one _ampere second_. [illustration: fig. 545.--gas voltameter for determining the strength of current by the volume of gas evolved. to use, connect up as shown in the illustration. adjust so that the zero position of the burette is about one-half inch below the level of the top of the u tube. pour acidulated water into the mouth of the burette till the water in the u tube is about one-half inch from the top. with the electrodes inserted through the corks, _carefully_ place each one in position by giving a slight twist to the right as the cork enters. the water level in the u tube and burette should now be the same, or further adjustment must be made to attain this result. the level in the burette does not necessarily have to correspond with the zero graduation, but must not be below it. unclamp the burette and hold it nearly horizontal. the liquid will not run out if the corks be tight, so that this is the _air leakage test_. attach the connectors and wires from the current source (which should have a pressure of 2 or more volts) placing a switch in the circuit. when the switch is closed, bubbles of gas will rise in the u tube from both electrodes, displacing the water and forcing it up the burette. hydrogen will be liberated over the negative electrode, and oxygen over the positive electrode in the proportion of twice as much hydrogen as oxygen. to calculate the current strength, _divide the volume of gas liberated by the time in seconds, and by the volume of gas liberated (in cubic centimeters) by one ampere in one second and by .1733_; that is: amperes = volume of gas liberated ã· (time in seconds ã� .1733).] example.--if an arc lamp require a current of 8 amperes, how much electricity does it consume per hour? since one coulomb = one ampere second, the quantity of electricity consumed per hour is 8 amperes ã� ( 60 ã� 60 ) = 28,800 coulombs. voltameter.--a voltameter is an electrolytic cell employed to measure an electric current by the amount of chemical decomposition the current causes in passing through the cell. there are two classes of voltameter: 1. weight voltameters; 2. gas voltameters. ques. what is the difference between these two classes of voltameter? ans. in one, the current strength is determined by the weight of metal deposited or weight of water decomposed, and in the other by the volume of gas liberated. fig. 544 shows a weight voltameter and fig. 545 a gas voltameter. ques. how should the plates of a weight voltameter be treated before use? ans. they must be thoroughly cleaned and polished with sandpaper, the sand being afterwards removed by placing them in running water. _the fingers must not be placed on any part of the plate which is to receive the deposit._ ques. what form of voltameter has been selected to measure the international ampere? ans. the silver voltameter arranged as here specified: the cathode on which the silver is to be deposited shall take the form of a platinum bowl, not less than 10 cms. in diameter, and from 4 to 5 cms. in depth. the anode shall be a disc or plate of pure silver some 30 sq. cms. in area, and 2 or 3 cms. in thickness. this shall be supported horizontally in the liquid near the top of the solution by a silver rod riveted through its center. to prevent the disintegrated silver which is formed on the anode falling upon the cathode, the anode shall be wrapped around with pure filter paper, secured at the back by suitable folding. the liquid shall consist of a neutral solution of pure silver nitrate containing about 15 parts by weight of the nitrate to 85 parts of water. ques. what is the value of the international ampere as measured with the silver voltameter? ans. the international ampere is represented sufficiently well for practical use by the unvarying current which, when passed through a silver voltameter (as described above) deposits _silver at the rate of .001118 gramme per second_. [illustration: fig. 546.--single contact and short circuiting key. this key is intended especially for use with d'arsonval galvanometers in zero deflection methods. the key is connected in circuit with the galvanometer so that whenever the key is not depressed, the galvanometer is short circuited, and its oscillations quickly damped out by the currents induced in its coil. the back end of the spring is held in a slot in a rubber block attached to the base.] ohm's law and the ohm.--the various tests here described depend for their truth upon the definite relation existing between the electric current, its pressure, and the resistance which the circuit offers to its flow. this relation was fully investigated by ohm in 1827. using the same conductor, he proved not only that the current varies with the pressure, but that it varies in direct proportion. ohm's law has already been discussed in a previous chapter and the several ways of expressing it are repeated here for convenience: volts 1. amperes = -------; ohms 2. volts = amperes ã� ohms; volts 3. ohms = ---------. amperes various values have been assigned, from time to time, to the ohm or unit of resistance, the unit in use at the present time being known as the _international ohm_. this was recommended at the meeting of the british association in 1892, was adopted by the international electrical congress held in chicago in 1893, and was legalized for use in the united states by act of congress in 1894. the international ohm in graphically defined in fig. 548. the previous values given to the ohm which were more or less generally accepted are as follows: the siemens' ohm.--a resistance due to a column of mercury 100 cm. long and 1 sq. mm. in cross section at 0â° c. b. a. (british association) ohm.--a resistance due to a column of mercury approximately 104.9 cm. long and 1 sq. mm. in cross section at 0â° c. legal ohm.--a resistance due to a column of mercury 106 cm. long and 1 sq. mm. in cross section at 0â° c. this unit was adopted by the paris conference of 1884. ohm table[a] | | inter-| | | | | date |national| legal|b. a. |siemens'| | | ohm| ohm|ohm | ohm| -------------------------+------+--------+--------+--------+--------| international ohm |1893-4| 1.0000| 1.0028| 1.0136| 1.0630| legal ohm | 1884 | .9972| 1.0000| 1.0107| 1.0600| b. a. ohm | 1864 | .9866| .9894| 1.0000| 1.0488| siemens' ohm | | .9407| .9434| .9535| 1.0000| [a] note.--in the above table to reduce, for instance, british association ohms to international ohms, multiply by .9866, or divide by 1.0136; to reduce legal ohms to international ohms, multiply by .9972, or divide by 1.0028, etc. [illustration: fig. 547.--double contact key. it is of especial value in connection with a wheatstone bridge. when used with the latter it forms a combination battery and galvanometer key. the battery is wired to the top leaves of the key and the galvanometer to the lower leaves. hence, when operated, the battery circuit will be closed before the galvanometer circuit, as it is desirable to avoid undue disturbance of the needle.] [illustration: fig. 548.--the international ohm. it is defined as the resistance of 14.452 grammes of mercury in the form of a column of uniform cross section 106.3 centimeters in length, at a temperature of 0â° c. this is approximately equivalent to a column 106.3 cm. long, having a uniform cross section of 1 sq. mm. in the figure the international ohm is defined graphically. the resistance of the external circuit and the standard one volt cell is assumed to be zero.] [illustration: fig. 549.--leeds and northrup one ohm standard resistance (reichsanstalt form); adjusted at 20â° c. low resistance standards may be properly divided into two classes: 1. those which are designed primarily as resistance standards, and 2. those designed as current carrying standards. those of the first mentioned class are often used to measure currents up to their capacity. the above standard has both pressure and current terminals. the binding posts for the former are mounted on high posts so as to be easily accessible when the standard is immersed in oil. when used as a resistance standard of precision, it should not be subjected to a current of more than one ampere, and when used as a current carrying standard of lesser accuracy, a current of 2 or 3 amperes may be used.] practical standards of resistance.--the column of mercury as shown in fig. 548, is the recognized standard for resistance, however, in practice, it is not convenient to compare resistances with such a piece of apparatus, and therefore secondary standards are made up and standardized with a great degree of precision. these secondary standards are made of wire. the material generally used being manganin or platinoid. [illustration: fig. 550.--direct deflection method of testing resistances; a useful and simple method which may be used in numerous tests. galvanometer readings are taken through the known, and unknown resistances, and the current being proportional to the deflections, the value of the unknown resistance is easily calculated.] resistance measurement.--_resistance is that which offers opposition to the flow of electricity._ ohm's law shows that the strength of the current falls off in proportion as the resistance in the circuit increases. this gives a basis for measuring resistance. there are various methods by which an unknown resistance may be measured, as by the: 1. direct deflection method; 2. method of substitution; 3. fall of potential method; 4. differential galvanometer method; 5. drop method; 6. voltmeter method; 7. wheatstone bridge method. direct deflection method.--this method is based on the fact that the greater the current through a galvanometer the greater the deflection of the needle; it is a simple method and is capable of extended application. the apparatus required consists of battery, galvanometer, known resistance, and double contact key. the connections are made as in fig. 550. the known resistance is put in circuit with the galvanometer and after noting the deflection, the key is moved so as to cut out the known resistance and throw into circuit the unknown resistance. the deflection of the galvanometer is again noted and compared with the first deflection. [illustration: fig. 551.--charge and discharge key, adapted to condenser testing where the condenser is to be alternately charged and discharged. the insulated handle enables the key to be used without insulating the body.] [illustration: fig. 552.--pohl commutator. this is equivalent to a two pole double throw switch. the depressions in the base are filled with mercury into which the contacts dip in closing the circuit.] if the deflections be proportional to the current, the unknown resistance will be as many times the known resistance as the deflection with the known resistance is greater than the deflection with the unknown resistance. method of substitution.--this is the simplest method of measuring resistance. the resistance to be measured is inserted in series with a galvanometer and some constant source of current, and the galvanometer deflection noted. a known adjustable resistance is then substituted for the unknown and adjusted till the same deflection is again obtained. the value of the adjustable resistance thus obtained is equal to that of the resistance being tested. [illustration: fig. 553.--substitution method of testing resistances. the connections and apparatus are the same as in fig. 550, except that a resistance box is used in place of the known resistance. in making the test, first note deflection with unknown resistance in circuit, then press key so that the current will pass through the resistance box, and adjust the resistance in the box so that the deflection of the galvanometer is about the same as with the unknown. now switch from one circuit to the other, changing the resistance in the box until equal deflections are obtained. when this obtains, the resistance in the box is the same as the resistance being tested.] ques. what kind of adjustable resistance is used in making the above test? ans. a resistance box. ques. describe a resistance box. ans. it consists of a box containing numerous resistance coils with their ends connected to terminals and provided with plugs so that they may be thrown into or out of circuit at will, thus varying the resistance in the circuit. [illustration: fig. 554.--ordinary resistance box. it contains sets of standard resistances consisting of coils of insulated wire having low conductivity and small temperature coefficient. the ends of the coils are joined to the section of the bar between the plugs. the insertion of a plug cuts out a coil. in using, care should be taken to put the plugs in with a slight twist so that there shall be no resistance introduced by poor contact.] fall of potential method.--this is a very simple method of measuring resistances, and one that is convenient for practical work in electrical stations because it requires only an ammeter, voltmeter, battery and switch--apparatus to be found in every station. the connections are made as shown in fig. 555. in making the test the ammeter and voltmeter readings are taken at the same time, and the unknown resistance calculated from ohm's law. accordingly, since: (1) amperes = volts / ohms solving for the resistance, (2) ohms = volts / amperes [illustration: fig. 555.--fall of potential method of testing resistances; a convenient method for testing at stations, requiring only the usual instruments to be found at a station. the resistance of the voltmeter must be very high, otherwise the test must be made as in fig. 556.] example.--if in fig. 555 the readings show 6 volts and 2 amperes how many ohms is the resistance being tested? substituting in formula (2) ohms = 6/2 = 3 ques. can this test be made with any kind of voltmeter? ans. its resistance must be very high to avoid error. when a voltmeter having small resistance is used, it should be connected so as to measure the fall of pressure across both ammeter and unknown resistance as shown in fig. 556. [illustration: fig. 556.--fall of potential method of testing resistances; diagram showing connections for testing with low resistance voltmeter. the resistance measured with this connection will be the sum of the resistances of the coil and the ammeter. the resistance of the ammeter is usually known and can be subtracted from the sum to obtain the required resistance.] differential galvanometer method.--this is what is known as a _nil_ or zero method, that is, a method of making electrical measurements in which comparison is made between two quantities by reducing one to equality with the other, the absence of deflection from zero of the instrument scale showing that the equality has been obtained. the test is made with a differential galvanometer, and resistance box connected as in fig. 557. the current then will divide so that part of it flows through the resistance being tested and around one set of coils of the galvanometer while the other part will flow through the resistance box and the other set of coils as indicated. when the resistance box has been so adjusted that its resistance is the same as the unknown resistance the current in the two branches will be equal, and the needle of the galvanometer will show _no deflection_. [illustration: fig. 557.--differential galvanometer method of testing resistances. in making the test, the resistance box is adjusted till the galvanometer needle shows no deflection. when this condition obtains, the resistance in circuit in the resistance box is equal to the unknown resistance, hence, a reading of the box gives the value of the unknown resistance.] ques. what name is given to this method of testing? ans. it is called a _zero_ method, distinguishing it from _deflection_ methods. ques. for what kind of resistance is the method adapted? ans. since it is a nil or zero method, it is better adapted to the measurement of non-inductive than of inductive resistances. ques. what precaution should be taken with inductive resistances? ans. the current must be allowed to flow until it becomes steady to overcome the influence of self-induction. ques. what may be said with respect to the differential galvanometer method? ans. with an accurate instrument it is very reliable. [illustration: fig. 558.--drop method of testing resistances. the apparatus is connected as shown and readings taken with voltmeter across known and unknown resistance. the unknown resistance is then easily calculated.] drop method.--this is a convenient method, and one which may be used for measuring either high or low resistances with precision. it is used for many practical measurements, and requires only a voltmeter, battery, known resistance and a two way switch. the instruments are connected as in fig. 558, and in making the test, the voltmeter is switched into circuit across the known resistance and then across the unknown resistance, readings being taken in each case. the value of the unknown resistance, is then easily calculated from the following proportion: drop across known resistance known resistance -----------------------------= ----------------- drop across unknown resistance unknown resistance from which unknown known resistance ã� drop across unknown resistance resistance = -------------------------------------------------------- drop across known resistance [illustration: fig. 559.--leeds and northrup portable galvanometer (pointer type a). the sensitiveness of this instrument is such that it may be substituted in numerous cases for the non-portable reflecting type of galvanometer; as for instance, in the checking of ammeters and voltmeters to an accuracy of .2% by the potentiometer method, and on almost all wheatstone bridge measurements to commercial accuracies. a current of 2 micro-amperes will cause the pointer to move 1 mm. over the scale, that is, it has a sensibility of 500,000 ohms. the method of suspending the moving system is such as to practically eliminate initial friction which is of value in all zero deflecting methods. the suspensions and moving system are guarded by springs, which together with the solid construction of the case render the instrument capable of withstanding rough usage. overall dimensions are 5-1/4" ã� 2-5/8" ã� 3-1/2"; weight about 3 pounds.] ques. what may be substituted for the voltmeter? ans. a high resistance galvanometer, whose deflections are proportional to the current, the value of the deflections being substituted in the formula. ques. what precaution should be taken in making the test? ans. the current used should not be strong enough to appreciably heat the resistance, and if the current be not very steady, several readings should be taken of each measurement and the average values used in the formula. ques. how are the most accurate results obtained? ans. by selecting the known resistance as near as possible to the supposed value of the unknown resistance. [illustration: fig. 560.--voltmeter method of testing resistances. knowing the resistance of the voltmeter, turn switch to the left and from reading calculate resistance corresponding to one division of the scale. turn switch to right and multiply reading by resistance required for deflection of one division. this gives resistance of voltmeter and unknown resistance; subtracting from this the resistance of voltmeter gives value of the unknown resistance.] voltmeter method.--this is a direct deflection method and consists in determining first the resistance that will deflect the needle through one division of the scale on a given battery current, then with this as a basis for comparison the voltmeter is connected across the unknown resistance whose value is easily calculated from the reading. in making the test, the instruments are connected as in fig. 560. the current from battery is first passed through the galvanometer by turning switch as shown. [illustration: fig. 561.--megohm box or set of standard high resistances. the box contains five resistances of 200,000 ohms each. the six pillars are petticoat insulated, the resistances being placed between each pair of pillars. there is a double contact post on top of each pillar so that these can be connected together with copper links.] assuming that the resistance of the instrument is 8,000 ohms and that the current deflects the needle through 10 divisions of the scale, then for a deflection of one division the resistance is 8,000 ã� 10 = 80,000 ohms. accordingly, if, when the switch is moved to the right, connecting the voltmeter across the unknown resistance, the needle be moved through 6 divisions of the scale, the combined resistance of the voltmeter and unknown resistance is 80,000 ã· 6 = 13,333-1/3 ohms, and subtracting the resistance of the voltmeter, the value of the unknown resistance is 13,333-1/3 8,000 = 5,333-1/3 ohms. ques. for what kinds of test is the voltmeter method best adapted? ans. for measuring high resistances, as the insulation of wires, etc. ques. what may be said with respect to the current used? ans. its voltage should be as high as possible within the limits of the voltmeter scale. [illustration: fig. 563.--standard resistance box: 100,000 ohms, in four units of 10,000, 20,000, 30,000, and 40,000 ohms. an "infinity" plug separates each coil from the ones adjacent. segments are elevated from the hard rubber top by special washers in order to increase insulation. binding posts are so arranged as not to be in the way when plugs are used.] [illustration: fig. 562.--standard high resistance box: 100,000 ohms. it is mounted in a brass box with a hard rubber top. connections should be made to terminals marked 3 and 4. when the flexible cord is on plug 1, the box is short circuited, but when on plug 2, the resistance of 100,000 ohms is in series. the box is especially suited to rapid cable testing.] ques. in testing cable insulation what is desirable with respect to voltmeter and current? ans. a low reading voltmeter should be used in connection with a large battery. [illustration: fig. 564.--diagram showing principle of wheatstone's bridge. a, b, c, and d, are the four members which constitute the bridge. the current from the battery divides at p, part traversing dc, and part traversing ba. the galvanometer connected to m and n will indicate when the currents are equal in the two branches by giving _no_ deflection. this is then a _zero_ or _nil method_ of testing. the resistances and keys required in testing are shown in fig. 565. in the actual instrument, the members a, b, c, and d are known by the names given in the figure.] wheatstone bridge method.--for accurate measurements of resistance this method is almost universally used. the so-called "wheatstone" bridge was invented by christie, and improperly credited to wheatstone, who simply applied christie's invention to the measurement of resistances. [illustration: fig. 565.--diagram showing arms of wheatstone bridge, resistances and method of connecting galvanometer, battery and unknown resistance.] the bridge consists of a system of conductors as shown in fig. 564. the circuit of a constant battery is made to branch at p into two parts, which re-unite at q, so that part of the current flows through the point m, the other part through the point n. the four conductors a, b, c, d, are spoken of as the _arms_ of the balance or bridge. it is by the proportion existing between the resistances of these arms that the resistance of one of them can be calculated when the resistances of the other three are known. when the current which starts from the battery arrives at p, the pressure will have fallen to a certain value. the pressure in the upper branch falls again to m, and continues to fall to q. the pressure of the lower branch falls to n, and again falls till it reaches the value at q. now if n be the same proportionate distance along the resistances between p and q, as m is along the resistances of the upper line between p and q, the pressure will have fallen at n to the same value as it has fallen to at m; or, in other words, if the ratio of the resistance c to the resistance d be equal to the ratio between the resistance a and the resistance b, then m and n will be at equal pressures. to find out if this condition obtain, a sensitive galvanometer is placed in a branch wire between m and n which will show _no_ deflection when m and n are at equal pressure or when the four resistances of the arms "balance" one another by being in proportion, thus: (1) a:c = b:d if, then, the value of a, b, and c be known, d can be calculated. the proportion (1) is reduced to the following equation before substituting. d = bc/a for instance, if a and c be, as in fig. 565, 10 ohms and 100 ohms respectively, and b be 15 ohms, d will be (15 ã� 100) ã· 10 = 150 ohms. [illustration: fig. 566.--diagram showing usual arrangement of resistances in arms of wheatstone's bridge. in practice the bridge is seldom or never made in the lozenge shape of the diagrams, figs. 564 and 565, these being given merely for clearness. the resistance box of fig. 554 is, in itself, a complete "bridge," the appropriate connections being made by screws at various points. the letters in the above diagram correspond with those in figs. 564 and 565, and the three figures should be carefully compared.] as constructed, wheatstone bridges are provided with some resistance coils in the arms a and c, as well as with a complete set in the arm b. the advantage of this arrangement is that by adjusting a and c, the proportionality between b and d can be determined, and can, in certain cases, be measured to fractions of an ohm. in fig. 565 resistances of 10, 100, and 1,000 ohms are included in the arms a and c. [illustration: fig. 567.--standard resistance box and wheatstone bridge. this pattern is a modification of the anthony form of bridge. all the resistances are wound upon metal spools. the bridge ratio coils are 1, 10, 100, 1,000, 10,000. the rheostat coils are arranged in five rows, of ten coils each. the ordinary decade plan (explained in fig. 570) is followed. the coils may be joined in series in multiple, or in any combination of series and multiple. the coils may thus be checked against each other in many combinations. for instance, all the ten ohm coils taken in parallel may be compared with any one ohm coil. the precision of adjustment is said to be 1/20th of 1% for the coils of the tenth ohm series, and 1/50th of 1% for the coils of the rheostat. the ratio coils are certified to be like each other to within 1/100th of 1%. the box is supplied with battery and galvanometer keys of substantial construction.] ques. describe the method of testing with the bridge. ans. fig. 567 illustrates the general arrangement of resistances to be found in an ordinary bridge. the connections are made as shown. in testing, first _depress_ the battery key, then _tap_ the galvanometer key. this should be repeated adjusting the resistances till no deflection is obtained. the resistance then in the arm b ã� (c ã· a) will give the value of the unknown resistance. [illustration: fig. 568.--ratio coils of wheatstone bridge. almost every box intended to serve as a wheatstone bridge is furnished with a set of coils which forms the arms of proportion or ratio arms of the bridge. there is a choice of several different ways of arranging these coils. the figure shows the simplest arrangement, which is employed in boxes not intended for the highest accuracy. the required ratio, as for instance 1:100, is obtained by withdrawing a plug from each arm a and b. ratios 1/1, 1/10, 1/100, 10/100, etc., or 1,000/1, 1,000/10, 1,000/100, 100/100, etc., are obtainable in this manner. this simple arrangement is open not only to the objection that the contact resistance of the plugs which remain in is always included with the resistance unplugged, but also to all other objections to be urged against the use of many plugs where a few will do. the method has the limitation that it is not possible to reverse the arms of the bridge, that is, to transpose the arms a and b.] ques. why should the battery key be depressed before the galvanometer key? ans. to avoid the sudden swing of the galvanometer needle, which occurs on closing circuit in consequence of self-induction. ques. how is it known whether too much or too little resistance be unplugged? ans. the galvanometer needle will be deflected to one side for too much resistance, and to the opposite side for too little resistance. [illustration: fig. 569.--method of reversing arms of wheatstone bridge with reversing blocks. the arrangement shown in the figure is classical, being that used in the english post office type of wheatstone bridge. it is open to the objections which apply to the use of several plugs, one of which is withdrawn to obtain the desired resistance.] ques. what is the meaning of "inf.," marked on the bridge? ans. it stands for "infinity," because the resistance coil at the point marked infinity is omitted so that adjacent sections of the arm are disconnected when the plug is taken out. in fact, the air gap interposed by the removal of the plug by no means provides an infinitely great resistance, but is usually called such because it is vastly greater than any of the other resistances of the bridge. [illustration: fig. 570.] [illustration: figs. 570 and 571.--diagrams illustrating the decade plan of combining resistance coils. in this method the coils are connected in series and the arrangement avoids the disadvantage of the ordinary wheatstone bridge in that the latter requires a large number of plugs to short circuit the resistances not in use, which introduces an element of uncertainty as to resistance of the plug contacts and the necessity of adding up the values of all the unplugged resistances in order to determine the total resistance in circuit. the necessary regular succession of values in a rheostat built on the decade plan can be obtained with either nine or ten coils per decade. the chief reason for using the latter number is found in the facility with which all the coils of one decade can be compared with one coil of the next higher decade, thus permitting the coils of a rheostat to be checked among themselves. thus, the ten 1 ohm coils can be checked with a 10 ohm, the ten 10's with a 100, etc. in some sets the ten coils of a decade can be connected in series or in parallel, and it then becomes an advantage to have ten coils to a decade, since the coils in one decade in parallel equal one of the coils of the next lower decade. when these latter advantages are not required, and especially when dials or sliding switches are used, there is little or no advantage in using more than nine coils per decade, as shown in fig. 570. here all the coils of the set are connected in series so that the circuit is never open. thus it is a slight advantage to have permanent connections _a_, _b_, and _c_, because all the coils of a decade can be thrown in circuit by simply pulling out a plug, it not being necessary to insert it again, as would be the case if the _a_, _b_, and _c_ connections were not used. moreover, if any plug make bad contact, its effect is somewhat lessened by having this bad contact shunted by the remaining coils of the decade. again, there are occasions where violent deflections of a galvanometer are prevented by not having the circuit entirely open when a plug is taken out.] the decade plan.--in this method of combining resistance coils, there are 9 or 10 one ohm coils for the units place, 9 or 10 ten ohm coils for the tens place, 9 or 10 one hundred ohm coils for the hundreds place and so on. each series of coils of the same value is designated a _decade_. the connections are usually made as shown in figs. 570 and 571. it is apparent from the figure that any value in any one decade can be obtained by inserting between a bar and a block, only one plug; moreover if several decades be in series, any value up to the limit of the set can be read off directly from the position of the plugs without having to add up the unplugged resistance as in the ordinary arrangement. [illustration: fig. 572.--two plug arrangement of ratio coils. each of the ratio coils has one of its terminals connected to a common center which corresponds to the block marked c in the figure. the other terminal of each coil is connected to an individual block, there being one block for each coil. the bar b on one side of these blocks is joined to the rheostat and the bar a on the other side to an x post. in the ordinary use of this set of ratio coils two plugs only are used. one plug is inserted between the bar a and one of the blocks, 1, 1', 10, 10', etc., of the central row of blocks. the other plug is inserted between the bar b and any one of the other blocks of the central row. there are two ratio coils of each value. to obtain an even ratio as 1,000 to 1,000', one plug is inserted between the block 1,000 and the bar a, and the other plug between the 1,000 block and bar b, the ratio arms are reversed; that is, the 1,000 ohm coil is connected to the x post and the 1,000 ohm coil to the end of the rheostat. when uneven ratios are used, the same ratio can be obtained by four different combinations. to obtain the ratio one to ten, insert a plug between a and 1, and another between b and 10, or between a and 1', and b and 10, and get 1:10, or between a and 1, and b and 10', and get 1:10', or again, between a and 1' and b and 10', and get 1' to 10'. other ratios are obtained in a similar manner. by using more than two plugs and connecting certain of the coils in parallel combinations, a large number of other ratios may be obtained. this arrangement offers a convenient method of measuring the sensibility of a bridge and galvanometer combination that is frequently applicable. if for instance the one ohm coil is used on either side after a balance has been obtained the one ohm may be shunted with the 1,000 ohm on the same side. this will make a variation of 1/10 of 1% and the galvanometer deflection may be noted for this variation. similarly, the 1 ohm may be shunted with the 100 for a variation of 1%, or with the 10,000 for a variation of 1/100 of 1%. the ten ohm coil may be shunted with the 1,000 for a variation of 1% and with the 10,000 for a variation of 1/10 of 1%. in the arrangement of ratio coils, errors due to plug contacts are negligible because only two plug contacts enter the circuit, and with an even ratio, it is only the difference in the resistances of the two plug contacts that can affect the result. in measuring any of the ratio coils while in the box it is only necessary to connect to the bar c and to either the bar a or b and plug in the coils to be measured.] [illustration: fig. 573.] [illustration: figs. 573 and 574.--the leeds and northrup decade. the object of this arrangement is to reduce the number of coils required. in fig. 573, the 1, 3', 3 and 2 are connected in series. let the terminals of the 1 ohm and 2 ohm coils be numbered (1), (2), (3), (4) and (5) (fig. 573). the current enters at point (1) and leaves the coils at the point (5), traversing 1, 3', 3, 2 = 9 ohms in all. if this series be multiplied by any factor _n_, then _n_ (1 + 3' + 3 + 2) = _n_ 9 ohms. it will be seen that if the points (1) and (5) be connected, all the coils are short circuited, and the current will traverse zero resistance. if the points (2) and (5) be connected, the 3', 3 and 2 ohm coils will be short circuited and the current will traverse 1 ohm. by extending the process so as to connect two and only two points at a time it is possible to obtain the regular succession of values n (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the last being obtained when no points are connected. fig. 574 shows leeds and northrup's method of connecting these points two at a time with the use of a single plug. the circles in the diagram represent two rows of ten brass blocks each. to the first two blocks at the top of the rows, the points (5) and (1), fig. 573, are connected; to the second two, the points (2) and (5) are connected, etc., no points being connected to the last pair of blocks. hence, if a plug be inserted between blocks 1 and 5, fig. 575, the points (1) and (5) of diagram fig. 573 are connected, giving the value of 0, if between the blocks 2 and 5 the points (2) and (5) are connected, giving the value 1, and so on. the value 9 is obtainable when the plug is in the last pair of blocks, which have no connections. fig. 572 shows a top view of the blocks of a simple decade constructed upon this plan.] ques. what other advantages are gained with the decade arrangement? ans. the single plug used with each decade is never out of use, being either in the zero position or set on some value, and is therefore not easily lost by being laid aside. the use of only one plug in a decade makes it easy to ascertain that the plug is making good contact as only one block in a row is plugged at a time, the other blocks are not kept under a strain by having plugs forced tightly between them. this strain on the blocks, which always exists in those sets in which a resistance is thrown in by removing a plug, tends to separate or loosen them and often to warp the hard rubber upon which they are mounted. another advantage of the decade plan is that it permits obtaining a succession of values by means of sliding contacts or dial switches, a method which is becoming deservedly more appreciated. [illustration: fig. 575.--leeds and northrup dial wheatstone bridge. rotating switches are used instead of plugs, which permits quicker adjustment of the resistances, adapting it to rapid working. the ratio coils are arranged as in fig. 568. there are four dials which form the rheostat. the units dial contains 9 one ohm coils; the tens dial, 9 ten ohm coils; the hundreds dial, 9 one hundred ohm coils, and the thousands dial 9 one thousand ohm coils. the values of the ratio coils are 1, 1, 10, 10, 100, 100, 1,000, 1,000, 10,000, 10,000.] ques. what is the difference between "plug out" and "plug in" types of resistance box? ans. in the plug out type, resistance is put in the circuit by removing plugs, as in fig. 565; in the plug in type, resistance is put in the circuit by inserting plugs as in figs. 570 and 571. [illustration: fig. 576.--queen acme portable testing set. it consists of a wheatstone bridge, with reversible arms, battery of four dry cells, d'arsonval galvanometer, battery and galvanometer keys. there are sixteen resistance coils, having a combined resistance of 11,110 ohms. each bridge arm is provided with three coils of 1, 10, 100 ohms, and 10, 100, 1,000 ohms respectively. the commutator admits of a ratio of 1 to 1,000 on either bridge arm, giving the set a theoretical range from .001 of an ohm to 11,110,000 ohms. for resistances above 1,000,000 ohms, the normal battery power must be increased. the contact keys are located as shown. the battery key has single contact, but the galvanometer key has double contact; depressing it closes the galvanometer circuit, and releasing it short circuits the galvanometer, bringing the latter quickly to rest.] testing sets.--for convenience in testing, a combination of the instruments used is put up in a neat and substantial case, and known as a testing set. there are innumerable forms of testing set, a few of which are shown in the accompanying illustrations. the usual combination is a wheatstone bridge, galvanometer, battery and necessary keys and connections. [illustration: fig. 577.--connections and circuits of queen acme portable testing set. there are three rows of blocks, ll', mm', nn'. ll is connected to nn' by means of a heavy copper bar, joining l' and n'. ll' and nn' constitute the rheostat, from which any resistance from 1 ohm to 11,110 ohms may be obtained by removing the proper plugs. the block n of the rheostat is connected to the lower line post d. the upper line post c is connected to the block x of the commutator. the block c has no other permanent connection, except key g. the block r of the commutator is connected to the block l of the rheostat, and has no other connection, excepting by plugs. each half of mm' constitutes a bridge arm, designated a and b respectively. beginning at the lower line post d, the connections form a continuous circuit through the rheostat, thence through the bridge arm b, thence through the bridge arm a, thence to the upper line post c. the commutator serves merely to reverse the bridge arms a and b. the battery terminals are connected as shown: the positive terminal directly to the common junction of the two bridge arms, and the negative terminal through the battery key to the rheostat. the positive terminal of the galvanometer is connected through the galvanometer key with the block x, and the negative terminal with the block r of the commutator or what is equivalent, with the block l of the rheostat. the commutator blocks a, b, r and x, are connected by plugs as shown. when the commutator plugs are in the position pq, the bridge arm b is connected to the rheostat and the bridge arm a is connected to the line, the ratio between the bridge arms ratio being a ã· b = x ã· r but when the plugs are in the position st, the bridge arms are reversed in position a, being connected with the rheostat and b, with the line, and the bridge arm ratio becomes a ã· b = r ã· x. the connections of the testing set may be more readily understood from the simplified diagram fig. 578.] [illustration: fig. 578.--simplified diagram showing connections of queen acme portable testing set.] ques. describe the operation of the queen acme testing set figs. 576 and 577, in measuring resistance. ans. connect the terminals of the resistance to be measured to the line posts c and d. place the battery connections on the two upper tips 0 and 1, thus throwing one end of the battery into circuit, which is sufficient until an approximate balance is obtained. employ the 100 ohm coil in each bridge arm, and place the commutator plugs in the position pq, or in the position st. then remove plugs from the rheostat until the value of total resistance employed, or nearly as may be guessed is equal to that of the unknown resistance. now press the battery key ba, and holding it down momentarily, press the galvanometer key ga. if the galvanometer needle swing to the right toward the symbol + the resistance employed in the rheostat is too high and must be reduced. if the needle swing to the left toward -, the resistance employed is too low and must be increased. by altering the resistance of the rheostat accordingly, a value will soon be found, which when varied slightly either way, will reverse the deflection of the galvanometer needle. now remove the battery connection from tip 1, and place it on the tip 4, thus throwing the whole battery into circuit. then press the keys again as before, first the battery key, then the galvanometer key. this will increase the deflection of the galvanometer needle for the same variation in the rheostat, thus enabling the making of a more accurate adjustment. the measurement thus made will be the best result that can be obtained with bridge arms of equal value, but by selecting more suitable values of the two arms from the following table of bridge ratios a much higher degree of accuracy may be obtained. [illustration: fig. 579.--diagram of the queen dial decade portable testing set. its dimensions are 9-1/2" long, 7" wide and 7" deep, and weighs 11-1/2 pounds. the resistances are arranged upon the dial decade plan, being placed in circuit by means of a rotating switch contact. the switches are so constructed that they may be turned in either direction, thereby permitting them to be turned quickly from the highest resistance in any dial to the lowest resistance in the same dial. this arrangement avoids the necessity of turning back through all the remaining resistances in any particular group of coils and is of value in locating swinging crosses or conditions of momentary balances. the connections for the various tests are made by the manipulation of one small knife switch (w.b.--m.l.) and the switch ba.; these are plainly lettered, thus avoiding the necessity of referring to a diagram of connections. in construction, the dial switches are made up of eight laminations of no. 28 b. & s. phosphor bronze, and the form is such as to prevent wearing grooves on the top of the contact studs. in this instrument the electrical circuits are soldered throughout excepting the switch contact whose resistance is negligible. the resistances are wound with manganin. the battery comprises six cells sub-divided which are easily replaceable. the galvanometer is the same as in the queen acme set, but has the addition of an ayrton shunt, which is useful in making insulation measurements. the necessary keys, binding posts, and switches are provided so as to facilitate the use of the instrument for the various measurements that can be made with it.] table showing the best values of bridge arms for measuring any desired resistance | | position of| | best values of| commutator| | | plugs as| value of resistance being measured | | | shown in| | a = | b = | fig. 582| --------------------------------------+-------+-------+------------| below 1.5 ohms | 1| 1,000| pq| between 1.5 and 11 ohms | 1| 100| pq| " 11 and 78 ohms | 10| 100| pq| " 78 and 1,100 ohms | 100| 1,000| pq| " 1,100 and 6,100 ohms | 100| 100| pq or st| " 6,100 and 110,000 ohms | 1,000| 100| st| " 110,000 and 1,110,000 ohms | 1,000| 10| st| " 1,110,000 and 11,110,000 ohms | 1,000| 1| st| ques. in testing with the queen acme set how should the plugs be placed in the commutator? ans. always make the arm a the smaller except when the two arms are of equal value. ques. if the resistance being measured is higher than 6,100 ohms, or lower than 1,100 ohms, how should the commutator plugs be placed? ans. if higher than 6,100 ohms, they should be placed in the position st; if lower than 1,100 ohms, in position pq. when the plugs are placed in the st position, the unknown resistance is found by dividing the value of the larger bridge arm by that of the smaller, and multiplying the total employed resistance in the rheostat by the quotient. when the plugs are placed in the pq position, the employed resistance in the rheostat is divided by the quotient. [illustration: fig. 580.--queen portable silver chloride testing battery. the silver chloride cell has the advantage of long life, light weight, and compactness. the pressure of each cell when new is .8 volt.] direct deflection method with queen acme set.--to measure for instance, insulation resistance by direct deflection connect a known high resistance, say 100,000 ohms between the line post c (fig. 577), and the positive battery post. remove all plugs from the commutator, and place all plugs in the rheostat, as any employed resistance in the rheostat will be in circuit with the galvanometer and the battery. place the battery connection so as to throw only one cell into circuit. now press the keys and obtain a deflection of the galvanometer needle. for example: assume that the needle to be deflected about 8 divisions of the scale. since this deflection is due to the current from one cell passing through a resistance of 100,000 ohms, then 100,000 ã� 8 = .8 megohms represents the resistance through which one cell will produce a deflection of one division on the scale. hence, .8 megohms is the constant of the galvanometer. [illustration: fig. 581.--ohmmeter. it consists essentially of a slide wire wheatstone bridge, with the scale divided to read either directly in ohms, or in per cent. of a fixed resistance value. a galvanometer is mounted on the containing case of each, and battery and galvanometer keys are provided. in the direct reading type, the scale is so cut that when the galvanometer is balanced, the pointer of the instrument indicates the value of the resistance between the x posts. the scale is calibrated for any desired range. these ohmmeters being slide wire bridges, the greatest accuracy is at the center of the scale, hence one should be selected that will bring the part of the scale likely to be the most used at or near the center. a convenient type is that in which the scale is cut in per cent., 100 per cent. being at the center of the scale. fixed coils of 1, 10, 100, 1,000 and 10,000 ohms are contained in the instrument with a plugging arrangement allowing any one to be used. when a balance is obtained, the actual resistance is determined by multiplying the dial reading by the value of the fixed coil in use. this amounts simply to shifting the decimal point. for instance, if the 100 ohm coil were being used, and the pointer were at .875, the resistance would be 87.5 ohms.] now, replace the known high resistance (100,000 ohms) by the unknown resistance (for instance such as a cable) the value of which is to be determined. add enough cells to produce as large a deflection of the needle as possible. assume that 75 cells give a deflection of 1.5 scale division. then, the galvanometer constant multiplied by the number of cells and the product divided by the deflection will give the insulation resistance of the cable; or 0.8 megohm ã� 75 cells = 60.0; and 60.0 ã· 1.5 = 40 megohms as the resistance of the cable. [illustration: fig. 582.--commutator plug setting for comparing electromotive forces by the fall of potential method with queen acme set.] fall of potential method with queen acme set.--to compare electromotive forces by this method, place the battery connection (fig. 577), so as to throw into circuit all the cells, taking care not to reverse them by crossing the battery cords. plug the commutator as shown in fig. 582, and remove 1,000 ohms from bridge arm b. place all plugs in arm a. from the rheostat unplug 5,000 ohms. then connect one of the cells being tested, with its positive terminal to the + battery post and its negative terminal to the line post c. when the keys are pressed, the galvanometer needle will swing either to the right or to the left. if it swing toward +, reduce the resistance in the rheostat; if it swing toward -, add resistance to the rheostat. when a value is found wherein a variation of an ohm either way reverses the deflection, add to this value the resistance unplugged in arm b, and divide the sum by the resistance in arm b. the result gives the ratio between the voltages of the testing set battery and cell being tested respectively. the division is decimal and may be readily accomplished by merely pointing off as many places as there are ciphers in the resistance employed from arm b. this operation repeated with any number of different cells, will give their voltages in terms of the voltage of the testing set battery, and from these ratios their relative values may be readily obtained. [illustration: fig. 583.--diagram of apparatus for measuring low resistances based on the principle of the kelvin double bridge. in the diagram ab represents a heavy piece of resistance metal of uniform cross section and uniform resistance per unit of length; cd is another piece of resistance metal of smaller cross section, and the two are joined together by a heavy copper bar, ac, into which both are silver soldered; ll are the current terminals and pp are the pressure terminals. the resistance of ab between the marks 0 and 100 on the scale s is .001 ohm. from the point 1 on the resistance cd to 0 on ab is also .001 ohm, from 2 to 0 is .002 and so on, and from 9 to 100 is .01 ohm. the slider m moves along the resistance ab and its position is read on the scale s which is divided into 100 equal parts and can be read by a vernier to thousandths. subdivided in this way the resistance between the tap off points pp may have any value from .001 to .01 ohms by steps of .000001 ohm.] if the testing set battery be replaced by a standard cell, the first measurement gives at once the voltage of the cell tested. if the voltage of the cell or battery being tested exceed that of the testing set battery, reverse the position of the two batteries, and the subsequent operations, as outlined above, will give the desired results. how to check a voltmeter with the queen acme set.--in using a set as in fig. 576, first remove about 10,000 ohms from the rheostat, plug the commutator as shown in fig. 582, remove 100 ohms from the arm b, of the bridge, and connect a standard cell with the positive terminal to the + battery post and the negative terminal to the line post c. then, connect the circuit to the battery posts of the testing set the positive lead to the + post and the negative lead to the post. now, press both keys and note the direction of the deflection of the galvanometer needle. if it move toward +, the rheostat resistance is too high; if toward -, too low. [illustration: fig. 584.--kelvin bridge. this includes a low resistance standard of .1 ohm variable by steps of .00001 ohm, a set of ratio coils, and a holder for rods or wires to be measured, with a scale to measure their length. it is also provided with heavy flexibles to be used in measuring the resistances of irregularly shaped pieces. the connections are clearly shown in the diagram. the range of measurements of this bridge is: 1 ohm to .1 ohm by steps of .001 ohm readily estimated to .0001; .1 to .01 ohm by steps of .0001 ohm readily estimated to .00001; .01 ohm to .001 ohm by steps of .00001 ohm, readily estimated to .000001; .001 ohm down by steps of .00001 ohm, readily estimated to .000001 ohm.] change the rheostat resistance accordingly until the balance attained is such that a very slight variation of the rheostat resistance one way or the other will reverse the galvanometer deflection. to find the pressure on the circuit, add 100 to rheostat resistance and point off two places. multiply this value by the voltage and the product will be the desired voltage. if the voltage of the standard cell be exactly one volt, the total employed resistance represents the voltage on the circuit. [illustration: fig. 585.--queen slide wire bridge. it consists of a portable slide wire, wheatstone bridge arranged to read directly in ohms in addition to its use for locating crosses and grounds. it is complete with battery, galvanometer and telephone receiver. the bridge is balanced by moving the hand stylus until the galvanometer shows no deflection or until there is no sound in the telephone receiver. in order to provide a wide range of measurement and maximum accuracy, ratio coils or multipliers having values of 1, 10, 100, 1,000 and 10,000 are provided. the scale of the instrument is arranged in two parts, one of which indicates ohms and the other is divided into uniform divisions for use when locating crosses and grounds by the murray and varley loop methods. a small induction coil is included so as to furnish an alternating current when using the telephone receiver.] for instance, in making a measurement on a 110 volt circuit, assume that the employing of 7,840 ohms rheostat resistance produces balance, and that increasing or decreasing this resistance by two ohms, reverses the galvanometer deflection. this indication that the setting 7,840 is uncertain, about 1/40 of 1 per cent. since the rheostat coils are adjusted to an accuracy of only 1/5 of 1 per cent., that will be about the accuracy of the measurement. if the pressure of the standard cell be 1.018 volts, then 7,840 + 100 = 7,940. pointing off two places, gives 79.40, which multiplied by 1.018 gives 80.82 for the voltage on the circuit. to measure internal resistance of cell with queen acme set.--first compare its voltage on open circuit with the pressure of the testing set battery. then, shunt the cell with a known resistance, about 100 ohms, and again measure its terminal voltage. the difference between the two values thus obtained, divided by the value of the shunt resistance, will give the value of the current. to find the internal resistance, multiply the value of the shunt resistance by the ratio between the first and second measured values. [illustration: fig. 586.--evershed portable ohmmeter set. this testing set consists of a direct reading ohmmeter which indicates by direct reading the value of the resistance being tested, also a portable hand dynamo which provides at any required pressure the current necessary to make the test. it is adapted to the needs of supply stations, wiring contractors and dynamo builders. it is also useful in testing the insulation of underground and aerial cables, and is designed so that it can be used by ordinary workmen who are not experienced in handling delicate instruments and who, by its use, are able to obtain accurate results. the dynamo is wound for 100, 200, 500, or 1,000 volts, and is fitted with spring drum inside the case on which is coiled a twin flexible cord provided with a connector adapted for clamping under the ohmmeter terminals.] for instance, assume that the open circuit voltage of the cell being tested as compared with the voltage of the testing set battery is .212 of the latter, and that when it is shunted with a resistance of 1,000 ohms, its terminal voltage is .179. then, the total resistance is to the 1,000 ohms shunt resistance as .212 is to .179 or (.212/.179) ã� 1,000 = 1,184, from which deducting the 1,000 ohms shunt resistance, gives 184 ohms as the internal resistance of the cell. [illustration: fig. 587.--leeds and northrup fault finder. a lineman's instrument for the location of faults, crosses, grounds, and opens in telephone and telegraph circuits, and for the measurement of conductor and insulation resistance.] [illustration: fig. 588.--diagram showing arrangement and connections of leeds and northrup fault finder. it is used to measure conductor resistance, fault resistance, to locate faults by four different tests, and when used with a buzzer and telephone, to locate opens. the essential feature of the instrument is the uniform resistance ab, which lies in a circle and which has a value of about 100 ohms. by a special construction, it is so arranged that the contact can be made at any point along it, and it is therefore equivalent to a very high resistance slide wire. it has a moving contact c and a uniform scale of 1,000 divisions. in series with this, there are the two resistances e and r which may be short circuited by the switches u and v. e has exactly the same resistance as the wire ab. r has a resistance of 100 ohms, and is the fixed resistance of the bridge arrangement for resistance measurements. the resistances of 1,000 ohms and 9,000 ohms connected to the battery post are to protect the battery and the apparatus from excessive current. the 9,000 ohms may be short circuited by the switch w.] ammeter test with queen acme set.--connect a low resistance in series with the ammeter and run leads from it to the testing set, the positive lead to the + battery post and the negative lead to the line post c (fig. 577). insert a standard cell between the battery posts, with positive terminal to + battery post, and negative terminal to battery post. plug commutator as shown in fig. 582. remove 10,000 ohms from rheostat, and 100 ohms from bridge arm b. determine a balance in the usual way by changing the value of the resistance in the rheostat. this operation will balance the difference of pressure at the terminals of the shunt resistance against the standard cell, and its value is equal to (1.40 ã� 100) / (r + 100) = 140 / (r + 100) to determine the current flowing, divide the value of the difference of pressure thus obtained by the value of the shunt resistance. [illustration: fig. 589.--resistance measurement with leeds and northrup fault finder. the diagram shows the proper connections and switch settings for measuring conductor resistance. as in the ordinary slide wire bridge, the resistance x between the two posts 1 and 2 is obtained from the formula x = a ã· (1,000 a) ã� r. to avoid the necessity of solving in each case the fraction a ã· (1,000 a), a table is furnished with the instrument, giving the value of this fraction for each value of a. _the resistance is accordingly determined in each case by simply setting the contact c for a balance and reading from the table the resistance opposite the number corresponding to the scale reading and multiplying by 100, the value of r._ to use an outside battery, remove the inside battery and connect the outside battery between the posts gr and ba. the pressure of this battery should not exceed 110 volts. _if it exceed 25 volts, open switch w._ example--with an unknown resistance connected between the posts 1 and 2, the galvanometer showed a balance for a dial reading of 387. the number opposite 387 in the table is .6313; hence, x = .6313 ã� 100 = 63.13 ohms.] [illustration: fig. 590.--diagram of the queen standard potentiometer. the circuit arrangement is a method of sub-dividing the main potentiometer wire, mnopq, so as to provide for very accurate reading. the secondary voltage, or that used to supply current to the main potentiometer circuit, is adjusted by regulating rheostats, "fast," "medium," and "slow" so that the current flow is exactly .0001 ampere. it is noted that this instrument requires a very small current for its operation. the instrument is direct reading for voltage measurements, not exceeding 1.4+. in order to determine if the current flow through the potentiometer be exactly .0001 ampere, the terminals of the standard cell binding posts are connected in circuit so that the drop over points between which they are connected are exactly equal to the voltage of the standard cell used. binding posts are provided for connection with various standard cells. the unknown voltage to be measured is placed in opposition to the current flow in the potentiometer circuit by connecting to the binding post "xemf." observe that polarity is connected as required. the galvanometer with its shunt is placed in the standard cell circuit, or x circuit, by means of a double pole, double throw switch. the switch at t provides for standard cells of different values and the setting at u allows for temperature correction. the range of the instrument in volts can be increased by means of multipliers or volt boxes.] [illustration: fig. 591.] [illustration: fig. 592.] [illustration: fig. 593.] [illustration: figs. 591 to 594.--diagrams illustrating loop testing. to properly understand the murray or varley loop tests, consider a wheatstone bridge (fig. 591) the arms of which are equal. in loop testing, the rheostat is replaced by a length of cable and the unknown resistance also by a length of cable, as in fig. 592, both being similar in resistance per foot. if both lengths be the same, their resistances are the same and the bridge balances. now shorten one cable and add resistance in series with it until the bridge again balances as in fig. 593. the added resistance equals that of the piece cut off. hence, if the resistance per foot be known, the length of the shorter piece can be easily calculated. in the murray and varley tests, the battery circuit is by ground connections instead of by wire. in the murray loop the arrangement is similar to fig. 592, the battery circuit being completed by ground connection through fault in defective cable. fig. 594 shows the general arrangement of the varley loop.] loop test.--this is a method of locating a fault in a telegraph or telephone circuit when there is a good wire running parallel with the defective one. in the process, the good and bad wires are joined at their distant ends and one terminal of the battery is connected to a wheatstone bridge, while the other terminal is grounded. there are different ways of making loop tests as by: 1. the murray loop; 2. the varley loop; 3. special loop. [illustration: fig. 595.--the murray loop test. the apparatus is connected as in the figure. the rheostat of the bridge is used in place of the second arm to permit large adjustment. x and y are the resistances of the cable between the fault and the points 1 and 2 respectively.] the murray loop.--in this test only one of the two regular bridge arms is used, the other being replaced by the rheostat giving an arm of large adjustment. the connections are shown in fig. 595. in making the test, close key and note the deflection of the needle due to pressure of chemical action at fault, if any. this is called the _false zero_. now apply the positive or negative pole of the battery by depressing the battery key, and balance to the false zero previously obtained by varying the resistance in arms a or b. then by wheatstone bridge formula: rx=ay, and l=x+y; y=l-x, whence x = a/(r+a) y = l(r/(b+a)) [illustration: fig. 596.--murray loop method of fault location with leeds and northrup fault finder. case i where there are two wires having equal resistance, in one of which there is a fault. connect and set switches as shown; join the good wire to post 1 and the faulty wire to post 2. the resistance of e is equal to that of ab. from the symmetry of the arrangement, it is evident that, if the fault were exactly at the junction between the good and bad wires, the contact point c would rest for a balance at 1,000 on the scale, or at 500 if the fault were half-way along the bad wire; hence, at whatever point it comes to rest, the reading divided by 1,000 and multiplied by the length of the bad wire is the distance from the instrument to the fault. example--in a pair of equal wires, 5.8 miles long, one is grounded. with the connections made as above, and the galvanometer balanced for the dial reading 124, the distance to the fault is (124 ã� 58) ã· 1,000 = .7192 miles.] [illustration: fig. 597.--murray loop method of fault location with leeds and northrup fault finder: case ii, where the good and bad wires are _unequal_. the figure shows the connections. it is the ordinary murray loop and it is evident that the resistance _a_, to the fault will be obtained from the formula _a_ = (a ã· 1,000) ã� r, where r is the resistance of the loop, and a is the reading of the contact c on its scale. the distance d, to the fault is obtained from the formula d = ar ã· (1,000 ã� m), where m is the resistance per mile of the faulty wire. example--a wire having a resistance m of 16.46 ohms per mile is grounded. this wire was looped with a wire of unknown resistance and the total resistance of the loop r was measured and found to be 54.07 ohms. connections were made as in the figure, and the reading a was found to be 332. substituting these values in the above formula: d = (332 ã� 54.07) ã· (1,000 ã� 16.46) = 1.09 miles.] ques. how may the distance from 2 to the fault be determined in knots or miles. ans. divide y by resistance per knot or mile. the varley loop.--this is a method of locating a cross or ground in a telephone or telegraph line or other cable by using a wheatstone bridge in a loop formed of a good wire and the faulty wire joined at their distance ends. one terminal of the battery is grounded and the other connected to a point on the bridge at the junction of the ratio arms. the rheostat arm then includes the resistance of the rheostat plus the resistance of the fault, while the unknown arm includes the resistance of the good wire plus the resistance of the bad wire beyond the fault. when the bridge is balanced, the unknown resistances may be readily determined by a simple equation. [illustration: fig. 598.--the varley loop test. the diagram shows the various connections. x and y are the resistances of the cable between the fault and the points 1 and 2 respectively. l is the resistance of the good and bad cable or x + y.] in making the varley loop test, the resistance of looped cable or conductors is measured, and then connected as in fig. 598. close the battery key and adjust r for balance. when earth current is present, the best results are obtained when the fault is cleared by the negative pole, and just before it begins to polarize. if x be the resistance from 2 to the fault, then x = (l r) / 2 also, x divided by the resistance of the cable or conductor per knot or mile gives the distance of fault in knot or miles. when the resistance of the good wire used to form a loop with the defective wire, together with that portion of the defective wire from the joint to the fault is less than the resistance of the defective wire from the testing station to the fault, the resistance r must be inserted between point 1 and the good conductor, the defective wire being connected directly to point. the formula in this case is x = (l + r) / 2 [illustration: figs. 599 and 600.--varley loop method of fault location with leeds and northrup fault finder. this method may be used as a check on the murray methods. connect the faulty wire to 1, and measure the resistance of the loop. then throw switches as shown in the fig. 600. let: a = resistance to fault, d = distance to the fault in miles, m = resistance of the faulty wire per mile, r = resistance of the loop, r = resistance of the coil r, or 100 ohms, t = a ã· (1,000 a) to be read from the table. from the wheatstone bridge relation: a = (r 100t) ã· (t + 1), and d = (r 100t) ã· (t + 1)m. example--a wire having a resistance of 16.46 ohms per mile is grounded. this was looped with a wire of unknown resistance and the resistance of the loop was found to be 54.07 ohms. connections were made as in the figure, and the reading a was found to be 234. from the table t = .3055, and substituting: d = (54.07 30.55) ã· (1.3055 ã� 16.46) = 1.094 + miles.] special loop.--this method may be used to advantage where the length of the cable or faulty wire only is known and where there are two other wires which may be used to complete the loop. it is not necessary that the resistance of the faulty wire and the length and resistance of the other wires be known. figs. 601 to 604 show the connections and method of testing. example.--all the wires in a cable 10,852 ft. long were found to be grounded so that none of them could be used as good wires. two wires were selected out of another cable going to the same place by a different route and securely joined to one of the grounded wires at the distant end. this grounded wire and one of the good ones were connected as shown in figs. 601 and 602 and the reading a was found to be 307. connections were then made as shown in figs. 603 and 604 and a was found to be 610. what is the value of d? according to formula d = al/a = (307 ã� 10,853)/610 = 5,461 ft. [illustration: figs. 601 and 602.--special loop test with leeds and northrup fault finder. for the first measurement connect the faulty wire to 2, either of the good wires, as z, to 1, the post gr to ground, and short circuit the coils r and e by closing switches u and v as in the figures. balance in the usual way and call the dial reading a. for the second measurement, connect the post gr. (disconnected from ground), to the other good wire y as shown in figs. 603 and 604, and get another balance; call this reading a'. the distance d, to the fault is determined from the simple formula d = al ã· a' where l is the length of the cable or faulty wire.] [illustration: figs. 603 and 604.--special loop test as made with the leeds and northrup fault finder. diagram showing connections for the second measurement. the special loop test may be used to advantage where the length of the cable or faulty wire only is known, and where there are two other wires which may be used to complete the loop. to use an outside battery, connect one pole to ba, and ground the other. the pressure of this battery must never exceed 110 volts; if it be over 25 volts, see that switch w is open.] the potentiometer.--for the rapid and accurate measurement of voltage, current, and resistance, the potentiometer can be recommended. those in charge of electric light and power companies, and also those who purchase large amounts of electrical energy are realizing, more and more, the necessity of having satisfactory primary standards with which to check their volt-, ampere-, and watt-meters. when it is realized that an error of one per cent. in a commercial instrument means an error of one dollar one way or the other in every one hundred dollars charged, the need of such standardization apparatus becomes at once apparent. the potentiometer, it should be noted, relies for its accuracy, only upon the constancy and accuracy of resistances and upon standard cells. with the materials now available, and the skill which has been acquired in their manufacture, both the resistances and the standard cells are obtainable which are remarkably constant, and both can be readily checked for accuracy. location of opens.--these measurements are based on the fact that the capacity of wires in a cable is ordinarily a measurable quantity, which, in wire of uniform diameter, is proportionate to length. in making these tests, a fault finder is used together with a buzzer, dry cells to operate it, small induction coil, and telephone receiver. these instruments are to be found in any telephone exchange. it is best to locate the buzzer at some distance from the fault finder in order that it cannot be heard by the operator. [illustration: fig. 605.--to use galvanometer of leeds and northrup fault finder in series with the battery: set switches as shown, and connect between posts gr. and 2 (see figs. 587 and 588). the galvanometer will have the maximum sensibility with the pointer at 1,000 and the minimum at zero.] [illustration: fig. 606.--to measure high resistances, such as the resistances of faults with leeds and northrup fault finder. _first method._--arrange the switches as shown in the figure. connect posts gr. and 2, turn the handle until the galvanometer needle comes to rest at an even deflection of ten divisions. call the reading a. connect in the unknown resistance between gr. and 2. now close the switch w, so that the figure 1 appears on the top of the block, and again bring the galvanometer to a deflection of ten divisions and call the reading b. then x = (10,000 b ã· a) 1,000. in case x be a high resistance, it will be found that the galvanometer will not deflect ten divisions for any position of the pointer. in such case, choose a number of divisions which is a factor of ten, such as 5, 2, or 1, and multiply (10,000 b ã· a) by ten divided by the number chosen, as 10/5, 10/2, 10/1. for example, for a deflection of two divisions: x = (10/2)(10,000 b ã· a) 1,000. the satisfactory range of the set for high resistance measurement may be increased by using an outside battery of higher voltage. with the contained battery, satisfactory measurements can be made up to 1 or 2 megohms. when outside battery is used, connect one terminal to the post ba, and the other to 2 for the reading a. connect the battery and unknown resistance in series between these posts for the reading b. when an outside pressure of 25 volts or over is used, the switch w should not be closed unless there be a resistance in series with the battery of 10,000 ohms or over. _second method._--for use as a voltmeter to measure high resistances. (more convenient but not quite as accurate as first method.) set the switches to rv, m and 10. turn the knurled nut on the galvanometer so as to set the needle to the extreme right hand side of the scale. connect the posts 2 and gr. with a short piece of wire. turn the rotating pointer on the scale until the galvanometer needle moves over about 20 scale divisions when the battery key is closed. remove the connection between 2 and gr. as the voltmeter terminals. this makes a simple way of testing for various kinds and amounts of trouble. on a wet cable a deflection of 10 to 15 divisions indicates heavy enough trouble to locate with the fault finder. with a little care, trouble showing only 5 or 6 divisions can be located.] before attempting locations for opens it is well to make the following measurements: 1. the insulation of the broken wire and the insulation of the good wire with which it is to be compared. this may be done as shown in fig. 606. it is best that the insulation resistance be fairly good, but experiments indicate that good results can be obtained by the methods which follow, even when the insulation is as low as 100,000 ohms, and fair results when as low as 50,000 to 100,000 ohms. [illustration: fig. 607.--diagram of connections in testing for opens with leeds and northrup fault finder. the apparatus required consists of fault finder, buzzer, dry cell to operate buzzer, small induction coil, and telephone receiver. connect the battery to the primary of the induction coil, one terminal of the secondary to the post ba, and the other to the connected wires as shown. set switches u and v so as to short circuit the two resistance coils.] 2. the resistance between the two sections of the broken wire should be measured. this may be done by joining the broken wire and a good wire at the distant end of the cable and measuring the resistance of the loop. to ensure close locations, this resistance should be over 100,000 ohms. fair locations can be made when the resistance is much lower and it is worth while to attempt it even if the resistance be as low as 10,000 ohms. the difficulty of determining the balance point increases as the resistance decreases. ques. describe the method of locating an open with a fault finder. ans. (_case i_) the broken wire will be one of a pair. select another pair in the cable that will have the same capacity per mile and join together the mate of the broken wire and one wire of the other pair. make the connections as shown in fig. 607, then depress the battery key and move the contact to the point of minimum sound in the telephone. the distance to the break is equal to la ã· (1,000 a), where l is the length of the good wire. example: a cable 1.45 miles long contained a broken wire. it was found that the insulation resistance of the end of this wire was over 10 megohms, as was that of the good pair selected to test against it. the resistance between the two pieces of the good wire was also over 10 megohms. connections were made as in fig. 607, and it was found that the balance point was 476. accordingly from the table a / (1,000 a) = 0.9084 and d = 1.45 ã� .9084 = 1.317 + miles. [illustration: fig. 608.--diagram of connections in testing with leeds and northrup fault finder for open wire in telegraph and other cables in which the wires are not grouped in pairs. connect the broken wire to 1. select a good wire and join to 2. connect all other wires and ground them, by connecting to the cable sheath. connect the distant end of the broken wire to the others. ground the end of the induction coil that is not connected to the post ba.] location of opens.--(_case ii_) open wire in telegraph or other cables in which the wires are not grouped in pairs. the connections are made as in fig. 608, and the measurement and calculation exactly as in the preceding case. the accuracy of the location of both of the above methods depends on the good and broken pair, or the good and broken wires having equal and uniform capacity per unit length. it is not always possible to select wires that are alike in this respect. in such cases, as for instance, where there is no good wire in the cable containing the broken wire, and a good wire has to be selected from another cable, the method of _case iii_ may be used. [illustration: fig. 609.--diagram of connections for reading in testing for opens with leeds and northrup fault finder, when broken wire and good wire are not in the same cable.] [illustration: fig. 610.--leeds and northrup potentiometer. it is direct reading from .000001 volt to 16 volts, and with accessories the range may be extended to 1600 volts, and currents may be measured up to 3000 amperes. the instrument has fifteen coils of 5 ohms each, which are in series with an extended wire about 190" long of equal resistance. the electrical circuits are shown in the diagram fig. 611. it is well for the user to open up the potentiometer and make himself familiar with its interior construction, in order to fully understand the operation of the rheostat and other parts. there are no contact resistances in the potentiometer circuit proper. the potentiometer has low internal resistance which gives it the maximum sensibility. compared with high resistance potentiometer, this is especially advantageous in measuring the electromotive force of thermocouples, and the fall of potential across standard low resistances. as constructed, the last one-tenth volt is covered by the extended wire and the handle which carries the contact point on the wire may be manipulated rapidly so that a fluctuating voltage may be accurately followed. when used with any cadmium cell, the potentiometer is direct reading. the accuracy of the potentiometer resistances can be verified with the facilities of the ordinary laboratory.] location of opens.--_case iii_, in which the broken wire and good wire are not in the same cable. connect the good wire and broken wire in the same way as shown in fig. 607, and set the pointer for a balance. call the reading a. then connect the good wire and the broken wire at the distant end and set the pointer for a new balance. call this a'. the connections for this reading are shown in fig. 609. the distance to the break will be d = (a a' l) / (1,000(a a') + a a') where l is the total length of the broken wire. [illustration: fig. 611.--diagram showing connections of leeds and northrup potentiometer. the coils in the series ad are each 5 ohms, and between each two there is a brass block with a reamed hole. a pair of flexible cords with taper plug terminals to fit these holes is furnished. these coils can be measured with an ordinary wheatstone bridge and thus compared with each other to a high degree of accuracy, even if the bridge be not accurate. for potentiometer work, the essential point is that they should be like each other, not that they should be accurately any particular value. in the same way the resistance of the extended wire can be compared with the resistance of the coils in ad. its resistance should be 1.1 times the value of any coil between a and d. outside connection with the extended wire may be made by using the posts marked br and -ba. this adjustment for balancing an unknown electromotive force is accomplished by the manipulation of the two contact points m and m'. the coils ad are arranged in a circle, a revolving switch moving m. a checking device enables the operator to set this switch without taking his eye from the galvanometer. the resistance s is of such value that when it shunts the wire ob, the total resistance between o and b is 1/10 of the same unshunted. when the shunt is applied, provided the total current remain the same, the drop between any two points on ab will be 1/10 of its previous value. the total current will remain the same provided the total resistance in the circuit remain the same. this is accomplished by making the coil k such that it exactly compensates for the reduction in resistance caused by plugging in the shunt coil s. the low scale is applied by moving the plug from the position 1 to the position .1. with this change the potentiometer reads from .16 volt down by indicated steps of .000005 volt. the reading is very simple. for instance, if m stand at 1.2 and m' at 1.35 revolutions, the reading is 1.2135 volts. the resistances of the instrument are wound upon metal spools, and are therefore able to dissipate a comparatively large amount of energy. this allows the potentiometer to be used for pressure measurements up to 16 volts without the use of a volt box.] [illustration: fig. 612.--diagram showing actual connections in the rheostat of leeds and northrup potentiometer. the figure corresponds to r of fig. 611. the rheostat is mounted in the end of the potentiometer as shown in fig. 610. rough adjustment of the potentiometer current is made by means of the variable resistance r. fine adjustment is made by means of the variable resistance r'. it will be noted that the 23 ohm resistance of this latter rheostat is shunted by a resistance of 6.1 ohms, making possible a very fine regulation. further, there is in series with the moving contact a resistance of 400 ohms, which makes the effect of variable contact resistance negligible. only one cell of storage battery should be used. when this battery is fresh, the plug shown in the figure at 2r should be inserted at r. this gives the greatest resistance in the rheostat circuit. as the cell runs down, the plug should be changed to 2r. when both plugs are in, the rheostat slide wires are in series with the potentiometer circuit.] example: a pair of wires containing one broken wire was connected with a good pair in a different cable as shown in fig. 607. the reading a was found to be 180. the good and bad wires were then joined at the distant end as in fig. 609, and the reading a was found to be 88. the total length of the bad wire mn was 1.44 miles. required, the distance to the break. substituting the values in the formula: d = 180 ã� 88 ã� 1.44 / 1,000(180 88) + 180 ã� 88 = .211 + mile. to pick out faulty wires in a cable.--short circuit the coils e and r with switches u and v. set the pointer at 1,000. connect the post gr. to ground or the cable sheath and apply the wires one after another to the binding post 2. the galvanometer will deflect for a faulty wire. [illustration: fig. 613.--diagram of the crompton potentiometer. in this instrument the resistance consists of fourteen coils, each of 10 ohms, in series with a straight wire, also 10 ohms resistance, thus forming a system of fifteen equal steps. across the whole a pressure of 1.5 volt is applied from a secondary cell, thus providing .10 volt per step. any fraction is then tapped off by means of a radial switch on the resistance coils and a sliding contact on the wire. the standardization is performed by adjusting a resistance in series with the whole until the standard cell employed indicates, by means of the galvanometer g, a balance at the point which represents its electromotive force on the basis given above.] ques. what is a potentiometer? ans. an arrangement of carefully standardized resistances for measuring voltages in comparison with a standard cell. it is used for accurate measurement of voltages, currents, and resistances. in place of a series of standardized resistances, a slide wire may be used as in fig. 614. ques. describe one form of potentiometer. ans. as shown in fig. 614, it consists of a fine german silver wire about 3 feet long stretched between the binding posts a, b, which are attached to a wooden base carrying a scale divided into 1,000 equal parts. there are three circuits, the terminal a being included in each, one including the battery, and the other two the galvanometer. a three point switch connects the galvanometer in series with the standard cell sc, or the cell to be tested c, the circuits being completed by leads terminating in the sliding contacts m and s. [illustration: fig. 614.--diagram of potentiometer showing method of measuring the voltage of a cell. the potentiometer is simply a high resistance wire of uniform diameter stretched between two binding posts, a and b, in such a way that contact can be made at its ends and along its length. necessary circuits are plainly shown in the figure; sc, is a _standard cell_ and c, the cell to be tested. m, and s are sliding contacts, connecting with the "slide wire."] ques. describe the method of measuring the voltage of a cell with a potentiometer. ans. fig. 614 shows a method of comparing a pressure with that of a standard cell and is applicable whether the pressure of the cell to be tested be greater or less than that of the standard cell. in making the test the switch f is first closed, then the other switch is moved to d, and m adjusted till galvanometer shows no deflection; similarly, the switch is moved to g, and s adjusted till galvanometer shows no deflection. then, c:sc = as:am. from which c = sc ã� as ã· am. example.--let 1.016 volts be the known voltage of the standard cell sc, and the scale reading of as be 657, and of am, 225 as in the figure, then c = (1.016 ã� 657) / 225 = 2.966 volts the arrangement may, however, be made direct reading, that is, the slide wire may have a scale of volts instead of lengths or resistances, as follows: suppose the standard cell to have a pressure of 1.434 volts, the sliding contact m is placed at the reading 1.434, and the adjustable resistance varied till the galvanometer shows no current. this means that the pressure between a and m is 1.434, and consequently the pressures all along the slide can be read off the scale _in volts_. hence, when s has been adjusted to balance, the pressure of c is read off the scale in volts. [illustration: fig. 615.--to measure a pressure greater than 1.6 volts with leeds and northrup potentiometer by using a volt box or multiplier. to measure high voltages it is necessary to connect the voltage across high resistance and to measure on the potentiometer a definite fraction of the total drop. in the figure, ab is a high resistance of which cb is .1th, db .01th, and eb .001th of the total resistance. the potentiometer reading is accordingly multiplied by 10, 100, or 1,000, depending upon whether the switch m is set on c, d, or e. resistance boxes for this purpose are called _volt boxes_, and are constructed to multiply the potentiometer readings by 10, 100, and 1,000. in using them, it is only necessary to connect the unknown e.m.f. at the posts so marked, and the potentiometer to the posts marked p. the potentiometer reading is taken as above and multiplied by a factor depending upon the position of the switch m, which factor is indicated upon the box. _it is essential in making these connections that the polarity be carefully observed._] how to use a potentiometer.--all connections must be made as indicated by the stamping on the instrument. particular attention must be given to the polarity of the standard cell, of the battery, and of the voltage, the corresponding + and signs being marked. if used with a wall galvanometer having a telescope and scale, it will be found convenient to place the potentiometer so that the telescope is directly over the glass index of the extended wire, thus permitting the observer to read the galvanometer deflections and potentiometer settings without changing his position. potentiometer current.--a medium sized storage cell will be found desirable, producing a steady current. errors in measurements are frequently made by using an unsteady current. setting for standard cell.--set the standard cell to correspond with the certified pressure of the standard cell as given in its certificate. in using the potentiometer shown in fig. 610, place the plug in hole 1, and see that it is always in this position when checking against the standard cell. place the double throw switch at std. cell. adjust the regulating rheostat until the galvanometer shows no deflection. in making the first adjustment use the key marked r_{1}; when a balance is almost attained, use key r_{2}, and for the final adjustment use key marked r_{0}. this cuts out the resistance in series with the galvanometer and gives the maximum sensibility. measurement of unknown pressure.--the potentiometer (fig. 610), as ordinarily used, gives direct readings for voltages up to and including 1.6 volts. for pressures higher than 1.6 volts, a volt box or multiplier should be used. after obtaining the standard cell balance, as previously described, place the double throw switch in the position marked e.m.f. the balance for the unknown e.m.f. is obtained by manipulating the tenths switch and rotating the contact on the extended potentiometer wire. the final position of the two contacts in conjunction with the position of the plug at the left of the instrument indicates the voltage under test. as directed above, use key r_{1} for rough adjustment, r_{2} for intermediate adjustment, and key r_{0} for final adjustment. plug at 1 gives readings for voltage directly from settings of tenths switch and extended wire contact. plug at .1 shunts the potentiometer circuit so that the voltage measured is .1 of the reading taken directly from the scale. hence, the readings taken from the setting of the tenths switch and the slide wire contact must be divided by 10. to balance galvanometer for unknown voltage.--place plug in hole 1 (fig. 610) for voltages up to 1.6, and in hole .1 for voltages up to .16. rotate the tenths switch until a condition of balance is obtained exactly or approximately. to secure an exact balance, rotate the contact on the extended wire. the unknown voltage can now be read directly from the position of the tenths switch and the extended wire contact if plug be at 1, or by dividing by 10 if plug be at .1. example.--a balance was obtained with the tenths switch at 1.3, the extended wire contact at 176 and the plug at 1. the voltage under test, therefore, is 1.3176. if the plug at .1 had been used, the same reading would have indicated .13176. to ascertain if the current in the potentiometer circuit has altered during a measurement, it is only necessary to plug in at 1, place the double throw switch on std. cell and close the galvanometer key. no deflection indicates that the current has not changed. if the galvanometer deflect, the regulating rheostat must again be adjusted until the galvanometer shows no deflection. to measure voltages from 1.6 to 16.--pressures up to 16 volts may be measured by using a greater voltage across the ba posts (fig. 615). for this purpose a battery of about 20 volts should be used. insert the large plug at .1 and throw the switch to std. cell, then balance the galvanometer by means of the regulating rheostat. when the rheostat has been set to secure a balance, insert the large plug at 1, set the switch on e.m.f. and read the voltage in the usual manner. multiply the reading by 10. [illustration: fig. 616.--measurement of current with potentiometer. this is done by measuring the drop in volts across a known low resistance. in the figure s is the standard resistance, and on it are the pressure terminals pp, and the current terminals cc. the potentiometer is connected to the shunt through the posts marked p. the resistance between the points pp is adjusted to an even fraction of an ohm. these resistances are so chosen that in order to determine the current passing through the shunt, after having obtained a potentiometer balance it is only necessary to multiply the potentiometer reading by a simple factor. for instance, in using a .01 ohm standard. it is only necessary to multiply the potentiometer reading by 100, which gives the current reading in amperes; similarly, a .1 ohm requires multiplication by 10, and a .001 ohm by 1,000.] care of potentiometer.--the slide wire, although protected to a great extent by the hood, in time accumulates dust and dirt with a thin film of oxide. this will tend to increase the resistance in this part of the circuit owing to poor contact. this wire should, therefore, be cleaned occasionally. to do this, unscrew the stop against which the hood strikes when turned to read zero; then remove the hood and rub the entire slide wire vigorously with a soft cloth dipped in vaseline. _do not use emery or sand paper as this will destroy the uniformity of the slide wire._ clean also the steel contact which rubs on the wire, as this becomes glazed after much use. when the potentiometer is not in use, the hood should be screwed all the way down, and the lid put in place to exclude dust. if it be used in a chemical factory, laboratory, or any place where acid fumes are prevalent, this latter precaution is important, because the fumes may attack the slide wire. it is also well to keep the contact surfaces of the switch studs clean and bright by wiping them occasionally with a soft cloth dipped in vaseline. [illustration: _res. of leads on each end is equal to 10 divisions of slide wire. the slide wire is divided into 1000 parts--20 for the leads, or 980 divisions. calibrated scale on galvanometer:_ fig. 617.--diagram of leeds and northrup bridge for locating faults in power circuits, showing arrangement of the connections including the lead cables and galvanometer contacts. make connections as shown. the clamps must be so fastened at a and c that the contact resistances will be very small. this contact resistance will figure as an error in the measurement. if, for instance, the contact resistance were equal to .001 of an ohm, and the wire were of such a size that .001 of an ohm were equal to the resistance of 20 feet of the cable, there would be an error of 20 feet in the location of the fault. for this reason all contact resistances throughout the loop from a to c must be extremely small. the battery is to be connected to the posts marked ba., and the post marked gr. is to be grounded. it will very frequently happen that the ground is to the cable sheath or some other conductor. in this case, the binding post gr. should be grounded to this conductor. sufficient battery should be used to give a readable deflection on the galvanometer for a small movement of the contact on the bridge wire. the fault is located by the usual murray formula. if, for instance, the galvanometer show no deflection when the contact is at 300 on the scale, it would indicate that the fault is at a distance from a equal to .003 of the total length of the loop from a to c. a testing current of five amperes may be used with this bridge. in cases of necessity, this current may be increased to eight amperes, but when this current is used it should not be allowed to pass through the bridge for a longer time than is necessary. it frequently happens that small faults which have a very high resistance develop in high pressure cables. such faults are likely to break down and result in damage and should be located. it is usually impossible to locate these faults until they have been partially carbonized. this must be done by applying a sufficiently high voltage between the cable and the sheath (or whatever it is grounded on) to break down the fault. in order to prevent the breaking down process from resulting in a serious burn out a high resistance must be placed in the circuit which will prevent an excessive current, or the circuit must be carefully fused. the former procedure is the better.] location of faults where the loop is composed of cables of different cross sections.--faults in loops of this character may be located with the same degree of accuracy as those in loops of a uniform cross section, provided the length and cross section of each length of cable are known. an example will illustrate the method: in the diagram, fig. 617, assume the length of the cable ae to be 550 yards of 25,000 cir. mil., ef, 500 yards of 40,000 cir. mil., and fc, 1,050 yards of 30,000 cir. mil. these lengths must be reduced by calculation to equivalent lengths of one size, and for this purpose it is best to select the largest size. the results of this calculation are as follows: 550 yds. of 25,000 cir. mil. = 880 yds. of 40,000 cir. mil. 500 " " 40,000 " " = 500 " " 40,000 " " 1,050 " " 30,000 " " = 1,400 " " 40,000 " " this makes the total resistance of the loop equivalent to 2,780 yards of 40,000 cir. mil. if the contact show a balance for a reading of 372.5, this indicates that the fault is at a distance of 372.5/1,000 of 2,780 = 1,035.5 equivalent yards. of this, 880 are in the stretch a e. consequently the fault is: 1,035.5 880 = 155.5 yards from e. [illustration: fig. 618.--the fischer portable cable testing set, designed for locating crosses, grounds and breaks in cables, also for conductor and liquid resistance measure. the distinguishing feature of the set is the _master switch_. by means of this switch, connections can be made for the various tests by a single movement, thus avoiding the labor and time which have to be expended in interchanging the connections and memorizing the rather complicated scheme of connections.] chapter xxviii ammeters, voltmeters and wattmeters. an ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current _in amperes_. a good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed; the needle should be dead beat, and sufficiently sensitive to respond to minute variations of current. according to the principle of operation, ammeters and voltmeters are classified as: 1. moving iron; 2. moving coil; 3. solenoid or plunger; 4. magnetic vane; 5. hot wire; 6. electrostatic; 7. astatic; 8. inclined coil; 9. fixed and movable coil. again, they are divided according to their use into two classes: 1. portable type; 2. switchboard type. _milli-ammeters_ or milli-voltmeters are instruments in which the scale is graduated to read directly in thousandths of an ampere or thousandths of a volt respectively. ques. describe the moving iron type instrument. ans. the arrangement of the working parts are shown in fig. 620. a soft iron needle n, is pivoted inside of a coil c, and is held out of line with the axis of the coil by means of a permanent magnet m, when the instrument is idle. in this position, a pointer p, which is attached to the needle, stands at the zero mark of the scale s. if a current be passed through the coil, magnetic lines of force are set up in its center, which tend to pull the needle into line with them, and therefore with the axis of the coil. this pull is resisted by the permanent magnet m, and the amount of deflection of the needle from the zero position depends upon the strength of the current or the voltage according as the coil is wound to indicate amperes or volts. [illustration: fig. 620. moving iron type instrument. the essential parts are: n, soft iron needle; c, coil; m, permanent magnet; p, pointer; s, scale. current passing through the coil acts on the needle, causing it to turn against the restraining force due to the influence of the permanent magnet.] [illustration: fig. 621.--moving coil type instrument. the essential parts are: a, spiral spring; c, coil; k, soft iron core; m, permanent magnet; p, pointer; s, scale. current passing through the coil causes the moving system to turn against the restraining force due to the influence of the permanent magnet.] ques. describe a moving coil instrument. ans. this type of instrument is shown in fig. 621. it consists of a moving coil c, to which is attached the pointer, and which is pivoted between the poles of a permanent magnet m. the coil moves between these poles and a fixed soft iron core k, and is held in the normal position by two spiral springs a, above and below the core. the springs also serve to make electrical connection with the coil c. when a current passes through the coil, magnetic lines are set up in it which are at an angle to those passing from one pole of the permanent magnet to the other. the lines of force, which formerly passed from one pole of the magnet to the other by straight lines or by short curved ones, are "stretched" on account of the field produced by the current in the coil, and, in trying to shorten themselves, tend to twist the coil through an angle. this tendency to move is resisted by the two spiral springs, hence the coil moves until equilibrium is established between the two opposing forces. the amount of deflection of the pointer depends, either upon the current strength, or the voltage according to the winding of the coil. [illustration: fig. 622.--keystone voltmeter; view showing the moving element being withdrawn by loosening one screw. these instruments are constructed on the d'arsonval system, the moving element being shown in detail in figs. 623 and 624. the entire system is mounted upon a solid metal base plate. the permanent field magnet is made of a single piece of magnet steel, and the pole pieces are of soft steel, permanently secured to the magnet in order that the distribution of the magnetic flux will not be changed by removal and replacement of the pole piece. accordingly the moving mechanism is mounted separately from the field, so that it can be readily lifted from the field without removing the pole pieces. the function of the core is to secure a uniform field. the moving coil is wound upon a form of aluminum, which serves the purpose of damping by the generation of eddy currents. the winding of the coil is of fine copper wire, to which current is conveyed by means of the controlling springs and which, in the case of a voltmeter, is connected in series with a resistance, and in the case of an ammeter, across the terminals of a shunt.] ques. how does the winding differ in ammeters and voltmeters? ans. an ammeter coil consists of a few turns of heavy wire (when designed to carry the full current), while a voltmeter coil is wound with many turns of fine wire. thus, the ammeter is of low resistance, and the voltmeter of high resistance. [illustration: fig. 623.--new moving element of keystone instruments, weight 1.2 grams. fig. 624.--moving element of keystone instruments assembled in bearing. the moving element consists of coil, counterpoise and pointer. the mechanical connections are made by means of screws and steady pins. in order to adjust for slight set or subset of spring under long use a zero adjuster is provided by means of which this set can be connected and the pointer brought back to zero.] ques. why is a high resistance coil used with a voltmeter? ans. as actually constructed, most voltmeters are simply special forms of ammeter. from ohm's law, the current through a given circuit equals the pressure at its terminals divided by its resistance. hence, if a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the voltage at its terminals, and the instrument may be calibrated to read volts. [illustration: figs. 625 and 626.--connections for series and shunt ammeters. when the construction is such that all the current passes through the instrument, it is connected as in fig. 625, but where the instrument is designed to take only a fraction of the current, it is connected across a shunt, as in fig. 626, a definite proportion of the current passing through the instrument and the remainder through the shunt.] ques. into what two classes may ammeters be divided? ans. they are classed as series or shunt according to the way they are designed to be connected with the circuit. ques. what determines the mode of connecting ammeters? ans. when the wire of the ammeter coil is large enough to carry the whole current, it is connected in the circuit _in series_ as shown in fig. 625. if, however, the wire be small, the instrument is connected _in parallel_ with a shunt of low resistance, so that it only carries a small part of the current, as in fig. 626. for circuits which carry large currents, the shunt connection is always used, because otherwise the coil of the ammeter would have to be very heavy and the instrument correspondingly bulky. ques. how are shunt ammeters arranged to correctly measure the current? ans. the coil is arranged so that a definite proportion of the whole current passes through it. a large conductor of low resistance is connected directly between the two terminals or binding posts of the instrument; the coil is connected as a shunt around a definite part of this main conductor; then, since the two are connected in parallel and each branch has a definite resistance, the current divides between the two branches directly in proportion to their relative conductivities, or inversely according to their resistances. the coil, therefore, takes a definite part of the whole current, and the force moving it and its pointer away from the zero position is directly proportional to the whole current. hence, by providing a proper scale, the value of the entire current will be indicated. [illustration: figs. 627 and 628.--westinghouse ammeter shunts. these shunts are used where heavy currents are to be measured. the shunt is connected in series with the bus bar or circuit to be measured, and its terminals are connected by means of small leads to the ammeter or other instrument. these shunts are designed to have approximately 50 millivolts drop at full rated current. they are intended primarily for westinghouse meters, but can be used satisfactorily with any meter requiring 50 millivolts for full scale deflection.] ques. how is a voltmeter connected? ans. a voltmeter is always connected to the two points, whose difference of potential is to be measured. for instance, to measure the voltage between the two sides a and b of the circuit shown in fig. 629, one terminal of the voltmeter is connected to wire a, and the other to wire b. if the "drop" or difference in voltage through a certain length of wire l, of a circuit, as from a to b in fig. 630 is to be determined, one terminal of the voltmeter is connected to a and the other to b. in a similar manner is found the drop through a lamp. [illustration: fig. 629.--voltmeter connection for measuring the pressure in an electric circuit. the voltmeter is connected in parallel in the circuit at the point where the voltage is to be measured. fig. 630.--voltmeter connection for measuring the "drop" or fall in voltage in a certain length of wire, as for instance, the length between the points a and b. the voltmeter is shunted between the two points whose pressure difference is to be measured.] ques. what is the difference between a voltmeter and an ammeter? ans. a voltmeter measures pressure, while an ammeter measures current. as actually constructed, most voltmeters are simply special forms of ammeter. if a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the pressure or voltage at its terminals and the instrument may be calibrated to read volts. ques. explain the term "calibrate." ans. to calibrate a measuring instrument is to determine the variations in its readings by making special measurements, or by comparison with a _standard_. [illustration: fig. 631.--weston ammeter; view showing shunt enclosed within the instrument. weston instruments are direct reading and dead beat. although the scales have practically uniform divisions, it is not assumed in the calibration that they are uniform, and the scales are not printed or engraved. the method of calibration consists in laying out each large division of the scale by comparing the instrument with a standard, and then inking in the division lines so found. the smaller divisions between the large ones are then equally spaced and marked by a mechanical method.] [illustration: fig. 632.--weston portable voltmeter, inspector's style. this instrument is provided with a reversing key. instead of the regular binding posts, pins are used with which connections are made by means of contact cups attached to flexible cords. these contact cups are convenient in making connections, or in changing quickly from one range to the other, if the instrument have a double scale. connections for the different ranges are made in precisely the same way as with the regular double scale voltmeters. for the upper scale values, the contact pin to the right and the front contact pin to the left being taken, and for the lower scale values, the left contact cup is changed to the rear contact pin.] ques. describe a solenoid or plunger ammeter. ans. this type consists of a "plunger" or soft iron core arranged to enter a solenoid. current being passed through the wire of the solenoid causes the core to be more or less attracted against a restraining force of gravity or springs. a pivoted pointer attached to the core indicates the current value on a graduated dial as shown in fig. 633. ques. what are the objections to plunger instruments? ans. they are not reliable for small readings, and are readily affected by magnetic fields. [illustration: fig. 633.--plunger type instrument. the current to be measured passes through the solenoid, producing a magnetic effect on the soft iron plunger which tends to draw it into the coil, and thus cause the pointer to move over the graduated scale. the distance the rod moves depends on the value of the restraining force (which may be springs or gravity), the coil winding, and strength of current. the winding consists of a few turns of heavy wire for an ammeter, and a large number of turns of fine wire when constructed as a voltmeter. since the iron has a certain amount of residual magnetism, the deflection with smaller following large currents is more than would be produced by the same current following a smaller one. the instrument therefore is less reliable than the usual types.] ques. describe a magnetic vane instrument. ans. it consists of a small piece of soft iron or _vane_ mounted on a shaft that is pivoted a little off the center of a coil as shown in fig. 634. the principle upon which the instrument works is that a piece of soft iron placed in a magnetic field and free to move will move into such position as to conduct the maximum number of lines of force. the current to be measured is passed around the coil producing a magnetic field through the center of the coil. the magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible. a pointer, attached to the vane shaft moves over a graduated dial. [illustration: fig. 634.--magnetic vane instrument. a soft iron vane, eccentrically pivoted within a coil carrying the current to be measured, is attracted toward the position where it will conduct the greatest number of magnetic lines of force against the restraining force of a spring or equivalent.] ques. describe an inclined coil instrument. ans. as shown in fig. 635, a coil carrying the current, is mounted at an angle to a shaft to which is attached a pointer. a bundle of iron strips is mounted on the shaft. a spring restrains the shaft and holds the pointer at the zero position when no current is flowing. when a current is passed through the coil, the iron tends to take up a position with its longest sides parallel to the lines of force, which results in the shaft being rotated and the pointer moved on the dial, the amount of movement depending upon the strength of the current in the coil. the coils for large sizes are generally wound with a few turns of flat insulated copper ribbon. the instruments are adapted to either direct or alternating currents but are recommended for alternating currents. [illustration: fig. 635.--thompson inclined coil ammeter. it is constructed on the magnetic vane principle in which an iron vane is attracted by the magnetic field due to the coil, so as to turn itself parallel with the axis of the coil, the latter being inclined with respect to the axis of the vane. the voltmeter of this type has a similarly placed stationary coil, but in place of the iron vane, is provided with a moving coil in series with the other coil. the restraining force in each case being that due to springs. figs. 636 and 637 show the actual construction of inclined coil instruments.] [illustration: figs. 636 and 637.--thompson inclined coil portable indicating instruments. fig. 636, ammeter type p interior; fig. 637, wattmeter, type p, interior. these instruments, though primarily designed for use on alternating current circuits, may also be used on direct current circuits, by making reversed readings and taking the mean as the true indication. the voltmeters and wattmeters are constructed on the dynamometer principle and the ammeters, on the magnetic vane principle. the voltmeters and wattmeters are provided with a contact key which may be locked in position, enabling the instruments to be left constantly in circuit. the movements of the pointer are damped by means of an air vane; there is also a friction damping device operated by a small button to check excessive oscillations of the pointer. the inclined coil instruments are so designed that the torque is sufficiently high to insure the pointer assuming a definite position with each change in current value.] ques. what is the principle of the hot wire instrument? ans. its action depends upon the heating of a conductor by the current flowing through it, causing it to expand and move an index needle or pointer, the movements of which, by calibration, are made to correspond to the pressure differences producing the actuating currents. ques. what are the characteristics of hot wire instruments? ans. voltmeters of this type are not affected by magnetic fields, and as their self-induction is small, they can be used on either direct or alternating currents; but they possess certain serious defects: they consume more current than the other types; cannot be constructed for small readings; are liable to burn out on accidental overloads; and are somewhat vague in the readings near the zero point and are sometimes inaccurate in the upper part of the scale. ques. describe the construction and operation of the whitney hot wire instruments. ans. as shown in fig. 638, a wire ax, of non-oxidizable metal, of high resistance and low temperature coefficient, passes over a pulley b mounted on the shaft c. the ends of the wire are attached to the plate e at its ends f and g, the wire being insulated from the plate at g. a spring h holds the wire in tension and takes up the slack due to the expansion caused by the heating of the wire when a current passes through it. the current flows only in the portion of the wire marked a, between the plate e and the pulley b up to the point k where the connection is shown. when a current flows through the wire a, the spring takes up the slack, pulls a around b, and causes b to rotate upon its shaft c. it is clear, that a pointer attached to c, would indicate on a scale the movement of b and c, but as this movement is very slight, a magnifying device will be required. this device consists of a forked rod l, rigidly attached to the shaft c, and carrying at its lower end a silk fibre fastened to the fork and passing around a pulley m, to which a pointer n is attached. for direct current measurements only an electromagnetic system is used. [illustration: fig. 638.--diagram showing principle and construction of the whitney hot wire instruments. the action of instruments of this type depends on the heating of a wire by the passage of a current causing the wire to lengthen. this elongation is magnified by suitable mechanism and transmitted to the pointer of the instrument.] ques. what is the principle of electrostatic instruments? ans. the action of these instruments depends upon the fact that two conductors attract one another when any difference of electric pressure exists between them. if one be delicately suspended so as to be free to move, it will approach the other. [illustration: fig. 639.--kelvin electrostatic voltmeter; a form of instrument designed for measuring high pressures up to 200,000 volts. the instrument, as illustrated, consists of fixed and movable vanes with terminals connecting with each. these vanes which act as condensers take charges proportional to the potential difference between them, resulting in a certain attraction which tends to rotate the movable disc against the restraining force of gravity. in the figure _aa_ and _b_ are two fixed vanes and _c_ a movable vane, carrying a pointer and having a proper weight at its lower end.] ques. describe the kelvin electrostatic voltmeter. ans. a simple form consists, as shown in fig. 639, of a metal case containing a pair of highly insulated plates, between which a delicately mounted paddle shaped needle is free to move. when the needle is connected to one side of a circuit and the stationary plates to the other side, the needle is attracted and moves between them as indicated by the pointer. adjusting screws at the lower end of the needle allow it to be balanced so that its center of gravity is somewhat below the center of suspension. gravity then is the restraining force. the range of the instrument may be changed by hanging different weights upon the needle. by increasing the number of blades the instrument can be made to measure as low as 30 volts. the form having two stationary blades and one movable blade is suitable for measuring from 200 to 20,000 volts. the quadrant electrometer or laboratory form will measure a fraction of a volt. [illustration: fig. 640.--thompson astatic instrument without cover. when current passes through the coils of the moving element, the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale. this action is, of course, opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal. these astatic instruments have no controlling springs. the two small silver spirals which conduct the current to and from the armature are made of untempered silver and exert no force as springs. the actuating and restraining forces are dependent upon the same electromagnets. the damping effect in these instruments is produced by an aluminum disc moving in a magnetic field, and is proportional to the square of the magnet strength.] ques. explain the construction and principle of the thompson astatic instruments. ans. the fields of these instruments are electromagnets wound for any specified voltage and provided with binding posts separate from the current posts of the instrument. the moving coils are mounted upon an aluminum disc and are located in a magnetic field which is parallel to the shaft and astatically arranged. two small pieces of magnetic metal are rigidly mounted on the shaft and the astatic components of the magnetic field, which are perpendicular to the shaft, tend to keep the pieces of magnetic metal in their initial positions. when current passes through the coils of the moving element, the lines of force parallel to the shaft produce a torque which tends to turn the shaft and cause the needle to travel across the scale. this action is, of course, opposed by the magnetic field at right angles to the shaft acting on the two pieces of magnetic metal. there are thus no restraining springs, current being conveyed to the moving coil by torsionless spirals of silver wire. thompson astatic instruments can be provided with polarity indicators, a red disc showing on the scale card where the poles are reversed. the effect of external fields is eliminated by the astatic arrangement of the fields and the moving parts. a field which tends to increase the torque on one side of the armature diminishes it to a corresponding degree on the other side. the damping effect in these instruments is produced by an aluminum disc moving in a magnetic field. [illustration: fig. 641 to 642.--multipliers for western standard portable voltmeters. multipliers are resistance boxes, the coils in which are highly insulated, and are adjusted so that the readings of the instrument may be multiplied by any desired constant. multipliers are usually constructed so that the indications of the pointer, multiplied by 2, 5, 10, 20 or 50, will give the voltage of the circuit. by the use of multipliers the range of voltmeters may be increased to any practical limit.] [illustration: fig. 643.--portable multiplier for portable voltmeter. a multiplier is used for increasing the readings of voltmeters, and consists of resistance coils placed in a portable case. a multiplier is connected in series with the voltmeter and must be adjusted for the instrument with which it is to be used, because the resistance coil must be a multiple of the voltmeter resistance. for instance, a multiplier with a value of 10, used with a 6 volt voltmeter or 521 ohms would measure about 5,215 ohms; one with a value of 40, would equal about 20,860 ohms. the multiplier 10 would give a total scale value of 60, and the multiplier 40, a total scale value of 240 volts to the 6 volt instrument. a multiplier is of considerable value in that it does away with the necessity of having a number of voltmeters of different ranges. the instrument here illustrated has a range of 150 volts.] ques. what are multipliers? ans. these are extra resistance coils which are connected in series with a voltmeter for increasing its capacity or readings. they are put up in portable boxes, and must be adjusted for each particular voltmeter as the resistance of a multiplier coil must be a multiple of the resistance of the voltmeter itself. ques. what is an electro-dynamometer? ans. an instrument for measuring amperes, volts, or watts by the reaction between two coils when the current to be measured is passed through them. one of the coils is fixed and the other movable. [illustration: figs. 644 to 645.--western standard portable shunts. the milli-voltmeters used in connection with these shunts read directly in amperes. shunts of different capacities can be adjusted to the same instrument, and it can, therefore, be used to measure a current of 2,000 amperes with the same degree of accuracy as a current of 1 ampere. in selecting shunts of different capacities for use in connection with one instrument it should be considered that the higher ranges must be even multiples of the lower one in order to suit the same scale on the instrument.] ques. describe the siemens' electro-dynamometer. ans. the essential parts are shown in fig. 646. the fixed coil a, composed of a number of turns of wire is fastened to a vertical support, and surrounded by the movable coil b of a few turns, or often of only one turn. the movable coil is suspended by a thread and a spiral spring c, below the dials which are fastened at one end to the movable coil and at the other end to a milled headed screw d, which can be turned so as to place the planes of the coil at right angles to each other, and to apply torsion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils. the ends of the movable coil dip into two cups of mercury e, e', located one above the other and along the axis of the coils so as to bring the two in series when connected to an external circuit. the arrows show the direction of current through the two coils. an index pointer f is attached to the movable coil. the upper end of this pointer is bent at a right angle, so that it swings over the dial between two stop pins g, g', and rests directly over the zero line when the planes of the coils are at right angles to each other. a pointer h is attached to the torsion screw d, and sweeps over the scale of the dial. the spring is the controlling factor in making the measurement. [illustration: fig. 646.--diagram of siemens' electro-dynamometer. it consists of two coils on a common axis, but set in planes at right angles to each other in such a way that a torque is produced between the two coils which measures the product of their currents. this torque is balanced by twisting a spiral spring through a measured angle of such degree that the coils shall resume their original relative positions. if the instrument be used for measuring _current_, the coils are connected in series, and the reading is then proportional to the square of the current. if used as a _wattmeter_, one coil carries the main current and the other a small current, which is proportional to the pressure. the reading is then proportional to the power in the circuit. fig. 647.--diagram showing connections of siemens' electro-dynamometer as arranged to read watts.] [illustration: figs. 648 to 650.--wright demand indicator. this is a device for registering the maximum ampere demand of appreciable duration in any electrical circuit. it may be used on either direct or alternating current circuits. the essential features and principle are as follows: a liquid is hermetically sealed in a glass vessel consisting of two bulbs connected by a "u" tube, and a central tube called the "index" tube, connected to the upper end of the right hand side of the "u." around the left hand or heating bulb, is placed a band of resistance metal, through which the current to be measured is passed, or a definite shunted portion of it. the heating effect of the current increases the temperature of the left hand bulb, causing the air to expand which forces the liquid up the right hand side of the "u" tube and into the index tube, where it remains until the indicator is reset. the height of the liquid in the index tube as shown by the scale, indicates the maximum current which has passed through the indicator. it is the difference in temperature of the air in the two bulbs which causes the flow of the liquid. any change in external temperature causes equal effect in both bulbs and therefore does not affect the reading.] [illustration: figs. 651 and 652.--weston illuminated dial station voltmeter and ammeter. the voltmeter has two indices, a pointed index for close readings and an index called the _normal index_, which enables a slight deviation from the normal voltage to be seen from a long distance. the "normal index" is inside the case and terminates in a circular disc of blackened aluminum. the disc is adjusted from the outside of the case by hand, by means of the knurled knob seen on the front of the case, so that it is directly below the point of normal voltage. when the indicating index reaches the point of normal voltage, the disc of the normal index appears in the center of the circular opening of the indicating index, a narrow ring of white being visible, encircling the disc of the normal index. the ammeter depends for its operation upon the fall of potential between two points of the circuit carrying the main current, and requires a difference of only about .05 volt to give a full scale deflection. when a maximum deflection is secured, the current passing through the instrument is never more than .07 ampere irrespective of the total capacity of the instrument. a separate shunt is used which is placed at the back of the switchboard. in many cases, a special shunt can be dispensed with and a short section of the mains on the switchboard, or the mains leading from the dynamo, can be used instead. on the basis of one square inch cross section per 1,000 amperes, a length of about 5 feet of copper conductor would be required as a shunt, in which case however, this section of the conductor must be adjusted with precision.] ques. explain the operation of the siemen's electrodynamometer. ans. in fig. 646, when a current is passed through both coils, the movable coil is deflected against a stop pin, then the screw d is turned in a direction to oppose the action of the current until the deflection has been overcome and the coil brought back to its original position. the angle through which the pointer of the torsion screw was turned is directly proportional to the square root of the angle of torsion. to determine the current strength in amperes, the square root of the angle of torsion is multiplied by a calculated constant furnished by the makers of this instrument. [illustration: fig. 653.--thompson watt hour meter (type c-6). this form is furnished with side connections, the line wires entering at the left and the load wires at the right. both sides of the system are carried through the meter in all sizes up to and including the 50 ampere size. in meters of larger ampere capacities, a voltage tap is used.] ques. how is the electrodynamometer adapted to measure volts or watts? ans. when constructed as a voltmeter, both coils are wound with a large number of turns of fine wire making the instrument sensitive to small currents. then by connecting a high resistance in series with the instrument it can be connected across the terminals of a circuit whose voltage is to be measured. when constructed as a wattmeter, one coil is wound so as to carry the main current, and the other made with many turns of fine wire of high resistance suitable for connecting across the circuit. with this arrangement, the force between the two coils will be proportional to the product of amperes by volts, hence, the instrument will measure watts. [illustration: fig. 654.--interior view of thompson watt hour meter (type c-6). capacity: 5 to 600 amperes, two wire, and 5 to 300 amperes, three wire; 100 to 250 volts. the meter is supported by three lugs, the upper one of which is keyholed, and the lower right hand one slotted. this permits rapid and accurate levelling as the top screw can be inserted and the meter hung thereon approximately level. the right hand screw may then be placed in position and the meter adjusted as may be required before forcing the screw home.] ques. describe briefly the construction of the thompson recording wattmeter. ans. it consists of four elements: 1, a motor causing rotation; 2, a dynamo providing the necessary load or drag; 3, a registering device, the function of which is to integrate the instantaneous values of the electrical energy to be measured; and 4, means of regulation for light and full load. [illustration: how to read a meter fig. 655.--recording dials of watt hour meter, illustrating method of reading electric meters. the unit of measurement of electrical energy is the watt hour. 1,000 watt hours make or equal 1 kilowatt hour. some electric meters have 4 dials, the extreme right hand dial of which registers in kilowatt hours, while others have 5 dials, the extreme right hand dial of which registers in tenths of kilowatt hours. in making out bills to customers the extreme right hand dial of a 5 dial meter is ignored in order that the "state of meter" shown on bills uniformly requires the addition of 3 ciphers to correctly express the registration in watt hours. each division on the right hand dial (ignoring the 5th dial mentioned) denotes 1,000 watt hours or 1 kilowatt hour; on the next dial 10 kilowatt hours, on the next dial 100 kilowatt hours and on the left hand dial 1,000 kilowatt hours. one complete revolution of any dial causes the hand on the dial immediately to its left to move forward one division. to take a statement from the meter begin at the left and set down for each dial the lower figure next to each hand, not necessarily the figure nearer the hand. in the above example the statement is 1,726 kilowatt hours or 1,726,000 watt hours. subtract the previous statement to arrive at registration for a given period. some meters are subject to a multiplying constant so stated on their face and the registration of such meters must be multiplied by the constant as shown, to determine the actual consumption of electrical energy. the constant is the measure of the mechanical adjustment in the register of the meter and is the ratio between the registration of the dial hands and the true consumption. this adjustment is made always by the manufacturer of the meter and is never changed in service.] ques. what is the action of the motor in the thompson watt hour meter? ans. it rotates at very slow speed, and since there is no iron in its fields and armature, it has very little reverse voltage. its armature current, therefore, is independent of the speed of rotation, and is constant for any definite voltage applied at its terminals. [illustration: fig. 656.--interior of thompson watt hour meter (type c-6) showing armature, small commutator and gravity brushes. a spherical armature moving within circular field coils is the construction adopted in this meter. the armature is wound on a very thin paper shell, stiff enough to withstand the strain due to winding and subsequent handling. the wire composing the armature is of the smallest gauge consistent with mechanical strength. the field coils, as before stated, are circular, and are placed as near each other as possible, one on either side of the armature, with the internal diameter just sufficient to give the necessary clearance for the rotating element. this construction prevents magnetic leakage. ribbon wire is employed for the field coils, thus economizing space and further carrying out the idea of concentration.] the torque of this motor being proportioned to the product of its armature and field currents, must vary directly as the energy passing through its coils. in order then, that the motor shall record correctly it is necessary only to provide some means for making the speed proportional to the torque. this is accomplished by applying a load or drag, the strength of which varies directly as the speed. ques. explain the operation of the thompson recording wattmeter. ans. there being no iron in either field or armature of the motor element, no considerations of saturation are involved. the torque or pull of the armature is dependent upon the product of the field and armature strength. the strength of the field--there being no iron--varies directly with the current in the field. thus the strength of the field with 10 amperes flowing to the load is exactly twice the strength of the field with 5 amperes flowing to the load. the strength of the armature is dependent on the voltage of the system to which it is connected, the armature element of the meter being practically a voltmeter. there is, therefore, a torque or pull varying directly with the strength of the armature multiplied by the strength of the field, or, in other words, varying directly with the watt load, and except in so far as influenced by friction, the speed of rotation varies directly with the torque or pull. the currents generated in the disc armature consist of eddy currents, which circulate within the mass of the disc. installation of wattmeters.--the various types of wattmeter differ so widely either in mechanical details, or operating principles, that it is customary for manufacturers to furnish detailed instructions for the installation of their meters. such instructions should be carefully followed in all cases, but the following will be found generally applicable to all types of motor meter: 1. after unpacking the meter, and before opening the case or cover, clean the latter carefully to remove all adhering particles of dust and excelsior. 2. the proper location for the meter should be one where there is no vibration. when this location has been selected, nail or screw upon the walls, a board somewhat larger than the dimensions of the back of the meter, and upon this board hang the meter by the top hanger. 3. after hanging the meter, open or remove the case or cover, and if necessary, put the mechanism in order according to instructions furnished by the manufacturer. 4. in order to operate satisfactorily, the meter should hang plumb, so that the spindle of the revolving element will be vertical, and the horizontal planes through the armature and retarding disc will be level. many complaints relative to meters being slow on light loads, are invariably due to the fact that the meters have been installed out of plumb[b]. 5. in making the circuit connections, be very careful that the _positive_ lead or wire is placed in the _positive_ binding post of the meter. this precaution is essential for insuring an accurate and sensitive measurement on small loads. 6. when a meter of the commutated motor type sparks at the brushes at starting, it is an indication that the commutator is dusty. clean it with a piece of closely woven cotton tape 1/4-inch in width. 7. meters should never be allowed to remain with their covers off, in the testing room, station, or any other place. in order to get the best service, and to give them long life they must be kept clean. [b] note.--the most practical and accurate method of plumbing a meter is to level it by means of a small brass weight placed upon the retarding disc. place the weight upon the front or back upper surface of the disc, close to the edge. if the disc and weight rotate toward the right, move the bottom of the meter in the same direction so as to raise the disc on the right. when the disc is level, the weight and disc will remain stationary when the weight is placed on either the front or the back of the disc. next, place the weight on the disc close to the edge on either side. if the disc rotate towards the front, swing the bottom of the meter away from the wall or board until the disc remains stationary when the weight is placed upon it on either side. if the disc rotate toward the back, raise it up on that side by bringing the top of the meter away from the wall or board. it is possible that the second levelling operation will alter the position of the disc obtained by the first operation, therefore, the first should be repeated, and after that the second also, until the disc remains stationary when the weight is placed at any point upon its surface. this method of levelling is more reliable than any method in which a spirit level is employed. [illustration: fig. 657.--interior view of thompson watt hour meter (type cq). the capacities of this type range from 50 to 400 amperes inclusive, two wire, and 50 to 200 amperes inclusive, three wire, and for voltages of from 100 to 600 volts. these meters are made with either front or back connections. in front connected meters the positions of the leading-in wires and cables are the same as in the type c-6, fig. 654, so that either type of meter may be installed in the same location.] [illustration: fig. 658.--specimen record from general electric recording ammeter. the record is made on a band of specially prepared paper four inches wide and sixty feet in length. on this paper are ruled lines corresponding to time, and the instrument calibration. the lines ruled across the paper represent time; those ruled lengthwise represent volts, amperes, or watts, depending upon the instrument construction. this form of paper has the advantage of permitting the use of time divisions of equal length throughout the entire range of the recording pen. the recording pen is attached to the moving element in such a manner that its motion is transmitted in a straight line parallel to the time division on the chart. as the paper is unwound and passed under the recording pen, it is paid into a space at the bottom of the instrument case. to assist in removing paper, the instrument is provided with a stripper, which enables the paper to be torn off evenly and without damage. the paper feeding mechanism is simple. by means of suitable gearing, the clock drives a drum having peg teeth which engage the holes located near the edge of the paper. these teeth not only feed the paper under the recording pen, but also give it a definite and accurate position along the axis of the drum. the feeding drum is driven by a friction clutch.] [illustration: fig. 659.--westinghouse type cw-6 watt hour meter with cover off. this meter is of the commutator type without iron in the magnetic circuit. the spherical armature is closely surrounded by circular field coils which provide the shortest magnetic path and smallest magnetic leakage, thus securing high torque with small consumption of energy. the armature winding is wound on a hollow sphere of prepared paper which is moulded in corrugated form to secure strength. uniform brush tension is maintained by gravity. each brush consists of two small round wires placed side by side and held against the commutator by a small counterweight whose distance from the fulcrum is adjustable. the current winding consists of two flat coils of strap copper, one clamped rigidly on either side of the central mounting frame which supports the armature bearings. these coils are connected either in series or parallel, depending on the capacity. in three wire meters one of the coils is connected in series with each side of the line. the retarding element consists of a light aluminum disc rotating between two pairs of permanent magnets. the magnets are prepared by a special aging process to insure permanence. full load adjustment is made by shifting the position of the permanent magnets. ample light load adjustment or friction compensation is provided by means of the movable coil, which can be shifted horizontally or radially on loosening one screw. the meter registers directly in kilowatt hours.] [illustration: fig. 660.--interior of thompson prepayment watt hour meter. the actuating force is a large flat coil spring enclosed in a barrel or drum to which its outside end is attached. the operating knob winds this main spring by turning the drum. the spring has many turns and as the operation of the device never equals one whole turn, the spring always exerts a practically constant force. the rate device consists of a small train of gears secured to the front of the frame directly back of the register. each device is marked with the price per kw-hr. for which it should be used. the switch is of the double pole double break type with leaf contacts. the coin receptacles are placed at the back of the meter. to make an advance payment, the winding knob is turned so that the arrow points upward. a quarter dollar is then inserted in the slot and the knob turned to the right, the coin serving as a key which operates the mechanism within the device, turning the registering wheel and placing the coin to the credit of the customer. if the circuit be open when the coin is deposited the same motion of the knob which moves the registering mechanism closes the circuit switch contained within the case. the dial contains a scale marked in plain figures over which a pointer passes indicating the number of coins remaining to the credit of the depositor. when the first coin is deposited and the knob turned closing the main switch, the pointer rests opposite the first division on the scale. if a second coin be deposited before the current purchased with the first coin has been consumed, a second motion of the knob will bring the pointer opposite the second division on the scale. twelve coins can thus be deposited consecutively, after which the slot is automatically closed and further prepayment cannot be made until the value of one or more coins has been consumed. whenever energy to the value of one coin has been delivered through the meter, the escapement is mechanically released turning the pointer back one division. this process continues until all the energy has been delivered for which payment has been made. thus the depositor can ascertain at any time how much energy can be obtained without further payment. when all energy has been delivered, the circuit switch is opened so that no more current can be obtained until one or more coins have been deposited. the indicating mechanism shows only the number of coins which stand to the credit of the customer, but, by consulting the meter dial, one can determine what fractional part of the prepayment next to be cancelled remains to the credit of the customer. a coin or washer larger than the coin for which the device is designed cannot be introduced into the receiving slot and a smaller one will not operate the device.] how to test a meter.--a simple test for ascertaining whether a customer's meter is fast or slow[c], may be applied as follows: 1. turn off the lamps and other power consuming devices in the house and then note the reading of the meter dial and the exact time of day; 2. turn on as quickly as possible about one-tenth of all the lamps in the house and allow them to burn for about two hours; 3. at the end of two hours, turn off the lamps as quickly as possible and note the reading of the meter dial. the difference between the first and second readings of the dial will be the indicated consumption of two hours, and if this be greater than the amount of power that ought to be consumed by the number of lamps turned on, the meter is fast, but if it be less, the meter is slow. the best results obtained by this method are only approximations, however, on account of the variations in the watts consumed by the different makes of lamp, the uncertainty as to the actual voltage on the line at the time of the test, and the lack of knowledge as to the age of the lamps. therefore, if the meter test within five per cent., or do not record more nor less than one-twentieth of the assumed lamp consumption it is safe to assume that the meter is correct as the result of the test is not likely to be any closer to the truth. [c] note.--a meter operates under more varied and exacting conditions than almost any other piece of apparatus. it is frequently subjected to vibration, moisture and extremes of temperature; it must register accurately on varying voltages and various wave forms; it must operate for many months without any supervision or attention whatever; and, in spite of all these conditions, it is expected to register with accuracy from a few per cent. of its rated capacity to a 50 per cent. overload. as a meter is a type of machine, its natural tendency is to run slow; but occasionally, through accident, a meter may run fast. when a meter runs fast the consumer is paying a higher rate per kilowatt hour than his contract calls for. he is being discriminated against. the periodic testing of meters is therefore a necessity and is an indication of the honesty of intention of the manager toward the customers of his company. meters controlling a very large amount of revenue may be tested as often as once a month, while the ordinary run of meters should be tested at least once a year, once in eighteen months, or once in two years, the period varying with different companies, different types and different civic requirements. commutator type meters, having comparatively heavy moving elements with consequent rapid increase in friction due to wear on the jewel and bearings, and a commutator also increasing in friction with age, must have frequent and expert attention to insure their accuracy under all conditions. ques. how should a roughened commutator be cleaned and smoothed? ans. by means of tape. [illustration: fig. 661.--internal connections of sangamo watt hour meter (type d). a, copper disc armature, submerged in mercury; b, bridge wire between binding posts, for main load current, when both sides of the line are carried through the meter; ct, compounding series turns around pressure circuit magnet, building up field as load increases, to compensate for falling off in speed otherwise found; d, aluminum damping, or brake disc, controlling speed of meter; e, copper contact ears, imbedded in insulating wall of mercury chamber, leading current into and out from armature; f, hardwood float on armature proportioned to give slight "lift" to entire moving system, when armature and float are immersed in mercury; h, soft steel disc above permanent magnets, riveted to fine pitch screw working in bracket above, so that adjustment of the disc up or down gives variation in damping effect of permanent magnets, and therefore of main speed. k, clamp slider with thumb screw, for obtaining light load adjustment by moving k to right or left, as may be necessary. k spans and connects parallel wires of light load adjustment, br and rr'. mm, powerful permanent magnets, acting on disc d, giving main speed control for meter. n, high resistance heavy wire, forming part of series adjustment between armature and any shunt with which meter may be used, to set drop through meter correct for drop of the shunt. p, spirally laminated soft steel ring, moulded in mercury chamber above the armature space, to act as a return for magnetic lines of force from and to energizing magnet below. r, resistance card unit, in series with pressure circuit coils; in 110 volt meters one card is used, in 220 volt meters two cards, or one card and a thermocouple. br, small brass wire, connected to ingoing end of pressure circuit coils and forming rr' and the slides k the light load adjustment. rr', high resistance wire having opposite ends connected to ears ee by low resistance wires. current energizing the pressure circuit coils sc passes from rr' through k to br and thence to the coils, and if k be near the end of rr' and br, the least compensation is obtained; if near right end, maximum light load compensation is obtained. s, shaft or spindle. in actual meter s is divided, the lower shaft carrying armature a, and the upper shaft damping disc d. sa, series resistance adjustment, for setting meter to correct drop for shunt. sc and sc', pressure coils connected in series. tt, binding posts at bottom of meter. y, laminated soft steel yoke, carrying coils sc and sc', and giving a powerful and concentrated magnetic field on the armature. w, worm, driving recording dial train. ww, worm wheel.] [illustration: fig. 662.--interior view of columbia watt hour meter (type d), showing construction and principal parts and connections. the armature winding consists of three coils approximately circular in shape. the coils are form wound, interlocked with one another and with the light impregnated fibre disc which serves as a spacer for them. the aluminum damper disc has the conventional anti-creep provision in the shape of the three small soft iron plugs, mounted close to the central staff, which the illustration shows. these in their revolution come successively within the influence of an adjustable iron screw which is magnetized by an extension from one of the damper magnets. the angular relationship of the armature windings and of the three iron plugs is such that at the time that the armature is exerting a maximum torque the magnetized screw is exerting the maximum pull to hold back a given plug and conversely when the armature pull is a minimum the magnetic screw is attracting a plug with the maximum effort to cause ahead rotation. the irregularities of torque are in this way smoothed out. the commutator has three segments and is made of chemically pure silver. each brush is formed of a length of phosphor bronze wire bent like a hair pin and secured at its "u" end to a brass sleeve, which in turn is secured to an insulated stud by a set screw. an extension on the sleeve carries a micrometer screw brush adjustment. the main speed adjustment is secured by providing a soft iron bridge plate, bridging over the extremities of each magnet end and adjustable, by means of a set screw and lock nut, to any desired distance therefrom. this gives a regular micrometer means of varying the effective magnet strength. interposed between the series coil and the permanent magnets is a heavy soft iron shield to guard the magnets against disturbance by short circuiting. light load adjustment is obtained by providing in the coil circuit a series of small resistance spools, equipped with pin terminals, to which connection can be selectively made by means of a split bushing terminal on a flexible cord. this series of spools is strung on a metal arbor located within the case.] [illustration: fig. 663.--diagram showing internal connections of the duncan watt hour meter. its operation depends upon the principle of the well known electro-dynamometer, in which the electromagnetic action between the currents in the field coils and an armature produces motion in the latter. it also embodies the other two necessary watt hour meter elements required for the speed control and registration of the revolutions of the armature, these being embodied in the drag magnet and disc, and the meter register respectively. the motion of the armature is converted into continuous rotation by the aid of a commutator and brushes, the commutator being connected to the armature coils and carried on the same spindle therewith.] waste of electricity in lighting.--in large residences where a good many servants are employed or in any place where the power consumed is not directly under the supervision of the person who must pay the bills, a great deal of waste usually occurs. if the meter be read before retiring, the reading in the morning will show how much energy was consumed during the night, which will show in turn how many lamps were burning all night. a great deal of light can be saved by placing the lamps so that they will throw the light where it is needed and by placing small lamps such as 8 candle power and 4 candle power in places where not much light is needed, such as bathrooms, halls, cellars, etc. when the lamps get old and dim they should be replaced with new ones, as it costs about the same to burn an old lamp as a new one. an old 16 candle power lamp which is very dim will give only about 8 candle power and use about as much current as is required for a new 16 candle power. if the dim light be light enough, it should be replaced by an 8 candle power lamp, which will not consume as much power as the old 16 candle power. chapter xxix operation of dynamos before starting a dynamo or motor.--when the machine has been securely fixed, it should be carefully examined to see that all parts are in good order. the examination should be made as follows: 1. the field magnet circuit should first be inspected to see that none of the wires or connections have broken or have become loose, and that the coils are correctly connected; 2. the caps of the bearings should be taken off, and these and the journals carefully cleaned of all grit and dirt. they should then be oiled, and the caps replaced and screwed up by hand only; 3. the gaps between the outer surface of the armature and the polar faces should be examined in order to ascertain whether any foreign body, such as a small screw or nail has lodged therein. if such be the case, it should be carefully removed with a bit of wire; 4. the guard plates protecting the armature windings should be removed, and the windings carefully inspected by slowly rotating the armature, to see that they are not damaged, and that the insulation is perfect. the armature should then be finally rotated by hand to see that it revolves freely, and that the bearings are securely fixed; 5. the commutator should be examined to see that it is not damaged in any way through one or more of the segments being knocked in, or the lugs being forced into contact with one another; 6. the brush holders and brushes should be inspected to see that the former work freely on the spindle, and that the hold off catches work properly, are clean and make good contact with the brush holders or flexible leads; 7. having ascertained that the machine is not injured in any way, and that the armature revolves freely, the brushes should be adjusted. in the subsequent working of the dynamo it will of course be unnecessary to follow the whole of these proceedings every time the machine is started, as it is extremely unlikely that the machine will be damaged from external causes while working without the attendant being aware of the fact. adjusting the brushes.--the _adjustment of the brushes_ upon the commutator requires careful attention if sparking is to be avoided. there are two adjustments to be made: 1. for pressure; the brushes must bear against the commutator segments with sufficient pressure for proper contact. 2. for lead. the brushes must have the proper angular advance (positive or negative, according as the machine is a dynamo or motor) to prevent sparking. ques. at what point on the commutator should the brushes bear? ans. the points upon the commutator at which the tips of the brushes (carried by opposite arms of the rocker) bear, should be, in bipolar dynamos, at opposite extremities of a diameter. in multipolar dynamos the positions vary with the number of poles and the nature of the armature winding. ques. what provision is made to facilitate the correct setting of the brushes? ans. setting marks are usually cut in the collar of the commutator next to the bearing. [illustration: figs. 664 and 665.--diagrams illustrating how to set brushes. some brush holders require brushes set _with_ the direction of rotation of the commutator, and others, set _against_ the direction of rotation. in fig. 664 is shown a brush holder of the first class, which must always be set as indicated by the arrow. if set in the opposite direction, trouble will ensue, as an inspection of the figure will show, because the surface of the commutator and the brush would form a toggle joint, and the brush would tend to dig into the commutator and either break itself or bend the brush rigging. in fig. 665 is shown a brush holder of the second type. this brush is set against the direction of rotation, but an inspection of the cut will show that there is, in this case, no tendency for the brush to dig into the commutator surface. each type of brush holder, of which there are several, should be adjusted as recommended by the manufacturer to secure proper working.] ques. how are the brushes set by these marks? ans. the tips of all the brushes carried by one arm of the rocker are set in correct line with the commutator segments marked out by one setting mark, and the tips of the brushes carried by the other arm or arms are set in correct line with the segments marked out by the other mark or marks. if one or more of the brushes in a set be out of line with their setting mark, it will be necessary to adjust the brushes up to this mark by pushing them out or drawing them back, as may be required, afterwards clamping them in position. when adjusting the brushes, the armature should always be rotated, so that the setting marks are horizontal. the rocker can then be rotated into position, and the tips of both sets of brushes conveniently adjusted to their marks. in those brush holders provided with an index or pointer for adjusting the brushes, the setting marks upon the commutator are absent, length of the pointer being so proportioned that when the tips of the brushes are in line with the extreme tips of the pointers, the brushes bear upon the correct positions on the commutator. [illustration: fig. 666.--method of soldering cable to carbon brush. drill a hole in the end, also in the side of the brush, as shown in the sketch, and after thoroughly tinning the "pig-tail," place it in the end hole and fill the holes up with solder through the side hole. another method is to drill a hole through the carbon so that the cable will just slip through, countersink the edge of the hole a little, clean the cable thoroughly and pass it through the hole. then with any good flux and solder, fill the countersunk part on both sides.] ques. what should be done after adjusting the brushes to their correct positions upon the commutator? ans. their tips or rubbing ends should be examined while in position to see that they bed accurately on the surface of the commutator. in many instances it will be found that this is not the case, the brushes sometimes bearing upon the point or toe, and sometimes upon the heel, so that they do not make contact with the commutator throughout their entire thickness and width. the angle of the rubbing ends will therefore need to be altered by filing to make them lie flat. ques. how is the proper brush contact secured? ans. when the brushes do not bed properly they should be refitted to secure proper contact. ques. how is the pressure adjustment made? ans. this is effected by regulating the tension of the springs provided for the purpose upon the brush holders. ques. with what pressure should the brushes bear against the commutator? ans. the tension of the springs should be just sufficient to cause the brushes to make a light yet reliable contact with the commutator. the contact must not be too light, otherwise the brushes will vibrate, and thus cause sparking; nor must it be too heavy, or they will press too hard upon the commutator, grinding, scoring and wearing away the latter and themselves to an undesirable extent, and moreover, giving rise to heating and sparking. the correct pressure is attained when the brushes collect the full current without sparking, while their pressure upon the commutator is just sufficient to overcome ordinary vibration due to the rotation of the commutator. [illustration: figs. 667 to 669.--method of winding cables with marlin. when connecting the feeders and dynamo and service leads to a switchboard, the wires are often _served_ with marlin. by serving is meant to tightly wrap the wires of each set together with marlin. a tool for serving may be made as in fig. 667, using a piece of oak 2 ins. wide, 7/8 in. thick and 14 ins. long, having four holes drilled through it, as shown. the marlin is passed through the holes, commencing at the hole nearest the handle, the object being to cause a strain on the marlin at the point where it passes around the wire, so that the marlin may be wrapped tightly. it is necessary to serve the first four or five inches by hand, pushing the winding into the conduit as far as possible. this acts as an additional protection to the wires where they leave the conduit. the serving is continued, as in fig. 668, to within four or five inches of the first lug by means of the serving tool, passing the ball of marlin around the wires with the serving tool. the wires are then bent in shape, as in fig. 669. to serve the wires properly it is necessary to tie the ends of the wires taut. the wires should be straightened and run together so as to be parallel, being bound with tape at different points to keep them so. when the serving is complete the marlin should be thoroughly painted with a moisture resisting compound. the marlin serving will stiffen the wires and they can be bent very neatly to avoid touching the bus bars of the board. when painted the marlin hardens so that it is difficult to bend the wires after the paint has dried. it then requires a strong pressure to bend them. the marlin acts as an additional insulation and mechanical protection to the wires, and while no harm would result from the wires coming in contact with the bars while thus protected, it looks better to bend them so as to avoid touching the bars.] direction of rotation.--this is sometimes a matter of doubt and often results in considerable trouble. as a general rule, a dynamo is intended to run in a certain direction; either right handed or left handed according to whether the armature, when looked at from the pulley end, revolves with or against the direction of the hands of a clock. dynamos are usually designed to run right handed, but the manufacturers will make them left handed if so desired. it may be necessary to reverse the direction of rotation of a dynamo, if the driving pulley to which it has to be connected happen to revolve left handed, or if it be necessary to bring the loose side of the belt on top of the pulley, or to place the machine in a certain position on account of limited space. the direction of rotation of ordinary series, shunt, or compound bipolar dynamos may be reversed by simply reversing the brushes without changing any of the connections, then changing the point of contact of the brush tips 180â°. in multipolar dynamos, a similar change, amounting to 90â° for a four pole machine, and 45â° for an eight pole machine, will reverse their direction of rotation. it will be understood that under these conditions, the original direction of the current and the polarity of the field magnets will remain unchanged. this rule does not apply to arc dynamos and other machines, which have to be run in a certain direction only, in order to suit their regulating devices. if the direction of current generated by a dynamo be opposite to that desired, the two leads should be reversed in the terminals, or the residual magnetism should be reversed by a current from an outside source. [illustration: fig. 670.--method of assembling core discs. for this operation two wooden "horses" should be provided to support the core at a convenient height, as shown in the illustration.] starting a dynamo.--having followed the foregoing instructions, all keys, spanners, bolts, etc., should be removed from the immediate neighborhood of the machine, and the dynamo started. [illustration: figs. 671 and 672.--tinning block for electric soldering tool. it is made with two soft bricks. one brick is used to support the soldering tool, and the other to contain the tinning material and to furnish a material which will keep the copper bit bright enough to receive its coating of "tin." fig. 671 represents a section of the tinning brick, which is scooped out on top as shown by the lower line. into one end of the hollow in the brick, some sal-ammoniac is placed to help tin the copper bit. sal-ammoniac is a natural flux for copper and aids greatly in keeping the tool well tinned. next, some melted solder is run into the hollow of the brick, and lastly enough resin to fill the cavity nearly to the top. when the tool is not in use, the electricity is switched off and the tool permitted to lie in the resin. if it be desired to repair the tin coating a little when the tool is in use, the latter is rubbed on the brick below the layer of solder, and the layer of resin. if the tool be in very bad condition, it may be pushed into the sal-ammoniac once or twice and then rubbed in the solder again. it requires but little heat to keep the brick and its contents ready for use. in fact, the brick is a fair non-conductor of heat and prevents the escape of heat from one side of the tool. when momentarily not in use, the tool remains in the solder which becomes melted underneath the layer of resin. when the copper bit becomes too hot, it will begin to volatilize the resin, thus calling attention to this fact, whereupon, the electricity should be turned off from the tool.] ques. how should a dynamo be started? ans. a dynamo is usually brought up to speed either by starting the driving engine, or by connecting the dynamo to a source of power already in motion. in the first case, it should be done by a competent engineer, and in the second case by a person experienced in putting on friction clutches to revolving shafts, or in slipping on belting to moving pulleys. [illustration: fig. 673.--connections for two shunt wound dynamos to run in parallel. the positive lead of each machine is connected to the same bus bar. in starting, if but one machine is to be used, the dynamo is first brought up to speed and the voltage regulated by means of the rheostat r and the voltmeter v. the main switch is then thrown in. the connections for the field are taken off the dynamo leads so that the opening of the main switch will not open the field circuit and for this reason the field will begin to build up as soon as the machine is started. when but one of the machines is running, the idle machine is brought up to speed with the main switch open, and the voltage regulated by means of the rheostat and voltmeter until the voltages of the machines are the same. then the main switch is thrown in and the load on the machines (which is ascertained by the ammeters) is equalized by means of the rheostats. should there be any great difference in voltages, the higher one will run the other as a motor without changing the direction of rotation. the field current will remain unchanged, and the armature current of the low dynamo will be reversed, which will cause it to run as a motor in the same direction as it ran as a dynamo. when dynamos feeding current to motors are to be shut down, the switches on the motors should first be opened. otherwise some of the motor fuses will blow. as the voltage goes down the motors will draw more current to do the work. if a plant be shut down with the motor switches "in" it will generally be found impossible to start a shunt dynamo, the low resistance in the mains not allowing enough current to flow around the shunt fields to energize them.] ques. should the brushes be raised out of contact in starting? ans. the brushes should not be in contact in starting if there be any danger of reverse rotation, as might happen when the dynamo is driven by a gas engine. aside from this, it is desirable that the brushes be in contact, because they are more easily and better adjusted, and the voltage will come up slowly, so that any fault or difficulty will develop gradually and can be corrected, or the machine stopped before any injury is done. [illustration: fig. 674.--connections for two shunt dynamos to run on the three wire system. the two machines are connected in series, three wires being carried from them, one from the outside pole of each machine and one from the junction of the two machines. the voltage between the outside wires is equal to the combined voltage of the two machines and the voltage between the outside and the central or neutral wire is equal to the voltage of the corresponding machine. if the load on each side of the system be equal, there will be no current in the neutral wire, while if the loads be unequal, the neutral wire will have to carry the difference in current between the two outside wires.] ques. how should a series machine be started? ans. the external circuit should be closed, otherwise a closed circuit will not be formed through the field magnet winding and the machine will not build up. ques. what is understood by the term "build up"? ans. in starting, the gradual voltage increase to maximum. [illustration: fig. 675.--connections for two compound wound dynamos to run in parallel. the series fields of the machines are connected together in parallel by means of wire leads or bus bars, which connect together the brushes from which the series fields are taken. this is known as the equalizer and is shown by the line running to the middle pole of the dynamo switch. by tracing out the series circuits it will be seen that current from the upper brush of either dynamo has two connections to its bus bar. one of these leads through its own field, and the other, by means of the equalizer bar, through the fields of the other dynamo. as long as both machines are generating equally there is no difference of pressure between the brushes of either, but should the voltage of one be lowered, current from the other would flow through its fields and thereby raise the voltage, and at the same time reduce its own until both are equal. the equalizer may then be called upon to carry much current, but to have the machines regulate closely it should be of low resistance. it may also be run as shown by the dotted lines, but this will leave all the machines alive when any one is generating. the ammeters should be connected as shown. if they were on the other side they would come under the influence of the equalizing current and would indicate wrong, either too high or too low. the equalizer switch should be closed a little before the main switches are closed.] ques. how should a shunt or compound machine be started? ans. all switches controlling the external circuits should be opened, as the machine excites best when this is the case. if the machine be provided with a rheostat or hand regulator and resistance coils, these latter should all be cut out of circuit, or short circuited, until the machine excites, when they can be gradually cut in as the voltage rises. when the machine is giving the correct voltage, as indicated by the voltmeter or pilot lamp, the machine may be switched into connection with the external or working circuits. ques. in starting a shunt dynamo, should the main line switch be closed before the machine is up to voltage or after? ans. if the machine be working on the same circuit with other machines, or with a storage battery, it is, or course, necessary to make the voltage of the machine equal to that on the line before connecting it in the circuit. if the machine work alone, the switch may be closed either before or after the voltage comes up. the load will be thrown on suddenly if the switch be closed after the machine has built up its voltage, thus causing a strain on the belt, and possibly drawing water over the engine cylinder. on the other hand, if the switch be closed before the voltage of the machine has come up, the load is picked up gradually, but the machine may be slow or may even refuse to pick up at all. ques. why does a shunt machine pick up more slowly if the main switch be closed first? ans. because the resistance of the main line is so much less than that of the field that the small initial voltage due to the residual magnetism causes a much larger current in the armature than in the shunt field. if this be too large, the cross and back magnetizing force of the armature weakens the field more than the initial field current strengthens it, and so the machine cannot build up. ques. if a shunt dynamo will not pick up, what is likely to be the trouble? ans. the speed may be too slow; the resistance of the external circuit may be too small; the brushes may not be in proper position; some of the electrical connections in the dynamo may be loose, broken or improperly made; the field may have lost its residual magnetism. [illustration: figs. 676 and 677.--diagrams of ground detectors. fig. 676, a ground detector switch suitable for mounting on a switch board. the two arms pivoted at their upper ends are connected with an insulating bar a and make contact at their lower ends with two brass strips and a contact button, which are connected to the bus bars and ground, respectively. when the arms are moved to the left, the positive bus bar is connected to the ground through the voltmeter v. in fig. 677 is another form of ground detector. this is known as a lamp ground detector. on a 110 volt system two ordinary lamps are connected in series, while the line connecting the lamps is connected to the ground through a snap switch s. when current is on, the two lamps will burn with equal brilliancy, but at a lower candle power. when the switch s is closed, if the two lines be clear, the brilliancy of the lamps will not be affected, but if there be a ground on the positive side, one lamp will burn brighter, the brightness depending on the resistance of the ground. if there be a dead ground, the lamp will burn to its full candle power.] ques. what is the indication that the connections between the field coils and armature are reversed? ans. if the machine build up when brought to full speed, the connections are correct, but if it fail to build up, the field coils may be improperly connected. [illustration: fig. 678.--method of correcting reversed polarity in large shunt dynamo by transposing the shunt field leads, and then starting up the machine. as soon as the voltmeter registers any voltage, the dynamo may be stopped and the field leads restored to their original position, when it will be found that the residual magnetism in the pole pieces will usually bring the dynamo up to its polarity and proper voltage. this method has the disadvantages, of the uncertainty as to the machine building up, and that a temporary wire must probably be run from the switchboard to one terminal of the field circuit, which is usually connected to a terminal back of the dynamo frame, so that the flow of current through the field coils may be reversed. with dynamos having laminated field magnet cores of comparatively low residual magnetism, this method may suffice, but in the case of solid field magnetic cores it is not practical. a better method is to disconnect the shunt field leads and temporarily extend them to some other source of direct current. if the current be of higher voltage than the coils are designed for, as for instance 110 volt dynamo and available current 500 volt, caution must be exercised and a suitable resistance be provided to protect the coils. a 500 volt coil, however, may be supplied from 110 volt circuit, providing the field winding to be energized is equipped with a cut off switch having a discharge resistance, so that it may be used to close and break the circuit when the temporary leads have been connected. if the field windings be not so provided, a bank of lamps or some other non-inductive resistance must be connected across the leads between the field magnet coils and the point at which the circuit is to be opened and closed. this is to provide a path for the discharge of the induced electromotive force. the circuit should not remain closed more than a few seconds if the full voltage can be applied. it is well, however, to leave the current on long enough to run the machine up to about half speed and make sure, by means of a voltmeter, that the polarity has been corrected. when this has been ascertained the dynamo should be stopped and the field winding leads returned to their proper terminals. then the voltage will be brought up in the right direction, provided the work has been done correctly.] this can be tested by connecting a voltmeter across the terminals of the armature, or by means of a magnetic needle placed at a short distance from one of the pole pieces in such a position that it does not point to the north pole. if the field coils be improperly connected, the current due to the initial voltage will weaken the field magnetism and thus prevent the machine building up, and when the field circuit is closed the voltmeter reading will be reduced, or the magnetic needle will be less strongly attracted. ques. what will be the result if the connections of some of the field coils of a dynamo be reversed? ans. if one-half the number of coils oppose the other half, the field magnetism will be neutralized and the machine will not build up at all; but if one of the coils be opposed to the others, the machine might build up, but the generated voltage will be low, and there will be considerable sparking at some of the brushes. ques. how may it be ascertained which coil is reversed? ans. in all dynamos there should be an equal number of positive and negative poles, and in almost all of them the poles should be alternately positive and negative. therefore, if a pocket compass be brought near the pole pieces, and it show that there are more poles of one kind than the other, the indication is that one or more of the coils are reversed, and the improper sequence of alternation will determine which one is wrongly connected. ques. when a dynamo loses its residual magnetism, how can it be made to build up? ans. by temporarily magnetizing the field. to do this a current is passed through it from another dynamo, or from the cells of a small primary battery. usually, this will set up sufficient initial magnetism to allow the machine to build up. the battery circuit should be broken before the machine has built up to full voltage. ques. what should be done if a dynamo become reversed by a reversal of its field magnetism due to lightning, short circuit, or otherwise? ans. the residual magnetism should be reversed by a current from another dynamo, or from a battery; but if this be not convenient, the connections between the machine and the line should be crossed so that the original positive terminal of the dynamo will be connected to the negative terminal of the line, and vice versa. [illustration: fig. 679.--method of correcting reversed polarity in compound wound dynamo. the polarity may be reversed without disconnecting or changing the wire. the figure shows two compound dynamos, and essential connections. the current from any machine connected to the equalizer bar by its equalizer switch will divide, a portion going through the series field winding of the other machines connected to the bus bar, the division being determined by the resistance of the different sets of coils. for instance, assume that no. 1 dynamo has had its polarity reversed and that no. 2 is running connected to the bus bar. the method of reversing the polarity of no. 1 machine is as follows: no. 1 machine should be at rest and then make sure that the circuit breaker and negative switch are open and that any other special connections to other machine or station lighting circuits are open. then close the positive and equalizer switches, thus allowing a part of the current from the other dynamo to pass through the equalizer connection and through the series field winding of no. 1 machine in the usual direction, which will magnetize the magnetic core. if no. 1 machine be a large unit and no. 2 a small unit, it will be necessary to cut out the resistance of the shunt field circuits by means of the rheostat, if it be desired to maintain its bus bar voltage at its normal point. this will rob the series winding of any other machines which may be connected to the bus bars and will lower the voltage slightly. no. 1 machine is then brought up to full speed when it will be found to have recovered its correct polarity. the positive switch may be readily opened, watching the bus bar voltage closely as it will rise when the current is restricted again to the series field winding of the other machines. the dynamo will then be ready to cut in with the other machines as soon as the voltage has been brought up to the proper point, or it may be shut down until required.] ques. can a dynamo be reversed by reversing the connections between the field coils and the armature? ans. no, for if these connections be reversed, the machine will not build up. ques. will a dynamo build up if it become reversed? ans. yes. ques. then what is the objection to a reversed dynamo? ans. since the direction of current of a reversed dynamo is also reversed, serious trouble may occur if it be attempted to connect it in parallel, with other machines not reversed. attention while running.--when a dynamo is started and at work, it will need a certain amount of attention to keep it running in a satisfactory and efficient manner. the first point to be considered is the adjustment of the brushes. if this be neglected, the machine will probably spark badly, and the commutator and brushes will frequently require refitting to secure good contact. ques. what may be said with respect to the lead of the brushes? ans. the lead in all good dynamos is very small, and varies with the load and class of machine. the best lead to give to the brushes can in all cases be found by rotating the rocker and brushes in either direction to the right or left of the neutral plane until sparking commences, increasing with the movement. the position midway between these two points is the correct position for the brushes, for at this position the least sparking occurs, and it is at this position that the brushes should be fixed by clamping the rocker. [illustration: fig. 680.--method of taking temperature. in taking the temperature of a hot part, it is convenient to use a thermometer in which the scale of degrees has been etched on the stem. bind this to the heated part, having first taken the precaution to cover the bulb with waste to prevent the radiation of heat and take the reading when the column of mercury has ceased to rise. the question which most often presents itself to the attendant is how hot can the various parts of a dynamo or motor become and yet be within the safe limit. the degree of heat can be determined by applying the hand to the various parts. if the heat be bearable it is entirely harmless, but if the heat become unbearable to the hand for more than a few seconds, the safety limit has been reached and the machine should be stopped and the fault located. of course when the solder begins to melt at the commutator connections and shellac begins to "fry out" of the armature and an odor of burnt cotton begins to pervade the air, the safe limit has been far exceeded, and in most cases, as a matter of fact serious damage is the result. to be more definite, _no part of the dynamo or motor should be allowed to rise in temperature more than 80 degrees f. above the temperature of the surrounding air_, excepting in the case of commutators where no solder has been used to connect the leads. these can be allowed to rise to a still higher temperature.] ques. how does the lead vary in the different types of dynamo? ans. in series dynamos giving a constant current, the brushes require practically no lead. in shunt and compound dynamos the lead varies with the load, and therefore the brushes must be rotated in the direction of rotation of the armature with an increase of load, and in the opposite direction with a decrease of load. in cases where the dynamos are subjected to a rapidly varying or fluctuating load, it is of course not possible to constantly shift the brushes as the load varies, therefore the brushes should be fixed in the positions where the least sparking occurs at the moment of adjustment. if at any time violent sparking occur, which cannot be reduced or suppressed by varying the position of the brushes by rotating the rocker, the machine should be shut down at once, otherwise the commutator and brushes are liable to be destroyed, or the armature burnt up. this especially refers to high tension machines. ques. what should be done if the brushes begin to spark excessively? ans. first, look at the ammeter to see if an excessive amount of current is being delivered; second, see if the brushes make good contact with the commutator, and if the latter have a bar too high, or too low, and an open circuit. [illustration: figs. 681 and 682.--remedies for leakage of oil from self-oiling bearings. if there be sufficient space, a metal ring may be attached to the shaft as in fig. 681. with this arrangement the high speed of the shaft will carry the oil outside of the ring and throw it off in the oil reservoir. another way is to insert a tin apron, as shown in fig. 682 at t, which will serve to drain the oil which may creep along the shaft, and also cut off the draft from the pulley which may suck the oil out of the bearing. sometimes a tin fan is attached to the pulley, which tends to drive the oil back into the bearing, and which also assists in keeping the box cool.] ques. what should be done if the current be excessive? ans. if the current exceed the rated capacity by more than 50 per cent., and continue for more than a few minutes, the main switch should be opened, otherwise the machine may be seriously injured. ques. how does an excessive current injure a dynamo? ans. by causing it to overheat which destroys the insulation of the armature, commutator, etc. lubrication.--the shaft bearings of dynamos may be lubricated by sight feed oilers or oil rings. the latter method is almost universally used. an oil well is provided in the hollow casting of the pedestals as shown in fig. 728. oil rings revolve with the shaft and feed the latter with oil, which is continuously brought up from the reservoir below. the dirt settles to the bottom and the upper portion of the oil remains clear for a long period, after which it is drawn off through the spigot and a fresh supply poured in through openings provided in the top. the latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated directly by means of an oil cup, if the rings fail to act or the reservoir become exhausted. [illustration: fig. 683.--imaginative view of a shaft showing its rough granular structure. in operation these minute irregularities interlock and act as a retarding force, or frictional resistance. hence, the necessity for lubrication--a lubricant presents a thin intervening film against which the surfaces rub.] ques. what kind of oil can should be used in filling the reservoir, or oil cups? ans. one made of some non-magnetic material such as copper, brass, or zinc. if iron cans be used, they are liable to be attracted by the field magnets, and thus possibly catch in the armature. ques. what is the indication of insufficient lubrication? ans. the bearings become unduly heated. ques. what precaution should be taken with new dynamos? ans. they are liable to heat abnormally and for the first few days they should be carefully watched and liberally supplied with oil. after a dynamo has been running for a short time under full load, its armature imparts a certain amount of heat to the bearings, a little more also to the bearing on the commutator end of shaft; beyond this there is no excuse for excessive heating. the latter may result from various causes, some of which are given with their remedies, as follows: 1. a poor quality of oil, dirty or gritty matter in the oil; 2. journal boxes too tight; 3. rough journals, badly scraped boxes; 4. belt too tight; 5. bearings out of line; 6. overloaded dynamo; 7. bent armature shaft. ques. what is the allowable degree of heating? ans. it may be taken as a safe rule that no part of a working dynamo should have a temperature of more than 80â° fahr. above that of the surrounding air. accordingly, if the temperature of the engine room be noted before applying the thermometer to the machine, it can at once be seen if the latter be working at a safe temperature. in taking the temperature, the bulb of the thermometer should be wrapped in a woolen rag. the screws and nuts securing the different connections and cables should be examined occasionally, as they frequently work loose through vibration. [illustration: fig. 684.--diagram illustrating forces acting on a dynamo armature. in the figure the normal field magneto-motive force is in the direction of the line 1, 2, produced by the field circuit g, if there were no current in the armature. but as soon as the armature current flows, it produces the opposing force 3, 4, which must be combined with 1, 2 to give the resulting force to produce magnetism and hence voltage. the resultant 1, 5, if 3, 4 be large enough, does not differ much from the original force 1, 2. or, expressed in a more physical way, the brushes e, f, rest on the commutator and all the turns embraced by twice the angle 6, 3, f, oppose the flow of flux through the armature core as well as all the turns embraced by twice the angle, 7, 3, e. the remaining turns distort the flux, making the pole corners at a and b denser, and at c and d rarer. so that all the effect is to kill an increase of flux, or voltage. this cross magnetism tends also to decrease the flow of flux, for the extra ampere turns required to force the flux through the dense pole tips are greater than the decreased ampere turns relieved by the reduction of flux at the other pole tips; this follows, since iron as it increases in magnetic density requires ampere turns greater in proportion than the increase of flux.] instructions for stopping dynamos.--when shutting down a machine, the load should first be gradually reduced, if possible, by easing down the engine; then when the machine is supplying little or no current, the main switch should be opened. this reduces the sparking at the switch contacts, and prevents the engine racing. when the voltmeter almost indicates zero, the brushes should be raised from contact with the commutator. this prevents the brushes being damaged in the event of the engine making a backward motion, which it often does, particularly in the case of a gas engine. on no account, however, should the brushes be raised from the commutator while the machine is generating any considerable voltage; for not only is the insulation of the machine liable to be damaged, but in the case of large shunt dynamos, the person lifting the brushes is liable to receive a violent shock. ques. what attention should the machine receive after it has been shut down? ans. it should be thoroughly cleaned. any adhering copper dust, dirt, etc., should be removed from the armature by dusting with a stiff brush, and the other portions of the machine should be thoroughly cleaned with linen rags. waste should not be used, as it is liable to leave threads or fluff on the projecting parts of the machine, and on the windings of the armature, which is difficult to remove. ques. what attention should be given to the brushes and brush gear? ans. they should be examined and thoroughly cleaned. if necessary the brushes should be refitted and readjusted. all terminals, screws, bolts, etc., should be carefully cleaned and screwed up ready for the next run. the brush holders should receive special attention, as when dirty, they are liable to stick and cause sparking. all dirt and oil should be removed from the springs, contacts, pivots, and other working parts. it is advisable at stated intervals to entirely remove the brush holders from the rocker arms, and give them a thorough cleaning by taking them to pieces, and cleaning each part separately with emery cloth and benzoline or soda solution. another point to which particular attention should be given is the cleaning of the brush rocker. this being composed wholly of metal, and the two sets of positive and negative brushes being only separated from it by a few thin insulating washers, it follows that if any copper dust given off by the brushes be deposited in the neighborhood of these washers, there is considerable liability for a short circuit of the machine to occur by the dust bridging across the insulation. ques. what further attention should be given? ans. it is a good plan, when the machine has been thoroughly cleaned and all connections made secure, to occasionally test the insulation of the different parts. if a record be kept of these tests, any deterioration of the insulation can at once be detected, localized and remedied before it has become sufficiently bad to cause a breakdown. as a means of protecting the machine from any moisture, dirt, etc., while standing idle, it is advisable to cover it with a suitable waterproof cover. chapter xxx coupling of dynamos series and parallel connections.--when it is necessary to generate a large and variable amount of electrical energy, as must be done in central generating stations, apart from the question of liability to breakdown, it is neither economical nor desirable that the whole of the energy should be furnished from a single dynamo. since the efficiency of a dynamo is dependent upon its output at any moment, or the load at which it is worked (the efficiency varying from about 95 per cent. at full load to 80 per cent. at half load), it is advisable in order to secure the greatest economy in working, to operate any dynamo as near full load as possible. under the above circumstances, when the whole of the output is generated by a single dynamo this can evidently not be effected, for the load will naturally fluctuate up and down during the working hours, as the lamps, motors, etc., are switched into and out of circuit; hence, although the dynamo may be working at full load during a certain portion of the day, at other times it may probably be working below half load, and therefore the efficiency and economy in working in such an arrangement is very low. ques. how is maximum efficiency secured with variable load? ans. it is usual to divide up the generating plant into a number of units, varying in size, so that as the load increases, it can either be shifted to machines of larger size, or when it exceeds the capacity of the largest dynamo, the output of one can be added to that of another, and thus the dynamos actually at work at any moment can be operated as nearly as possible at full load. ques. what should be noted with respect to connecting one dynamo to another? ans. it is necessary to take certain precautions (as later explained) in order that the other dynamos may not be affected by the change, and that they may work satisfactorily together. ques. what are the two methods of coupling dynamos? ans. they are connected in series, or in parallel. in coupling dynamos in series, the current capacity of the plant is kept at a constant value, while the output is increased in proportion to the pressures of the machines in circuit. when connected in parallel, the pressures of all the machines are kept at a constant value, while the output of the plant is increased in proportion to the current capacities of the machines in circuit. coupling series dynamos in series.--series wound dynamos will run satisfactorily together without special precautions when coupled in series, if the connections be arranged as in fig. 685. the positive terminal of one dynamo is connected to the negative terminal of the other, and the two outer terminals are connected directly to the two main conductors or bus bars through the ammeter a, fuse f, and switch s. if it be desired to regulate the pressure and output of the machines, variable resistances, or hand regulators r, r^1, may be arranged as shunts to the series coils as shown, so as to divert a portion or the whole of the current therefrom. series dynamos in parallel.--simple series wound dynamos not being well adapted for the purpose of maintaining a constant pressure, are in practice seldom coupled in parallel; the conditions or working, however, derive importance from the fact that compound dynamos, being provided with series coils, are subject to similar conditions when working in parallel, which is frequently the case. ques. what may be said with respect to coupling two or more plain series dynamos in parallel? ans. the same procedure cannot be followed as in the case of plain shunt dynamos, for the reason that if the voltage of the dynamo to be coupled be exactly equal to that of the bus bars when connected in parallel, the combination will be unstable. [illustration: fig. 685.--diagram showing method of coupling series dynamos in series. r and r' are two hand regulators which are placed in shunt across the coil terminals to regulate the pressure and output of the machine.] ques. why is this? ans. if, from any cause, the pressure at the terminals of one of the dynamos fall below that of the others, it immediately takes a smaller proportion of the load; as a consequence, the current in its field coils is reduced, and a further fall of pressure immediately takes place. this again causes the dynamo to relinquish a portion of its load, and again occurs a further fall of pressure. thus the process goes on, until finally the dynamo ceases to supply current, and the current from the other dynamos flowing in its field coils in the reverse direction reverses its magnetism, and causes it to run as a motor against the driving power in the opposite direction to that in which it previously ran as a dynamo. under such circumstances the armature is liable to be destroyed if the fuse be not immediately blown, and in any case it is subjected to a very detrimental shock. this tendency to reverse in series dynamos can be effectually prevented by connecting the field coils of all the dynamos in parallel. [illustration: fig. 686.--diagram showing method of coupling series dynamos in parallel. in the diagram a, a', are ammeters; f, f', fuses; s, s', switches.] ques. how are the field coils of all the dynamos connected in parallel? ans. this is effected in practice by connecting the ends of all the series coils where they join on to the armature circuit by a third connection, called the "equalizing connection," or "equalizer," as shown in fig. 686. ques. what is the effect of the equalizer? ans. the immediate effect is to cause the whole of the current generated by the plant to be divided among the series coils of the several dynamos in the inverse ratio of their resistance, without any regard as to whether this current comes from one armature, or is divided among the whole. the fields of the several dynamos being thus maintained constant, or at any rate being caused to vary equally, the tendency for the pressure of one dynamo to fall below that of the others is diminished. shunt dynamos in series.--the simplest operation in connection with the coupling of dynamos, and the one used probably more frequently in practice than any other, is the coupling of two or more shunt dynamos to run either in series or in parallel. when connected in series, the positive terminal of one machine is joined to the negative of the other, and the two outer terminals are connected through the ammeter a, fuses f, f', and switch s, to the two main conductors or omnibus bars as represented in fig. 687. the machine will operate when the connections are arranged in this manner, if the ends of the shunt coils be connected to the terminals of their respective machines. shunt dynamos in parallel.--the coupling of two or more shunt dynamos to run in parallel is effected without any difficulty. this method of coupling dynamos is one that is very frequently used. fig. 688 illustrates diagrammatically the method of arranging the connections. the positive and negative terminals of each machine are connected respectively to two massive insulated copper bars, shown at the top of the diagram, called _omnibus bars_, through the double pole switches, s, s', and the double pole fuses f, f'. ammeters, a, a' are inserted in the main circuit of each machine, and serve to indicate the amount of current generated by each. an automatic switch or cutout, ac, ac', is also shown as being included in the main circuit of each of the machines, although this appliance is sometimes dispensed with. the pressure of each of the machines is regulated independently by means of the hand regulators r, r', inserted in series with the shunt circuit. the shunt circuits are represented as being connected to the positive and negative terminals of the respective machines, but in many cases where the load is subjected to sudden variations, and when a large number of machines is connected to the bus bars, the shunt coils are frequently connected direct to these. in such circumstances this method is preferable, as by means of it the fields of the idle dynamos can be excited almost at once direct from the bus bars by the current from the working dynamos; hence, if a heavy load come on suddenly, no time need be lost in building up a new machine previous to switching it into parallel. the pressure of the lamp circuit is given by a voltmeter whose terminals are placed across the bus bars; and the pressure at the terminals of each of the machines is indicated by separate voltmeters or pilot lamps, the terminals of which are connected to those of the respective machines. [illustration: fig. 687.--diagram showing method of coupling shunt dynamos in series. the ends of the shunt coils may be connected to the terminals of their respective machine, or they may be connected in series as shown.] ques. describe a better method of parallel connection. ans. better results are obtained by connecting both the shunt coils in series with one another, so that they form one long shunt between the two main conductors, the same as in fig. 687. when arranged in this way, the regulation of both machines may be effected simultaneously by inserting a hand regulator (r) in series with the shunt circuit as represented. [illustration: fig. 688.--diagram showing method of coupling shunt dynamos in parallel.] switching dynamo into and out of parallel.--in order to put an additional dynamo in parallel with those already working, it is necessary to run the new dynamo up to full speed, and, when it excites, regulate the pressure by means of a hand regulator until the voltmeter connected to the terminals of the machines registers one or two volts more than the voltmeter connected to the lamp circuit, and then close the switch. the load upon the machine can then be adjusted to correspond with that upon the other machines by means of the hand regulator. ques. in connecting a shunt dynamo to the bus bars, must the voltage be carefully adjusted? ans. there is little danger in overloading the armature in making the connection hence the pressure need not be accurately adjusted. it is, in fact, common practice in central stations to judge the voltage of the new dynamo merely by the appearance of its pilot lamp. ques. how is a machine cut out of the circuit? ans. when shutting down a machine, the load or current must first be reduced, by gradually closing the stop valve of the engine, or inserting resistance into the shunt circuit by means of the hand regulator; then when the ammeter indicates nine or ten amperes, the main switch is opened, and the engine stopped. by following this plan, the heavy sparking at the switch contacts is avoided, and the tendency for the engine to race, reduced. ques. what precaution must be taken in reducing the current? ans. care must be taken not to reduce the current too much. ques. why is this necessary? ans. there is danger that the machine may receive a reverse current from the other dynamos, resulting in heavy sparking at the commutator, and in the machine being driven as a motor. ques. what provision is made to obviate this danger? ans. dynamos that are to be run in parallel are frequently provided with automatic cutouts, set so as to automatically switch out the machine when the current falls below a certain minimum value. dividing the load.--if a plant, composed of shunt dynamos running in parallel, be subjected to variations of load, gradual or instantaneous, the dynamos will, if they all have similar characteristics, each take up an equal share of the load. if, however, as is sometimes the case, the characteristics of the dynamos be dissimilar, the load will not be shared equally, the dynamos with the most drooping characteristics taking less than their share with an increase of load, and more than their share with a decrease of load. if the difference be slight, it may be readily compensated by means of the hand regulator increasing or decreasing the pressures of the machines, as the load varies. if, however, the difference be considerable, and the fluctuations of load rapid, it becomes practically impossible to evenly divide the load by this means. under such circumstances, the pressure at the bus bars is liable to great variations, and there is also liability of blowing the fuses of the overloaded dynamos, thus precipitating a general breakdown. to cause an equal division of the load among all the dynamos, under such circumstances, it is needful to insert a small resistance in the armature circuits of such dynamos as possess the straightest characteristics, or of such dynamos as take more than their share of an increase of load. by suitably adjusting or proportioning the resistances, the pressures at the terminals of all the machines may be made to vary equally under all variations of load, and each of the machines will then take up its proper share of the load. coupling compound dynamos in series.--since compound dynamos may be regarded as a combination of the shunt and series wound machines, and as no special difficulties are encountered in running these latter in series, analogy at once leads to the conclusion that compound dynamos under similar circumstances may be coupled together with equal facility. ques. how are compound dynamos connected to operate in series? ans. the series coils of each are connected as in fig. 685, and the shunt coils are connected as a single shunt as in fig. 687, which may either extend simply across the outer brushes of the machines, so as to form a double short shunt, or may be a shunt to the bus bars of external circuit, so as to form a double long shunt. [illustration: fig. 689.--coupling compound dynamos in series; short shunt connection. the dotted lines indicate the changes that would be made for long shunt connection.] compound dynamos in parallel.--machines of this type will not run satisfactorily together in parallel unless all the series coils are connected together by an equalizing connection, as in series dynamos. the method of arranging the connections as adopted in practice, being illustrated in fig. 690. by means of it idle machines are completely disconnected from those at work. ques. how is the equalizer connected? ans. the equalizer is connected direct to the positive brushes of all the dynamos, a three pole switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working. the two contacts of the switch are respectively connected to the positive and negative conductors, while the central contact is connected to the equalizer. [illustration: fig. 690.--diagram showing method of coupling compound dynamos in parallel.] switching a compound dynamo into and out of parallel.--if the characteristics of all the dynamos be similar, and the connections arranged as in figs. 690, or 691, the only precaution to be observed in switching a new machine into parallel is to have its voltage equal, or nearly equal to that of the bus bars previous to closing the switch. if this be the case, the new machine will take up its due share of the load without any shock. [illustration: fig. 691.--diagram showing another and better method of coupling compound dynamos in parallel. with this arrangement the idle machines are completely disconnected from those at work. the same reference letters are common in both diagrams. s, s' are switches; f, f' fuses; a, a' ammeters, which indicate the total amount of current generated by each of the machines; ac, ac', automatic switches, arranged for automatically switching out a machine in the event of the pressure at its terminals being reduced through any cause; r, r,' are hand regulators, inserted in the shunt circuits of each of the machines, by means of which the pressures of the individual machines may be varied and the load upon each adjusted. the pressure at the bus bars is given by the voltmeter v, one terminal of which is connected to each of the bars; a second voltmeter may be used, to give the pressure of any individual machine, by connecting "voltmeter keys" to the terminals of each of the machines, or a separate voltmeter may be used for each individual machine. the only essential difference between figs. 690 and 691 is, that in fig. 690 the equalizer is connected direct to the positive brushes of all the dynamos, while in fig. 691 the equalizer is brought up to the switchboard and arranged between the two bus bars, a switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working.] ques. how is a compound dynamo, running in parallel, cut out of circuit? ans. the load is first reduced to a few amperes, as in the case of shunt dynamos, either by easing down the engine, or by cutting resistance into the shunt circuit by means of the hand regulator, and then opening the switch. previous to this, however, it is advisable to increase the voltage at the bus bars to a slight extent, as while slowing down the engine the load upon the outgoing dynamo is transferred to the other dynamo armatures, and the current in their series coils not being increased in proportion, the voltage at the bus bars is consequently reduced somewhat. equalizing the load.--when a number of compound dynamos of various output, size, or make, are running together in parallel, it frequently happens that all their characteristics are not exactly similar, and therefore the load is unequally distributed, some being overloaded, while others do not take up their proper share of the work. note.--the action of an equalizing bar in equalizing the load on compound dynamos run in parallel may be explained as follows: the compound winding of a dynamo raises the pressure in proportion to the current flowing through it, and if, in a system of parallel operated compound dynamos without the equalizing connection, the current given by one machine were slightly greater than the currents from the others, the pressure of that machine would increase. with this increase in pressure above the other machines, a still greater current would flow, and so raise the pressure further. the effect is therefore cumulative, and in time the one dynamo would be carrying too great a proportion of the whole current of the system. with the equalizing connection, whatever the current flowing from each machine, the currents in the various compound windings are all equal, and so the added pressure due to the compound winding is practically the same in each machine. any inequality in output from the machines is readily eliminated by adjusting the shunt currents by means of the shunt rheostats. when compound wound dynamos are operated in parallel, the equalizer bar insures uniform distribution among the series coils of the machines. note.--to secure the best results in parallel operation, dynamos should be of the same design and construction and should possess as nearly as possible the same characteristics; that is, each should respond with the same readiness, and to the same extent, to any change in its field excitation. any number of such machines may be operated in parallel. the usual practice is to connect the equalizer and the series field to the positive terminal, though if desired, they may be connected to the negative terminal; both however, must be connected to the same terminal. the resistance of the equalizer should be as low as possible, and it must never be greater than the resistance of any of the leads from the dynamos to the bus bar. sometimes a third wire is run to the switchboard from each dynamo and there connected to an equalizer bar, but the usual practice is to run the equalizer directly between the dynamos and to place the equalizer switches on pedestals near the machines. this shortens the connections and leads to better regulation. the positive and equalizer switches of each machine differ in pressure only by the slight drop in the series coil, and in some large stations these two switches are placed side by side on a pedestal near the machine. in such cases, the equalizer and positive bus bars are often placed under the floor near the machines, so that all leads may be as short as possible. if all the dynamos be of equal capacity, all the leads to bus bars should be of the same length, and it is sometimes necessary to loop some of them. if the difference be small, it may be compensated by means of the hand regulator; if large, however, other means must be taken to cause the machines to take up their due proportion of the load. if the series coils of the several dynamos be provided with small adjustable resistances, in the form of german silver or copper ribbon inserted in series with the coils, the distribution of the current in the latter may be altered by varying the resistance attached to the individual coils. the effect of the series coils upon the individual armatures in raising the pressure may be adjusted, and the load thus evenly divided among the machines. shunt and compound dynamos in parallel.--it is not practicable to run a compound dynamo and a shunt dynamo in parallel, for, unless the field rheostat of the shunt machine be adjusted continually, the compound dynamo will take more than its share of the load. chapter xxxi dynamo fails to excite this trouble is of frequent occurrence in both old and new machines. if a dynamo fail to excite, the operator should first see that the brushes are in the proper position and making good contact, and that the external circuit is open if the machine be shunt wound, and closed if series wound. in starting a dynamo it should be remembered that shunt and compound machines require an appreciable time to build up, hence, it is best not to be too hasty in hunting for faults. the principal causes which prevent a dynamo building up are: 1. brushes not properly adjusted; 2. defective contacts; 3. incorrect adjustment of regulators; 4. speed too low; 5. insufficient residual magnetism; 6. open circuits; 7. short circuits; a. in external circuits; b. in dynamo. 8. wrong connections; 9. reversed field magnetism. brushes not properly adjusted.--if the brushes be not in or near their correct positions, the whole of the voltage of the armature will not be utilized, and will probably be insufficient to excite the machine. if in doubt as to the correct positions, the brushes should be rotated by means of the rocker into various points on the commutator, sufficient time being given the machine to excite before moving them into a new position. defective contacts.--if the different points of contact of the connections of the machine be not kept thoroughly clean and free from oil, etc., it is probable that enough resistance will be interposed in the path of the exciting current to prevent the machine building or exciting. each of the contacts should therefore be examined, cleaned, and screwed up tight. ques. which of the contacts should receive special attention? ans. the contact faces of the brushes and surface of the commutator. these are very frequently covered with a slimy coating of oil and dirt, which is quite sufficient to prevent the machine exciting. incorrect adjustment of regulators.--when shunt and compound machines are provided with field regulators, it is possible that the resistance in circuit may be too great to permit the necessary strength of exciting current passing through the field windings. accordingly, the fault is corrected by cutting out more or less of the resistance. the field coils of series machines are sometimes provided with short circuiting switches or resistances arranged to shunt the current across the field coils. if too much of the current be shunted across, the switch should be opened, or if there be a regulator, it should be so adjusted that it will pass enough current through the field windings to excite the machine. speed too low.--in shunt and compound dynamos there is a certain critical speed below which they will not excite. if the normal speed of the machine be known, it can be seen whether the failure to excite arises from this cause, by measuring the speed of the armature with a speed indicator. in all cases it is advisable, if the machine do not excite in the course of a few minutes, to slightly increase the speed. as soon as the voltage rises, the speed may be reduced to its regular rate. [illustration: fig. 692.--method of testing for break by short circuiting the terminals of the machine. if the external circuit test out apparently all right, and there be no defective contacts in any part of the machine, and all short circuiting switches, etc., be cut out of circuit, the machine still refusing to excite, short circuiting the terminals of the machine should be tried. this should be done very cautiously, especially in case of a high tension machine. it is advisable to have, if possible, only a portion of the load in circuit, and the short circuit should be effected as shown in the figure. the short circuit may be made by momentarily bridging across the two terminals of the machine with a single piece of wire. as this, however, is liable to burn the terminals, a better plan is to fix a short piece of scrap wire in one terminal, and then with another piece of insulated wire make momentary contacts with the other terminal and the short piece of wire. if the machine excite, it will be at once evident by the arc which occurs between the two pieces of wire. as the voltage of a series machine when induced to build in this manner generally rises very rapidly, great care should be taken that the contact is at first only momentary, merely a rubbing or scraping touch of the wires. the contact may be prolonged if the machine do not excite at the first contact. compound wound machines can often be made to excite quickly by short circuiting their terminals in this manner.] insufficient residual magnetism.--this fault is not of frequent occurrence; it takes place chiefly when the dynamo is new, and may be remedied by passing the current from a few storage cells, or from another dynamo, for some time in the proper direction through the field coils. if a heavy current, such as is obtainable from a storage battery, be not available, and the machine be shunt or compound wound, a few primary cells arranged as in fig. 693 will generally suffice. [illustration: fig. 693.--method of overcoming insufficient residual magnetism. the flexible "lead" l of the dynamo d is disconnected from the positive terminal of the machine, and is connected to the negative or zinc pole of the battery b, the other or positive carbon pole being connected to the terminal, from which the lead was removed, and shunt circuit s. as thus arranged, it will be seen that the battery b is in series with the armature and shunt circuit, and therefore its voltage will be added to any small voltage generated in the armature. when the machine is started, the combined voltages will probably be able to send sufficient current through the shunt to excite the machine. as the voltage rises and the strength of the current in the shunt windings increases, the flexible lead may be again inserted into the terminal from which it was removed. the battery will thus be short circuited, and may be cut out of circuit without any danger of breaking the shunt circuit, and thus causing the machine to demagnetize.] open circuits.--dynamos are affected by open circuits in different ways, depending upon the type. series machines require closed circuit to build up, while an open circuit is necessary with the shunt machine. an open circuit may be due to: 1, broken wire or faulty connection in the machine; 2, brushes not in contact with commutator; 3, safety fuse blown or removed; 4, circuit breaker open; 5, switch open; 6, external circuit open. if the trouble be due merely to the switch or external circuit being open, the magnetism of a shunt machine may be at full strength, and the machine itself may be working perfectly, but if the trouble be in the machine, the field magnetism will probably be very weak. open circuits are most likely to occur in: 1. the armature circuit; 2. the field circuit; 3. the external circuit. when the open circuit is due to the brushes not making good contact, simple examination generally reveals the fact. ques. what causes breaks in the field circuit? ans. bad contacts at the terminals, broken connections, or fracture of the coil windings. ques. how is the field circuit tested for breaks? ans. the flexible leads attached to the brushes are removed from their connections with the field circuit, and the latter is then tested for conductivity with a galvanometer. ques. where is a break likely to occur in a shunt machine? ans. in the hand regulator through a broken resistance coil or bad contact. very frequently the fault occurs in the connecting wires leading from the machine to the hand regulator fixed upon the switchboard, or in the short wires connecting the field coils to the terminals or brushes. the insulation of a broken wire will sometimes hold the two ends together so as to defy any but the most careful inspection or examination; therefore, in order to avoid loss of time, it is advisable to disconnect the wires if possible, and test each separately for conductivity with a battery and galvanometer connected, as in fig. 694. if the fault be not located in the various connections, the magnet coils should be tested with the battery and galvanometer coupled up as in fig. 706, care being first taken to disconnect the ends of each of the coils. a faulty coil will not show any deflection of the galvanometer. [illustration: fig. 694.--method of testing dynamo for short circuits. in the figure, one pole of the battery b is placed in contact with the frame of the machine at a point which has previously been well scraped and cleaned; the other pole is connected to one of the galvanometer terminals as shown. the other terminal of the galvanometer is connected to each of the dynamo terminals t t under test in turn. if a deflection of the needle be produced when the galvanometer terminal is in contact with either, the terminals are in contact with the frame, and they should then be removed, and the fault repaired by additional insulation or by reinsulating.] ques. at what point of a shunt coil does a break usually occur? ans. at the point where the wire passes through the flanges of the spool or bobbin. ques. how should the coil be repaired? ans. in most cases a little of the wood or metal of which the flange is made can be gouged or chipped out, and a new connecting wire soldered on to the broken end of the coil without much difficulty. if it be necessary to take the magnets apart at any time, care should be taken in putting them together again to wipe all faces perfectly clean, and screw up firmly into contact, and to see that the connections of the coils are made as they were before being taken apart. if the faulty coil cannot be repaired quickly, and the machine is urgently required, the coil may be cut out of circuit entirely, or short circuited, and the remaining coils coupled up so as to produce the correct polarity in the pole pieces. [illustration: fig. 695.--watson armature discs. each lamination is made from low carbon electrical steel of high magnetic permeability. each disc is annealed and afterwards varnished.] ques. what trouble is liable to be encountered in operating after cutting out a coil? ans. the remaining coils are liable to heat up to a greater extent than formerly, owing to the increased current, hence it is advisable to proceed cautiously in starting the dynamo, since the temperature may exceed a safe limit. if this occur, a resistance may be put in circuit with the field coils, or the speed of the dynamo reduced. [illustration: fig. 696.--fort wayne pedestal type commutator truing device. when this device is used, the armature is revolved in its own bearings by means of a handle clamped to the pulley. the tool has a horizontal travel of 21 ins., (being 3 ins. wide inside the fastening bolt in the base), and a vertical adjustment of 12 ins., adapting it to machines with commutators up to 36 ins. in diameter.] [illustration: fig. 697.--fort wayne yoke type commutator truing device for machines having brush mechanism mounted on a yoke carried by the field frame. it consists of a carriage for the tool holder having a screw feed and a bracket for attaching to the brush yoke. the bracket replaces two brush holder brackets on the brush yoke, and is made to fit the yoke of the particular machine on which it is to be used.] ques. what kind of dynamo is affected by breaks in the external circuit? ans. a series dynamo. ques. name the kind of break that is difficult to locate. ans. a partial break. short circuits.--in a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow. a shunt machine will not excite under these circumstances, for the reason that practically the whole of the current generated in the armature passes direct to the external circuit, and the difference of potential between the shunt terminals is practically nil. ques. what should be done if it be suspected that the failure to excite arises from this cause? ans. the main leads should be taken out of the dynamo terminals, then, if due to this cause, the machine will excite. ques. what parts of a dynamo are specially liable to be short circuited? ans. the terminals, brush holders, commutator, armature coils and field coils. ques. how are the terminals liable to be short circuited? ans. the terminals of the various circuits of the machine are liable to be short circuited, either through metallic dust bridging across the insulation, or through the terminals making direct contact with the frame of the machine. the various terminals should be examined, and if the fault cannot be located by inspection, they should each be disconnected from their circuits and tested with a battery and galvanometer arranged as in fig. 694. ques. what precaution should be taken with the brush holders? ans. since, they are liable to be short circuited through the rocker by metallic dust lodging in the insulating washers, they should be kept clean. ques. how are the brush holders tested? ans. a galvanometer and battery are connected in series with one terminal of the galvanometer connected to one set of brushes; the unconnected terminal of the battery is then connected with the other set of brushes. a deflection of the needle will indicate a short circuit. [illustration: fig. 698.--field coil testing with telephone receiver. in the method here shown, a telephone receiver is connected in series with two symmetrically placed coils a and b. very little sound will be heard when the flux through the two coils ab is the same; but if a short-circuited coil is being tested, the fluxes through the coils a, b will not be equal and a noise can be heard in the receiver.] ques. what is the effect of a short circuit in the field coils or field circuit? ans. the machine generally refuses to excite. ques. how are the field coils tested for short circuit? ans. by measuring the resistance of each coil with an ohmmeter or wheatstone bridge. the faulty coils will show a much less resistance than the perfect coils. the fault may also be discovered and located by passing a strong current from a battery or another dynamo through each of the coils in turn, and observing the relative magnetic effects produced by each upon a bar of iron held in their vicinity. the short circuit may be in the terminals or connections, and these should first be examined and tested. some series dynamos are provided with a resistance, arranged in parallel or shunt with the field coils, to divert a portion of the current therefrom, and thus regulate the output. when making a series dynamo excite, all resistances and controlling devices should be temporarily cut out of circuit by opening the shunt circuit. series machines have frequently a switch which short circuits the field coils. care should be taken that this is open, or otherwise the machine will not excite. [illustration: fig. 699.--watson armature complete. the armature coils are form wound, heavily insulated and so mounted on the core as to insure rapid dissipation of heat by ventilation. each coil is protected by an insulating sheath and tape covering before mounting. the armature is baked after the coils are mounted to drive out all moisture, then, while hot, is treated with insulating compound and again baked twelve hours.] wrong connections.--when a machine is first erected, the failure to build up may be due to incorrect connections. the whole of these latter should therefore be traced or followed out, and compared with the diagrams of dynamo connections given in figs. 190 to 198. sometimes errors are made in connecting the field coils, causing them to act in opposition. this may occur when the dynamo is a new one or the coils have been removed for repairs. it may be caused either through the coils having been put on the field cores the wrong way, or through incorrect coupling up. under these circumstances, the dynamo, if bipolar, will fail to excite; and if multipolar, poles will be produced in the yokes, etc. it may be remedied by removing one of the coils from the core and putting it on the reverse way, or by reversing its connections. the correctness of connections of all the coils should be verified. in compound dynamos it sometimes happens that the machine will excite properly, but that the series coils tend to reverse the polarity of the dynamo, thus reducing the voltage as the load upon the machine increases. this may be detected when the machine is loaded by short circuiting the _series coils_, not the _terminals_. if the voltage rise in doing this, the series coils are acting in opposition to the shunt coils, and the connections of the _series coils_ must be reversed. reversed field magnetism.--this is sometimes caused by the nearness of other dynamos, but is generally due to reversed connections of the field coils. under such conditions the field coils tend to produce a polarity opposed to the magnetization to which they owe their current, and therefore the machine will refuse to excite until the field connections are reversed, or a current is sent from another dynamo or a battery through the field coils in a direction to produce the correct polarity in the pole pieces. chapter xxxii armature troubles a large proportion of the mishaps and breakdowns which occur with dynamos and motors arise from causes more strictly within the province of the man in charge than in that of the designer. the armature, being a complex and delicately built structure, is subject in operation to various detrimental influences giving rise to faults. many of the faults which occur are avoided by operators better informed as to the electric and magnetic conditions which obtain in the running of the machine, especially the mechanical stresses on the copper inductors due to the magnetic field and the necessity of preserving proper insulation. the chief mishaps to which armatures are subject are as follows: 1. short circuits; a. in individual coils; b. between adjacent coils; c. through frame or core; d. between sections of armature; e. partial short circuits. 2. grounds; 3. breaks in armature circuit. short circuit in individual coils.--this is a common fault, which makes its presence known by a violent heating of the armature, flashing at the commutator, flickering of the light on lighting circuits, and by a smell of burning varnish or overheated insulation. when these indications are present, the machine should be stopped at once, otherwise the armature is liable to be burnt out. the fault is due either to metallic dust lodging in the insulation between adjacent bars of the commutator, or to one or more convolutions of the coils coming into contact with each other, either through a metallic filing becoming embedded in the insulation or damage to the insulation. [illustration: fig. 700.--method of locating short circuited armature coil. disconnect the external and field circuits from the armature, and pass a large current--say from 20 to 100 amperes--from a battery (b) or another dynamo through the whole armature by means of the brushes. then, having previously well cleaned the commutator, measure the difference of potential between adjacent segments all round the commutator (c), by means of a voltmeter or galvanometer (g), the terminals of which are connected to adjacent segments, as shown. the short circuited coil or coils will be located by the difference of potential between the corresponding segments being little or nothing. it may be remarked, however, that this is not always a decisive test. in some cases the short circuit may be intermittent, or may disappear as soon as the armature ceases to rotate. in such cases, the short circuit is caused by the wire coming into contact through the action of the centrifugal forces developed by the rotation of the armature.] ques. how is the faulty coil located? ans. when the machine is stopped, the faulty coil, if not burnt out, can generally be located by the baked appearance of the varnish or insulation, and by its excessive temperature over the rest of the coils, being detected also by the baked appearance of the varnish or insulation. ques. what should be done if the machine do not build, and it be suspected that the fault is due to short circuited armature coils? ans. the field magnets should be excited by the current from a storage battery or another dynamo, and, having raised the brushes from contact with the commutator, the armature should be run for a short time. in stopping, the faulty coil or coils may be located by the heat generated by the short circuit. when the dynamo is started for the purpose of localizing a short circuit, precautions should be taken, and the machine only run for a few minutes at a time until the faulty coil is detected. when the faulty coil has been located, the insulation between the segments of the commutator to which its ends are connected should be carefully examined for anything that may bridge across from segment to segment, and scraped clean. if the commutator be apparently all right, the fault probably lies in the winding. the insulation of the winding should be carefully examined, and any metallic filings or other particles discovered therein carefully removed, and a little shellac varnish applied to the faulty part. [illustration: fig. 701.--test for break in armature lead. clean the brushes and commutator, and apply current from a few cells of battery having a telephone receiver in circuit as shown in the figure. if the machine have more than two brushes, connect the leads to two adjoining brushes and raise the others. now rotate the armature slowly by hand and there will be a distinct click in the receiver as each segment passes under the brushes until one brush bears on the segment at fault, when the clicking will cease. in making this test, the brushes must not cover more than a single segment.] ques. if the insulation on adjacent conductors has been abraded, how should it be repaired? ans. a small boxwood or other hardwood wedge, coated with shellac varnish should be driven in tightly between the wire; this will generally be sufficient. [illustration: fig. 702.--bar to bar test for open circuit in coil or short circuit in one coil or between segments. if, in testing as in fig. 701, on rotating the armature completely around, the receiver indicate no break in the leads, connect the battery leads directly to the brushes, as shown in the above figure, and touch the connections from the receiver to two adjacent bars, working from bar to bar. the clicking should be substantially the same between any two commutator bars; if the clicking suddenly rise in tone between two bars, it indicates a high resistance in the coil or a break (open circuit).] ques. if a faulty coil cannot be quickly repaired and the dynamo be needed, what should be done? ans. the coil may be cut out of circuit, and the corresponding commutator segments connected together with a piece of wire (of a size proportionate to the amount of current to be carried), soldered to each. it will not be necessary to cut out and remove the entire coil. if the active portions only be separated so that they do not form a closed circuit, it will answer the purpose. if the wires be cut with a chisel at the point where they pass over the ends of the core, and the ends separated, it will be quite as effective as removing the entire coil. it is wise, of course, to rewind the coil at the first opportunity. [illustration: fig. 703.--alternate bar test for short circuit between sections. where two adjacent commutator bars are in contact, or a coil between two segments becomes short circuited, the bar to bar test described in fig. 702 will detect the fault by the telephone receiver remaining silent. if a short circuit be found, the leads from the receiver should then include or straddle three commutator bars, as here shown. the normal click will then be twice that between two segments until the faulty coils are reached, when the clicking will be less. when this happens, test each coil for trouble and, if individually they be all right, the trouble is between the two. to test for a ground place one terminal of the receiver on the shaft or frame of the machine, and the other on the commutator. if there be a click it indicates a ground. move the terminal about the commutator until the least clicking is heard and at or near that point will be found the contact. grounds in field coils can be located in the same manner.] short circuits between adjacent coils.--in ring armatures the presence of this fault does not necessarily imply that the machine will not build; in drum armatures, wound into a single layer of conductors, it entirely prevents this occurring. reference to a winding diagram will show that adjacent coils are during a certain period of the revolution at the full difference of pressure generated by the machine. hence, if any two adjacent coils be connected together or short circuited, the whole of the armature will be practically closed on itself, any current generated flowing within the armature only. large drum armatures wound with compressed and stranded bars and connectors are particularly susceptible to this fault, a slight blow generally forcing one or more of the strands into contact with the adjacent bars, thus short circuiting the armature, and rendering it practically useless so far as the generation of current is concerned. in this class of short circuit in drum armatures, the method of locating the faulty coils by exciting the field, and running the armatures on open circuit, does not apply, for the reason that the whole armature will be heated equally. a method of locating such fault is illustrated in fig. 704. this applies to drum wound armatures. faults of this description can frequently be discovered by a careful inspection of the windings of the armature without recourse to testing. when located, the fault can usually be repaired with a hardwood wedge, as explained above, or a piece of mica or vulcanized fibre cemented in place with shellac varnish. [illustration: fig. 704.--method of locating short circuits between adjacent armature coils. fasten a monkey wrench to the rim of the pulley, or a crank to the shaft. now, excite the fields, and, to make the effects more marked, connect the coils in parallel. when this has been done it will require considerable force to rotate the armature, and then it will move quite slowly, except at one position. when this position has been found, mark the armature at points in the center of the pole pieces at points a and b and at both ends of the armature. the explanation is that both halves of the armature oppose one another at this position; but when not at these points a continuous circuit is formed, and the resultant magnetic effect is considerable. the "cross" or "short" circuit is nearly always found on the commutator end in the last half of the winding, where the wires pass down through the first half terminals. this applies to an unequal winding. in armatures where the windings are equal, it is as liable to occur at one point as at another. with this method a defect can be found and remedied in a few moments, for it has always been a simple matter to repair it when discovered. these results can be observed in a perfect armature by connecting the opposite sections of the commutator.] short circuits between sections through frame or core of armature.--detection of this fault can be effected by the methods described above, and by disconnecting the whole of the armature coils from the commutator and from each other, and testing each separately with a battery and galvanometer coupled up as in fig. 705, one wire being connected to the shaft and the other to the end of the coil under test. as a rule, there is no way of remedying this fault other than unwinding the defective coils, reinsulating the core, and rewinding new coils. [illustration: fig. 705.--method of locating short circuits between coils through armature core. the galvanometer, battery and coil to be tested are connected in series as shown, and then the unconnected terminal of the galvanometer is brought into contact with the shaft. if then some portion of the insulation of the wire has been abraded or destroyed, thus bringing the bare wire into contact with the metal core, as at a in the figure, the needle of the galvanometer will be deflected since a closed circuit is formed through the core and wire. if the insulation be perfect, the needle will not be deflected. it will thus be seen that in the conductivity test (fig. 700) it is necessary that the needle should be deflected, or turned, to prove that all is right, while in the insulation test the converse holds good; if the needle be deflected, it proves that the insulation is broken down.] short circuits between sections through binding wires.--this fault is the result of a loose winding, and is caused by the insulation upon which the binding wires are wound giving way, thus bringing coils at different pressures together. as a consequence of the heavy current which flows, the binding wires are as a rule unsoldered or burned. the location of the fault can therefore be effected by simple inspection. to remedy, it will be necessary to unwind and rewind on new binding wires, on bands of mica or vulcanized fibre, soldering at intervals to obviate flying asunder. partial short circuits in armatures.--this is usually due to the presence of moisture in the windings. to remedy the fault, the armature should be taken out and exposed to a moderate heat, or subjected to a current equal to that ordinarily given by the dynamo. under the action of heat or of this current the moisture will be gradually dispersed. when thoroughly dry, and while still warm, a coat of shellac should be applied to the whole of the windings. [illustration: fig. 706.--method of testing for breaks. the instruments are connected as shown. b is the battery, g the galvanometer, and s the coil of wire being tested. one terminal of the battery is connected to a terminal of the galvanometer, and the other to one of the ends of the coil under test. the other terminal of the galvanometer is connected to the other end of the coil. if the connecting wires be making good electrical contact with the respective terminals, and the wire of coil being tested be unbroken, the needle of the galvanometer will be deflected as soon as a closed circuit is made by the end of the coil coming into contact with the galvanometer terminal. if the wire of the coil be broken in some part or the ends of the connecting wires do not make good electrical contact with the terminals, the needle will not be deflected. in order to prevent mistakes, it is advisable to test the battery and galvanometer connections and contacts by short circuiting or bringing the ends of the wire connecting the terminal of the galvanometer and negative pole or the battery together before starting to test the circuit or coil. if the needle be deflected, the connections are all right; if not deflected, there is a bad contact somewhere, which must be made good before the test can proceed.] burning of armature coils.--the reason for the burning of an armature coil may be explained as follows: the coil, segments, and the short circuit between the segments form a closed circuit of low resistance so that it is only necessary to have a low pressure set up in the active portion of the coil to force a very large current through the coil and the short circuited commutator bars. the heating effect of this current is sufficient to burn out the coil. [illustration: fig. 707.--watson field coils. automatic machinery is employed to wind these coils; after winding, they are bound with tape, then baked to expel all moisture, and while hot, are saturated with an insulating compound and again baked for twelve hours to make them practically oil and water proof. heavy flexible leads are brought out to avoid danger of breaking or other damage.] cutting out damaged armature coils.--to cut out a damaged coil from an armature, first, disconnect the coil from the commutator, and after cutting off the leads, insulate the exposed parts with tape. then connect the commutator bars (which were connected with the leads) with a wire of the same size as the wire winding. to remove the coil entirely, cut the band wire or remove the wedges, and lift up a sufficient number of leads and coils to permit of the removal of the damaged coil. grounds in armatures.--these faults occur when the armature coils become connected to the frame or core of the armature. when this grounding is confined to a single coil, it is not in itself liable to do damage. a simple method of locating a grounded coil is illustrated in fig. 708. [illustration: fig. 708.--method of locating grounded armature coil. b is a battery or dynamo circuit giving a current of a few amperes through the armature by its own brushes (1 and 2). at g is placed a roughly made galvanometer, to carry some 25 amperes or so, one terminal being in connection with the shaft of the armature, and the other attached to a movable brush 3. since the function of the particular galvanometer is simply to show a deflection when a current is passing, and to mark zero when there is none, a coil of thick wire with a pocket compass in the center will do all that is required, but care must be taken to remove it sufficiently far away from the disturbing effects of the armature magnetism. the manner of testing is as follows: assume a steady current to be flowing from battery b through the armature; touch the commutator with brush 3, and a current will flow through g. slowly rotate the armature or the brush 3 until the galvanometer g shows no deflection. the coil in contact with 3 will be found to be _grounded_. a hand regulator or rheostat r may be inserted in series with the battery or dynamo circuit to regulate the strength of the current passing.] ques. what is the advantage of this test? ans. the damaged coil can be located without unsoldering the coils from the commutator, which is sometimes a difficult operation without proper tools; further, the fault can frequently be repaired without disconnecting any of the wires if its exact position be determined. magneto test for grounded armatures.--a magneto test for grounded armatures is not to be recommended, as armatures often possess sufficient static capacity to cause a magneto to ring even though there be no leak. this is due to the alternating current given by the magneto for when the circuit has capacity it acts as a condenser and at each revolution of the armature of the magneto a rush of current goes out and returns, charging the surfaces of the conductor alternately in opposite directions, and ringing the bell during the process. [illustration: fig. 709.--method of binding armature winding. complete appliances for handling armatures in making repairs are usually not available with most street railway companies, since they are so seldom required. when needed, therefore, some temporary contrivance must be resorted to for help in the dilemma. should an armature burn out, some local concern that makes coils and rewinds armatures may be available to do the work; again, it will be necessary to send to the manufacturers for a man, as soon as coils can be made ready for the work. in no case should any but an experienced man be given charge of this work. but if there be any doubt as to whether the armature is really burnt out, let a competent man be the judge. when a large armature needs repairing, a pair of chain tongs can be used on some part of the shaft when putting in the coils, and a block and tackle, as shown, can be used, when putting on the band wires. do not finish one band and then cut off the wire, but run it over for the next, etc. then solder and trim off the wires.] breaks in armature circuit.--a partial or complete break in the armature circuit is always accompanied by heavy sparking at the commutator, but not, as a rule, by an excessive heating of the armature or slipping of the belt, and this enables the fault to be distinguished from a short circuit. the faulty part can always be readily located by the "flat" which it produces upon the surface of the commutator. the armature circuit being open at the faulty part, heavy sparking results at every half revolution as the brushes pass over it, and as a consequence the corresponding segments become "pitted" or "flattened" with respect to the others; they may easily be discovered on examination. breaks in the armature circuit may occur in either the commutator or in the coils of the armature. to ascertain whether it be in the latter, carefully examine the winding of the faulty coil. the defect may be sought for more particularly at the commutator end of the armature, as breaks in the wire are most frequent where the connections are made with the commutator segments. if no break can be discovered, try passing a heavy current through the faulty coil by means of the brushes. if a partial break exist with sufficient contact to pass a current, the coil will be heated at that point and may be discovered by running the fingers over the coil. when located, the fault may be repaired by rewinding the coil, or carefully cleaning the broken ends and jointing. the fault may also be temporarily repaired by soldering the adjacent commutator segments together without disconnecting the coil. chapter xxxiii care of the commutator and brushes for satisfactory operation, the brushes and commutator must be kept in good condition. to this end the main thing to be guarded against is the production of sparks at the brushes. if care be taken in the first instance to adjust the brushes to their setting marks, and to regulate their pressure upon the commutator, and afterwards to attend to the lead as the load varies, so that little or no sparking occurs, and also to keep the brushes and commutator free from dirt, grit, excessive oil, etc., the surface of the commutator will assume a dark burnished appearance and wear will practically cease. under these circumstances the commutator will run cool, and will give very little trouble. in order to maintain these conditions it will only be necessary to see that the brushes are kept in proper condition and fed forward to their setting marks, as they wear away, and that the commutator is occasionally polished. if the pressure of the brushes upon the commutator be too great, or their adjustment faulty, or the commutator be allowed to get into a dirty condition, sparking will result, and, if not at once attended to and remedied, the brushes will quickly wear away, and the surface of the commutator will be destroyed. as this action takes place, in the earlier stages, the surface of the commutator will become roughened or scored, resulting in jumping of the brushes, and increased sparking; in the later stages, the commutator will become untrue and worn into ruts, moreover, owing to the violent sparking which takes place through this circumstance, the machine will quickly be rendered useless[d]. [d] note.--in operating dynamos having metal brushes, it is of importance to keep the commutator smooth and glossy. to accomplish this, it is necessary to keep the commutator and brushes clean and free from grit, and to occasionally lubricate the commutator with some light oil, such as ordinary machine oil. this should be done daily if the machine be in constant use. keep the brushes resting upon the commutator with just enough pressure to insure a good firm contact. this will be found to be much less than the springs are capable of exerting. a good method to follow in cleaning the machine is as follows: loosen the brush holder thumb screws and tilt the brushes off the commutator (or, if box brush holders be used, take them out of their holders). then run the machine and hold a clean cloth against the commutator. after the commutator is clean, hold against it a cloth or piece of waste moistened with machine oil and reset the brushes. if for any reason the brushes begin to cut or score the commutator, it may be readily detected by holding the finger against the commutator; the ridge may be easily felt by the finger. this should be attended to at once in the following manner: tilt back the brushes (or if box brushes are used take them out of their holders), and hold lightly against the commutator a piece of no. 00 sandpaper well moistened with oil, passing it back and forth until the surface is perfectly smooth. then wipe off the commutator with a clean piece of cloth or waste and lubricate with another clean piece moistened with oil and reset the brushes. ques. how is the commutator easily tested as to the condition of its surface? ans. it is readily tested by resting the back of the finger nail upon it while in motion; the nail being very sensitive to any irregularities, indicates at once any defect. ques. what causes grooves or ridges to be cut in the commutator? ans. they result from using brushes with hard burnt ends which are not pliable; also by too great a pressure of the brush upon the commutator surface. sparking at the brushes is expensive and detrimental, chiefly because it results in burning the brushes and also the commutator, necessitating their frequent renewal. every spark consumes a particle of copper, torn from the commutator or brush. the longer the sparking continues, the greater the evil becomes, and the remedy must be applied without delay. ques. what kind of oil should be used on the commutator? ans. mineral oil. ques. what attention should be given to the brushes? ans. at certain intervals, according to the care taken to reduce sparking and the length of time the machine runs, the brushes will fray out or wear unevenly, and will therefore need trimming. they should then be removed from the brush holders and their contact ends or faces examined. if not truly square, they should be filed or clipped with a pair of shears, the course of treatment differing with the type of brush. if the machine be fitted with metal strip brushes, frayed ends should be clipped square with a pair of shears, the ends thoroughly cleaned from any dirt or carbonized oil, and replaced in their holders. gauze and wire brushes require a little more attention. when their position on the commutator has been well adjusted and looked after, so that little or no sparking has taken place, it is generally only necessary to wipe them, clean the brushes and clip off the fringed edges and corners with the shears, or a pair of strong scissors. if, however, the machine has been sparking, the faces will be worn or burnt away, and probably fused. if such be the case, they will need to be put in the filing clamp, and filed true. a convenient method of trimming carbon brushes, or of bedding a complete new set of metal brushes, is to bind a piece of sandpaper, face outwards, around the commutator after the current has been shut off, and then mount the carbon or metal brushes in the holders, adjusting the tension of the springs so that the brushes bear with a moderately strong pressure upon the sandpaper. then let the machine run slowly until the ends of the brushes are ground to the proper form. care should be taken, however, that the metal dust given off does not get into the commutator connections or armature windings, or short circuiting will result. if the contact faces of the brushes are very dirty and covered with a coating of carbonized oil, etc., it will be necessary to clean them with benzoline or soda solution before replacing. [illustration: fig. 710.--bissell brush gear. the brushes are held in the brushholders radially and work equally well with armature running in either direction. brushes can be renewed and adjustment made while machine is in operation.] ques. describe a filing clamp. ans. as usually constructed, it consists of two pieces of metal, both shaped at one end to the correct angle, to which the brushes must be filed. one of the pieces of metal (the back part) has a groove sufficiently large to accommodate the brush, which is clamped in position by the other piece of metal and a pinching screw. if the clamp be not supplied with the machine a convenient substitute can be made out of two pieces of wood about the same width as the brush. one end of each piece is sawn to the correct angle, and the brush placed between the two. in filing, the brush is fixed in the clamp, with the toe or tip projecting slightly over the edge of the clamp, and the latter being fixed in a vise, the brush is filed by single strokes of a smooth file made outwards, the file being raised from contact with the brush when making the back stroke. [illustration: fig. 711.--jig for filing brushes to the correct bevel; used with copper brushes to fit them to the commutator.] sparking.--in all well designed machines there are certain positions upon the commutator for the brushes at which there will be no sparking so long as the commutator is kept clean and in good condition. in other dynamos, badly designed or constructed, sparking occurs at all positions, no matter where the brushes are placed, and in such dynamos it is therefore impossible to prevent this no matter how well they are adjusted. [illustration: fig. 712.--commutator clamp; a useful device for holding the segments firmly in position in taking out the end rings of the commutator to repair for internal grounds. it is made of 2 ã� 1/8 inch sheet steel, with a 1/2 inch screw. the illustration clearly shows the adjustable fastening. the notches fit around rivets on one side of each fastening, which can be moved by removing the two cotters. the clamp is made loose or taut by screwing the bolt in the nut.] ques. what two kinds of sparking may be generally distinguished? ans. one kind of sparking is that due to bad adjustment of the brushes, and a second kind, that due to bad condition of the commutator. sparks due to bad adjustment of the brushes are generally of a bluish color, small when near the neutral plane, and increasing in violence and brilliancy as the brushes recede from the correct positions upon the commutator. when sparks are produced by dirty or neglected state of the commutator, they are distinguished by a reddish color and a spluttering or hissing. when due to this last mentioned cause, it is impossible to suppress the sparking until the commutator and brushes have been cleaned. in the former case, the sparks will disappear as soon as the brushes have been rotated into the neutral points. another class of sparks appear when there is some more or less developed fault, such as a short circuit, or break in the armature or commutator. these are similar in character to those produced by bad adjustment of the brushes, but are distinguished from the latter by their not decreasing in violence when the brushes are rotated towards the neutral plane. having distinguished the classes of sparks which appear at the commutator of a dynamo, it remains to enumerate the causes which produce them. these are: 1. bad adjustment of brushes; 2. bad condition of brushes; 3. bad condition of commutator; 4. overload of dynamo; 5. loose connections, terminals, etc.; 6. breaks in armature circuit; 7. short circuits in armature circuit; 8. short circuits or breaks in field magnet circuit. bad adjustment of brushes.--when sparking is produced by bad adjustment of the brushes, it may be detected by rotating or shifting the rocker, by the indication that the sparking will vary with each movement. to obtain good adjustment of the brushes, it will be necessary to rock them gently backwards and forwards, until a position is found in which the sparking disappears. ques. if, in rocking the brushes, a position cannot be found at which the sparking disappears, what is the probable cause of the trouble? ans. the brushes may not be set with the proper pitch, that is they may not be separated a correct distance, or the neutral plane may not be situated in the true theoretical position upon the commutator through some defect in the winding, etc. in this last named case, the brushes may be strictly adjusted to their theoretically correct positions before starting the machine; then, when the machine is started and the load put on, violent sparking occurs, which cannot be suppressed by shifting the rocker. if, however, one set of brushes only be observed, it will generally be found that, at a certain position, the sparking at the set of brushes under observation ceases or is greatly reduced, while sparking still occurs at the other set. when this position is found, the rocker should be fixed by the clamping screw, and the brushes of the other set at which sparking is still occurring adjusted by drawing them back or pushing them forward in their holders until a position is found at which the sparking ceases. correct position of the brushes and the suppression of sparking is a matter of importance, and any time spent in carefully adjusting the brushes will be amply repaid by the decreased attention and wear of the brushes and commutator. [illustration: figs. 713 to 715.--brushes making bad contact. a brush making a bad contact, as only at the shaded portion of figs. 713 and 714, will not allow the short circuited coil enough time to reverse, causing sparking and heating. the latter will also result from bad contact on account of the surface being too small for the current to be carried off. this form of bad contact is worse than that shown in fig. 715, where the area of contact surface only is lessened. if the brushes do not make good contact, they should be ground down.] bad condition of brushes.--if the contact faces of the brushes be fused or covered with carbonized oil, dirt, etc., there will be bad contact which is accompanied by heating and sparking. simple examination will generally reveal whether this be the case. the remedy is to remove the brushes, one at a time if the machine be running, clean, file if necessary, trim, and readjust. if the brushes be exceedingly dirty, or saturated with oil, it will be necessary to clean them with turpentine, benzoline, or soda solution, before replacing. bad condition of commutator.--if the surface of the commutator be rough, worn into grooves, or eccentric, or if there be one or more segments loose or set irregularly, the brushes will be thrown into vibration, and sparking will result. a simple examination of the commutator will readily detect these defects. a rough and uneven commutator is due to bad adjustment of brushes, bad construction of commutator, and to neglect generally. if allowed to continue, it results in heavy sparking at the brushes, and the eventful destruction of the commutator. the fault may be remedied by filing or re-turning the commutator. [illustration: fig. 716.--rough and grooved commutator due to improper brush adjustment and failure to keep brushes in proper condition.] ques. how is an untrue commutator detected? ans. if the commutator be untrue, the fact will be indicated when the machine is slowed down by a visible eccentricity, or by holding the hand, or a stick in the case of a high tension machine, against the surface while revolving, when any irregularity or eccentricity will be apparent by the vibration or movement of the stick. the only remedy for an untrue commutator is to re-turn it in the lathe. ques. what should be done in case of high segments? ans. they should be gently tapped down with a mallet, and if possible the clamping cones at the commutator end should be tightened. if it be impossible to hammer the segments down, they should be filed down to the same diameter as the rest of the commutator, or the commutator re-turned. for low segments, the only remedy is to pull out the segments, or turn commutator down to their level. ques. explain the term "flats on the commutator." ans. this is the name given to a peculiar fault which develops on one or more segments of the commutator. it is not confined to dynamos of bad design or construction, but frequently appears on those of the highest class, and may be recognized as a "pitting" or "flattening" of one or more segments. ques. what is the effect of flats on the commutator? ans. sparking at the brushes. ques. what are the causes which produce flats? ans. periodical jumping of the brushes due to a bad state of the commutator, bad joint in the driving belt, a flaw, or a difference in the composition of the metal of the particular bar upon which it appears. but more frequently flats may be traced to a more or less developed fault, such as a break, either partial or complete, in the armature coil. the break may occur either in the coil itself, or at the point where its ends make connection with the lug of the commutator, or at the point where the lug is soldered to the segment. ques. what should be done in case of flats? ans. the brushes should be examined to see if any periodical vibration take place. if such be the case, the cause should be removed, the flat carefully filed or turned out, and the brushes readjusted. if it be due to a difference in the composition of the metal of which the segment is made, the flat will exist as long as the particular segment is in use, and will need periodic attention. with hard drawn copper or phosphor bronze segments, this fault is rarely due to this last mentioned cause. it is more frequently due to bad soldering, of the conductors to the lugs, or of the lugs to the segments. in all cases of flats, if the disconnection in the armature circuit be not complete, and cannot be readily located, the effect of re-soldering or sweating the ends of the coils into the lugs should be tried. flats may also frequently be cured by drilling and tapping a small hole in the junction between the lug and the segment, and inserting a small screw, or bit of screwed copper or brass wire, afterwards filing down level with the surface of the commutator. [illustration: figs. 717 and 718.--method of repairing broken joint between commutator segment and lug. to repair such a break push asbestos in between adjacent bars, so that heat from the torch will not affect them. asbestos should also be worked in at the back if possible, for the purpose of keeping solder from places where it would cause trouble. then unsolder the armature leads from the lug and remove the latter. next, with specially made cape chisels, cut in a slot in the commutator bar for a new lug. care and skill are required not to destroy the mica insulation between the segments. the slot should be cut one-quarter to three-eighths inch deep. the connector is then soldered in place. with care a satisfactory connection can be made in this way, which will last well. if it do not last, the trouble in almost every case is due to poor soldering. short circuits sometimes occur after this operation, because of solder falling in at the back and lodging on lower connections. in large machines, the excessive current flowing is quite likely to melt this solder, and the machine may buck, throwing out the melted solder, after which it may be all right again. while the bar connector is out, however, asbestos should be packed in back of it to prevent this occurrence, which may be a serious affair. all surplus solder and the asbestos packing should be removed after the connection is finished, and the connections cleaned with compressed air. the armature should be turned over slowly, air being applied all the while.] segments loose or knocked in.--when the segments are loose, it is an indication that the clamping ring or cone has worked loose. this should therefore be tightened up, and the commutator re-turned if necessary. ques. how should low commutator segments be treated? ans. the commutator surface may be turned down to the level of the low segment, or the latter may be pulled out again to its former level, this latter being the preferable method, if it can possibly be effected. ques. how is a commutator segment pulled out to its correct position? ans. a hand vise is firmly clamped to the lug, or a loop of copper wire is passed round the conductor where it joins the commutator. a bar of iron, to act as a lever, is supported on a fulcrum over the commutator, and one end of the bar is passed through the loop or vise. pressure is applied to the other end which will generally bury the segment up to its proper position. how to re-turn a commutator.--in re-turning the commutator, the armature should first be carefully taken out of the armature chamber, avoiding knocks or blows of any kind. the whole of the winding should then be wrapped in calico or canvas before the armature is put into the lathe, to prevent any particles of metal becoming attached to the surface of the armature at the time the commutator is being turned. the armature should on no account be rolled upon the floor, or subjected to blows or knocks while being put into the lathe. in re-turning the commutator, a sharp pointed tool should be used with a very fine feed. a broad nosed tool ought not to be used, as it is liable to burr over the segments. after turning, the commutator should be lightly filed with a dead smooth file, and finally polished with coarse and fine sandpaper. after the commutator has been turned and polished, the insulation between the segments should be lightly scraped with the tang of a small file to remove any particles of metal or burrs which might short circuit the commutator. the points where the armature wires are soldered to the lugs should also be carefully cleaned with a brush, and should then receive a coat or two of shellac varnish. while the commutator is being turned, care should be taken that the setting marks for the adjustment of the brushes are not turned out if these be present. the same care should be used in putting the armature back into the armature chamber as was used in taking it out, otherwise the insulation may be damaged. [illustration: figs. 719 and 720.--bissell commutators. the segments are of hard drawn copper and are insulated from each other and from the shell by mica.] ques. should the commutator be run without any lubricant? ans. in most cases it will be found that a little lubricant is needed in order to prevent cutting the brushes, cutting the commutator; this is especially the case when hard strip brushes are used. the quantity of oil applied should be very small; a few drops smeared upon a piece of clean rag, and applied to the commutator while running, being quite sufficient. ques. what kind of oil should be used on the commutator? ans. mineral oil, such as vaseline, or any other hydrocarbon. animal or vegetable oils should be avoided, as they have a tendency to carbonize, and thus cause short circuiting of the commutator, with attendant sparking. [illustration: figs. 721 to 723.--method of repairing a large hole burned in two adjacent bars of a commutator. fig. 721 shows the hole. the first operation is to clean carefully and tin the surface of the hole. the two bars are then wedged apart and mica strips, a b, fig 722, of the requisite size and thickness forced in. the commutator must now be warmed up as much as possible by means of soldering irons, and strips of mica, c d, e f, fig. 723, placed at the front and back of the hole, being kept in position by pieces of wood w, solder is poured into the hole from a ladle, using a rough mica funnel to guide it.] overload of dynamo.--it may happen, through some cause or other that a greater output is taken from the machine than it can safely carry. when this is the case, the fact is indicated by excessive sparking at the brushes, great heating of the armature and other parts of the dynamo, and possibly by the slipping of the belt (if it be a belt driven machine), resulting in a noise. the causes most likely to produce overload are: 1. excessive voltage; 2. excessive current; 3. reversal of polarity of dynamo; 4. short circuits or grounds in dynamo, or external circuits. ques. what is the indication of excessive voltage? ans. it is indicated by the voltmeter, or by the brilliancy of the pilot lamp. [illustration: fig. 724.--method of smoothing commutator with a stone. the proper stone to use is made out of white sandstone similar to that used for grindstones, but a trifle softer. it is dove-tailed into a holder, as shown in the illustration, and held in place by a set screw. when being used, one knob is grasped in one hand and the other knob in the other hand, the stone being moved back and forth along the length of the commutator. as the stone will become coated with copper at first, it must be cleaned frequently by means of coarse sandpaper. the fine dust from the stone will get under the brushes and wear them to a very close fit. after using the stone, finish with fine sandpaper.] ques. what are the causes of excessive voltage? ans. over excitation of the field magnet or too high speed. in the former case, resistance should be introduced into the field circuit to diminish the current flowing therein if a shunt machine; or if a series machine, a portion of the current should be shunted across the field coils by means of a resistance arranged in parallel with the series coils; or the same effect may be produced in both cases by reducing the speed of the armature if this be possible. if due to excessive speed, which will be indicated by a speed indicator, the natural remedy is to reduce the speed of the engine driving the dynamo, or, if this be not easily done, insert resistance into the dynamo circuit, as described above. ques. what are the causes of excessive current? ans. if the dynamo be supplying arc lamps, the excessive current may possibly be caused by the bad feeding of the lamps. if this be the case, the fact will be indicated by the oscillations of the ammeter needle, and the unsteadiness of the light. if incandescent lamps be in the circuit, the fault may be caused by there being more lamps in circuit than the dynamo is designed to carry. under such circumstances, another dynamo should be switched into circuit in parallel, or, if this be not possible, lamps should be switched off until the defect is remedied. when motors are in the circuit, sparking frequently results at the dynamo commutator, owing to the fluctuating load. in such cases the brushes should be adjusted to a position at which the least sparking occurs with the average load. ques. what may be said with respect to reversal of polarity of dynamos? ans. when compound or series wound dynamos are running in parallel, their polarity is occasionally reversed while stopping by the current from the machines at work. loose connections, terminals, etc.--when any of the connecting cables, terminal screws, etc., securing the different circuits are loose, sparking at the brushes, as a rule, results, for the reason that the vibration of the machine tends to continually alter the resistance of the various circuits to which they are connected. when the connections are excessively loose, sparking also results at their points of contact, and by this indication the faulty connections may be readily detected. when this sparking at the contacts is absent, the whole of the connections should be carefully examined and tested. breaks in armature circuit.--if there be a broken circuit in the armature, as sometimes happens through a fracture of the armature connections, etc., there will be serious flashing or sparking at the brushes, which cannot be suppressed by adjusting the rocker. as a rule it results in the production of "flats" upon one or more bars of the commutator. [illustration: fig. 725.--sandpaper holder for commutator. the sandpaper is made fast on top by a clamp and screw. the two face blocks are pivoted and adjust themselves to the commutator, and will fit any size of commutator. if it have four brushes, the lower block will go in between the brush-holders.] ques. how may such sparking be reduced without stopping the machine? ans. by placing one of the brushes of each set a little in advance of the others, so as to bridge the gap. short circuits in armature circuit.--this fault is indicated by sparking at the commutator, and in bad cases by an excessive heating of the armature, dimming of the light and slipping of the belt, and in the case of a drum armature, by a sudden cessation of the current. [illustration: fig. 726.--sandpaper block. it is made to fit the surface of the commutator. at s is a saw cut into which the ends are pushed after being wrapped around the block. the latter should be cut down on the dotted lines to form a handle.] short circuits or breaks in field magnet circuit.--either of these faults is liable to give rise to sparking at the commutator. if one of the coils be short circuited, the fact will be indicated by the faulty coil remaining cool while the perfect coil is overheated. the fault may arise through some of the connections to the coils making contact with the frame of the machine or with each other. to ascertain this, examine all the connections, and test with a battery and galvanometer. a total break in one or more of the field coils may readily be detected by means of the battery and galvanometer. a partial break is not, however, so readily discovered, for the reason that the coil wires may be in sufficiently close contact to give a deflection of the galvanometer needle. the only methods of detecting this fault is by measuring the resistance of the coils with an ohmmeter or wheatstone bridge, or by placing an ammeter in circuit with each coil in turn, and comparing the amount of current flowing in each. if the partial break be not accessible, the only way to remedy the fault is to rewind the coil, and the same applies to a break in the interior of the coil. short circuits in commutator.--these are of frequent occurrence, and result in heating the armature and sparking at the brushes. they are caused either by metallic dust or particles lodging in the insulation between the segments, or by the deterioration of the commutator insulation. to remedy, the insulation between the segments should be carefully examined, and any metallic dust, filings, or burrs cleaned or scraped out. when the commutator is insulated with asbestos or pasteboard (as is oftentimes the case in dynamos of european make), short circuits very frequently occur through the insulation absorbing moisture or oil, which is subsequently carbonized by the sparking at the brushes. in faults of this description the only remedy is to expel all moisture from the commutator insulation by means of heat, and scrape out all metallic dust which may be embedded in the surface of the insulation. if this do not effect a cure, it will be necessary to dig out the insulation, as far as possible, with a sharp tool, and drive in new insulation. oil should not be used on commutators insulated with these materials, but only asbestos dust or french chalk. chapter xxxiv heating the excessive heating of the parts of dynamos and motors is probably the most frequent and annoying fault which arises in operation. when the machine heats, it is a common mistake to suppose that any part found to be hot is the seat of the trouble. hot bearings may cause the armature or commutator to heat, or vice versa. all parts of the machine should be tested to ascertain which is the hottest, since heat generated in one part is rapidly diffused. this is best done by starting with the machine cold; any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited. heating may be due to various electrical or mechanical causes, and it may occur in the different parts of the machine, as in: 1. the connections; 2. the brushes and commutator; 3. the armature; 4. the field magnet; 5. the bearing. ques. how is heating detected? ans. by applying the hand to the different parts of the machine if low tension, or a thermometer if high tension, and also by a smell of overheated insulation, paint, or varnish. ques. what should be done if the odor of overheated insulation, paint or varnish be noticeable? ans. it is advisable to stop the machine at once, otherwise the insulation is liable to be destroyed. ques. what is the allowable rise of temperature in a well designed machine? ans. it should not exceed 80â° fahr., above the surrounding air, and in the case of the bearings, this temperature ought not to be reached under normal conditions of working. if this limit be exceeded after a run of six hours or less, it indicates a machine either badly designed and probably with the material cut down to the lowest possible limit with a view to cheapness, or some fault or other which should be searched for and remedied as early as possible, otherwise the machine will probably be destroyed. ques. how should the rise of temperature be measured? ans. it is not sufficient to feel the machine with the hand, but special thermometers must be placed on the armature winding, immediately on stopping the machine, covering them with cotton or wool to prevent cooling. readings must be taken at short intervals, and continued till no further rise of temperature is indicated. heating of connections.--a rise of temperature of the connections may be due to either excessive current, or bad contacts, or both. the terminals and connections will be excessively heated if a larger current pass through them than they are designed to carry. this nearly always proceeds from an overload of the dynamo, and if this be rectified, the heating will disappear. if the contacts of the different connections of the dynamo be not kept thoroughly clean and free from all grit, oil, etc., and the connections themselves be not tightly screwed up, heating will result, and the connections may even become unsoldered. heating of brushes, commutator and armature.--when heating occurs in these parts, it may be due to any of the following causes: 1, excessive current; 2, hot bearings; 3, short circuits in armature or commutator; 4, moisture in armature coils; 5, breaks in armature coils; 6, eddy currents in armature core or conductor. [illustration: fig. 727.--ventilated commutator; sectional view showing air ducts. air is frequently circulated through a commutator in order to maintain it at a sufficiently low temperature, suitable openings being provided for this purpose as shown.] ques. what may be said with respect to excessive current? ans. when a dynamo is overloaded, the temperature of the armature will rise to a dangerous extent, depending upon the degree to which the safe capacity of the machine is exceeded, and heavy sparking of the brushes will also result. if the overload be not removed, the insulation of the armature may be destroyed. ques. state some causes of hot bearings. ans. lack of oil; presence of grit or other foreign matter in the bearings; belt too tight; armature not centred with respect to pole pieces; bearings too tight; bearings not in line; shaft rough or cut. [illustration: fig. 728.--self-oiling and self-aligning bearing. the self-oiling feature consists of rings which revolve with the shaft, and feed the latter with oil continually, which they bring up from the reservoir below. the dirt settles to the bottom, and the upper portion of the oil remains sufficiently clean for a long time, after which it is drawn off, and a fresh supply poured in through holes provided in the top. these latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated immediately by means of an oil cup if the rings fail to act or the reservoir become exhausted. the bearing is made self-aligning by providing the bearing proper with an enlarged central portion of spherical shape, held in a spherical seat formed in the pedestal by turning, milling, or by casting babbitt or other fusible metal around it, thus allowing the bearing to adjust itself to the exact direction of the shaft. the upper half of the box can be taken off to facilitate renewal, etc., and to permit the armature to be removed.] ques. what is the effect of hot bearings? ans. besides giving trouble themselves, the heat may be conducted along the armature shaft and core, thus giving rise to excessive heating of the armature. points relating to hot bearings 1. use good oil; 2. see that oil cups or reservoirs are full and all oil passages clear; 3. in self-oiling and splash systems where the oil is used over again, it should be kept in clean condition by frequent straining; 4. keep bearings clean and properly adjusted; 5. maintain bearings in good alignment; 6. avoid tight belts; 7. examine the air gap or clearance between armature and pole faces and see that they are uniform. ques. what troubles are encountered with short circuits in the armature or commutator? ans. this results in sparking at the brushes, and in the heating of one or more of the armature coils, and even in the burning up of the latter if a bad case. when the armature is overheated, and the defect does not proceed from an overload or the causes mentioned below, the dynamo should be immediately stopped and tested for this fault. ques. what will happen with an overheated commutator? ans. it will decompose carbon brushes and cover the commutator with a black film, which offers resistance and increases the heat. ques. what should be done if carbon brushes become hotter than the other parts? ans. use higher conductivity carbon. reduce length of brush by adjusting holder to grip brush nearer the commutator. reinforce brushes with copper gauze, sheet copper or wires, or use some form of combined metal and carbon brush. increase size or number of brush if necessary, so the current does not exceed 30 amperes per square inch of contact. brushes heat sometimes due to too much friction. they should not press against the commutator more than is necessary for good contact. ques. give some causes for heating of armature. ans. eddy currents; moisture; short circuits; unequal strength of magnetic poles; operation above rated voltage, and below normal speed. ques. what trouble is encountered with eddy currents? ans. considerable heating of the whole of the armature results, which may even extend to the bearings. [illustration: fig. 729.--eck manchester type motor. it is a very small size unit and is designed for special purposes where very little room is available. the motor occupies a space of 2-1/2" ã� 4-3/8" between bearings and develops 1/16 horse power at 2,000 r. p. m. the frame of this motor is made of high permeability steel so as to reduce the weight to a minimum. the armature is of the hand wound bipolar type built up of thin punchings. the armature, after being wound, is baked at high temperature for a prolonged period and then dipped while hot in insulating varnish. pulley is one inch in diameter and takes a 1/4 inch round belt. weight of motor 5-1/2 pounds.] ques. how can this be overcome? ans. there is no remedy for eddy currents other than the purchase of a new armature, or reconstruction. the fault may be detected by exciting the field magnets and running the machine on open circuit, with the brushes raised off the commutator for some time, when the armature will be found to be excessively heated. ques. how does moisture in the armature coils affect the armature? ans. the effect of this fault being to practically short circuit the armature, a heating of the latter results. in bad cases, steam or vapor is given off. ques. what is the effect of short circuits in the armature? ans. it produces overheating. ques. what trouble is likely to occur when the armature is not centered in the armature chamber? ans. a heating of the bearings is liable to be occasioned through the attractive forces developed by the center of the armature core not being parallel with the centre of the armature chamber or bore, or through the core being nearer one pole piece than the other. this may result from unequal wearing of the bearings, and therefore the bearings should either be relined or the bolt holes of the bearings readjusted, or the bearings packed up until the armature is correctly centered. ques. what happens in case of breaks in the armature coils? ans. this fault results in local heating of the armature, for the reason that resistance is interposed in the path of the current at the fracture. it always results in sparking at the brushes, and the heating being confined to the neighborhood of the break. ques. what are the effects of operation above the rated voltage and below normal speed? ans. voltage above normal is a possible cause of heating, and operation below normal speed calls for an increase of field strength and reduces the effective ventilation, thus tending to cause heating. [illustration: fig. 730.--forced system of lubrication as applied to engine of the generating set shown in fig. 443. in engines employing the forced system of lubrication the crank pit, which is formed by the columns, is accessible through doors in the front and back of the engine. the base of the engine forms an oil tank to which is attached a small plunger pump driven by an eccentric on the shaft. the lubricant is carried under pressure to the various parts of the engine by the mechanism shown in the accompanying diagram. the oil is forced by a pump to a groove in the main bearing, and a drilled hole in the shaft connects this groove with the crank pin. from the crank pin box the oil is further forced to the wrist pin through the pipe running along the side of the connecting rod. the passage in the crosshead allows the oil to be forced from the wrist pin to the guides. as the oil is forced from one bearing to another, it is quite important that the bearing caps be set tight, otherwise the oil will escape before reaching the last bearing. after passing through the bearings, the oil is collected in the base, strained and used again. the oil should be free from foreign substances, and to guard against the introduction of any foreign matter, a strainer, which may be taken out for examination or cleaning, is attached to the suction valve of the pump. an oil pressure of from 10 to 20 lbs. should be maintained, and may be regulated by adjusting the set screw on the relief valve of the oiling system. the pressure gauge need not remain in the circuit continuously. only mineral oils should be used for lubrication. a heavy oil gives better results and prevents knocking more effectively than thin oil. an oil which has been found to give good results, consists of two-thirds red engine oil and one-third heavy cylinder oil. as the oil passes through the bearings repeatedly, it gradually loses its lubricating properties, becoming thick and gritty, and should be occasionally run through a filter and mixed with new oil. the frequency of this change depends on the oil, as well as the number of hours the engine is in operation, and can easily be determined by observation. the oil in the reservoir should stand about 2 inches over the suction and discharge valves, and no water should be allowed to mix with it. should any water accumulate in the base, it should be drawn off by the cock provided for the purpose before starting the engine.] ques. how may the field magnets become heated? ans. by excessive field current; eddy current in pole pieces; moisture; short circuits. ques. what may be said with respect to excessive field current? ans. when heating results from this cause, all the exciting coils will be heated equally. it may be due to excessive voltage, in the case of shunt dynamos; or to an overload in the case of compound and series dynamos. in either case it may be remedied by reducing the voltage or overload. if due to the coils being incorrectly coupled up, that is, coupled up in parallel instead of in series, it will be necessary to rectify the connections or insert a resistance in series. ques. state the causes of eddy currents in the pole pieces? ans. this fault may be due to defective design or construction of the armature. slotted armatures are particularly liable to cause this fault, if the teeth and air gap be not properly proportioned. the defect may also be occasioned by variation in the strength of the exciting current. if due to this latter cause, it will be accompanied by sparking at the brushes. if a shunt dynamo, insert an ammeter into the shunt circuit, and note if the deflection be steady. if this be not the case, the variation in the current most probably proceeds from imperfect contacts thrown into vibration. ques. how is the insulation affected by moisture? ans. moisture tends to decrease the insulation resistance, thus in effect producing a short circuit with its attendant heating. ques. how is moisture in the field coils detected? ans. it is easily detected by applying the hand to the coils, when they will be found to be damp, and in addition steam or vapor will be given off where the machine is working. the fault may be remedied by drying and varnishing the coils. ques. what is the indication of short circuits in the field coils? ans. this fault is characterized by an unequal heating of the field coils. if the coils be connected in series, the faulty coil will be heated to a less extent than the perfect coils; if connected in parallel, the faulty coil will be heated to a greater extent than the perfect coils. the former can thus be easily located. chapter xxxv operation of motors in operating motors of any considerable size, whether connected to the public supply mains of a central generating station for combined lighting and power service, or to power service mains only, there are certain precautions to be observed in starting, stopping, and regulating the motor, in order that the efficiency of the supply, and indirectly the working of other motors and lamps connected to the mains in the immediate neighborhood, may not be affected by abnormal variations of pressure. these precautions should be observed also to prevent any danger of the motor itself being subjected to detrimental mechanical shocks and excessive temperatures in the working parts. before starting a motor.--the general instructions relating to inspection and adjustment, lubrication, etc., which have already been given, should be carefully followed preparatory to starting[e]. [e] note.--in starting a motor, first see that the bearings contain sufficient oil and that the brushes bear evenly on the commutator. if a circuit breaker be used, close it; then close the main switch. rotate slowly the handle of the starting rheostat as far as it will go. care should be taken, in starting the motor, that the handle of the rheostat be not rotated too fast. to stop a motor, open the circuit breaker or switch, which will cut in the resistance of the starting box. never attempt to stop a motor by forcibly pulling open the starting box, _disregard of these instructions may cause burning out of the field coils._ starting a motor.--in starting a motor, resistance must be put in series with the armature because, since there is no reverse electromotive force to counteract the applied voltage when the motor is at rest, the switching of the latter direct to the motor would result in an abnormal rush of current. this, in addition to being uneconomical and productive of a drop of voltage in the mains, would injure all except the smallest motors. hence motors above two horse power usually require a rheostat. ques. describe a rheostat or "starting box." ans. it consists essentially of a suitable resistance to be inserted at starting to reduce the initial rush of current, and which can be cut out in sections by successive movements of a lever as the speed increases. [illustration: fig. 731.--press for forcing on and removing a commutator. small commutators are pressed on to the shaft by a hand press. all of the larger commutators are pressed on by means of a power press. in the above figure is shown a hand press. the plate _b_ is used in removing old commutators. it is placed back of the commutator as at _x y_ with the slot _c_ over the shaft. bolts _a b_ are passed through the holes in the plate and secured by nuts. the commutator can then be forced off the shaft. in pressing on a commutator, a sleeve is placed over the shaft at _o_, and against the commutator. the rear end of the shaft is secured so it will withstand the pressure, and the commutator is forced on. the power presses are built on the principle of a hydraulic press. in pressing on a commutator a piece of babbitt metal or soft brass should be used against the end of the shaft. the shaft should be painted with white lead before having the commutator pressed on, in order to lubricate the shaft so that the commutator will press on easily. the wiper rings are pressed on after the commutator and then the armature is ready to be connected.] ques. describe what occurs in starting a motor. ans. when the lever of the starting box is moved to the first contact some of the resistance is cut out of the circuit and current flows through the motor. this produces a torque and starts the armature rotating. the movement of the armature induces a reverse voltage, which, as the speed increases, gradually reduces the applied current. with this reduction of current, the torque is reduced and the speed not accelerated as quickly as at first. when the applied current has been reduced to a certain value by the increasing reverse current, the handle of the starting box is moved to the next contact, and so on till all the resistance in the starting box has been cut out, the motor then attaining its normal speed. [illustration: figs. 732 to 735.--various starting resistances. the type of resistance used in motor starting rheostats of small size consists usually of tinned iron wire wound on asbestos tubes, as shown in fig. 732, the tubes being firmly supported by porcelain nipples, the ends of which fit into holes in the top and bottom of the enclosing case. in starters of larger size, cast metal grids, as shown in fig. 733, are used. in addition to these types of resistance, some forms of starter are equipped with what is known as "unit" type resistance. in this form, the resistance is built up of a number of separate sections, or units, which are connected to form the complete starting or regulating resistance as the case may be. a single unit consists of a moulded core of vitreous material upon which is wound the resistance wire, as shown in fig. 734. the surface of the unit is then coated with a special cement and baked. by this method the resistance material is protected from mechanical injury and is also made proof against moisture and other conditions which sometimes affect the ordinary type of resistance. in addition to units coated with cement only, there are still other types of units, as in fig. 735, which are provided with a sheet metal covering around the cement, as a further precaution against injury. each of the various types of resistance described possesses certain characteristics not shared by the others, the use of any particular type being largely governed by conditions of service.] ques. what is the difference between a starting box and a speed regulator? ans. motor starting rheostats or "starting boxes," are designed to start a motor and bring it gradually from rest to full speed. they are _not_ intended to regulate the speed and must not be used for such purpose. _failure to observe this caution will result in burning out the resistance which, in a motor starter, is sufficient to carry the current for a limited time only_, whereas in the case of speed regulators sufficient resistance is provided to carry the full load current continuously. [illustration: fig. 736.--view of cutler-hammer starter with slate front removed showing open wire coil resistance. the type of resistance here used consists of tinned iron wire wound on asbestos tubes. the bottom of the casing is perforated to secure ventilation.] ques. for what kinds of service are speed regulators used? ans. in cases when the speed must be varied, as in traction motors, organ blowers, machine tool drive, etc. ques. how long does it take to start a motor? ans. usually from five to ten seconds. ques. how is the starting lever operated? ans. it is moved progressively from contact to contact, pausing long enough on each contact for the motor to accelerate its speed before passing to the next. ques. what are the conditions at starting in a series motor? ans. there is a rush of current, the magnitude of which depends on the amount of resistance cut out at each movement of the starting lever. [illustration: figs. 737 and 738.--sliding contact starters. fig. 737, starter with button contacts; fig. 738, starter with renewable contacts. motor starters in which the successive steps of resistance are cut out by a pivoted lever carrying a contact shoe which slides over button contacts or over contact segments, are known as sliding contact starters. button contacts are usually furnished with motor starting rheostats of small size while contact segments are used on those of greater capacity. the contact segment being held in position by two screws, is readily renewable when worn by long service or damaged by arcing. the fixed button contact is not so easily renewed but being used only on small size starters is never likely to be subjected to severe service. some starters, however, have renewable button contacts.] ques. how are small series motors started on battery circuits? ans. by simply closing a switch to complete the circuit, the resistance of the battery being sufficient to prevent a great rush of current while starting. ques. how is a shunt motor started? ans. in starting a shunt motor, no trouble is likely to occur in connecting the field coils to the circuit. since the resistance of the armature is very low, it is necessary on constant voltage circuits to use a starting rheostat in series with the armature. the necessary connections are shown in fig. 756. the switch is first closed thus sending current through the field coils, before any passes through the armature. the rheostat lever p is then moved to the first contact to allow a moderate amount of current to pass through the armature. the resistance of the rheostat is gradually cut out by further movement of the lever p, thus bringing the motor up to speed. [illustration: figs. 739 and 740.--multiple switch starters. fig. 739, starter with no voltage release; fig. 740, starter with no voltage release and circuit breaker. the multiple switch type of starter is designed to overcome the arcing on sliding contacts which, in the case of large motors would be very severe. the cutting out of each step of resistance is accomplished in the multiple switch starter by a separate carbon contact switch which breaks the circuit with a quick snappy action.] ques. how does the reverse voltage affect the starting of a motor? ans. when a motor is standing still, there is no reverse voltage, and the current taken at first is governed principally by the resistance of the circuit. if the motor be series wound, there is a momentary reverse voltage, due to self-induction while the field is building up. if the motor be shunt wound, self-induction delays the current through the field coils, but that through the armature is not impeded by such cause. when the armature begins to revolve, reverse voltage is developed which increases with the speed. the resistance of the starting box may be gradually cut out as the armature comes to speed. thus the reverse voltage gradually replaces ohmic drop in limiting the current as the motor comes to speed. [illustration: fig. 741.--starting rheostat with no voltage and overload release. the no voltage release permits the starting lever to fly to the "off position" should the voltage fail momentarily, thus protecting the motor against damage should the voltage suddenly return to the line. the movement of the lever is due to a spring. the overload device causes the lever to back to the off position should the current exceed a predetermined maximum for which the release is adjusted. fig. 742.--compound starter. rheostats designed for the double duty of starting a motor and regulating its speed are commonly known as compound starters, the resistance provided being a combination of armature resistance for starting duty and shunt field resistance for speed regulation.] failure to start.--this fault, which is liable to occur in a motor of any description, is similar to failure to excite in a dynamo, and is liable to be produced by any of the causes mentioned in connection with the latter fault, excluding insufficient speed, and insufficient residual magnetism. when a motor fails to start, it should first be ascertained if a supply of electrical energy be available in the mains. this may readily be discovered by means of a voltmeter, or if low tension service, by means of the fingers bridging across the main terminals. if the supply of energy be present, the contact arm of the starter should be moved into such position that all resistance is inserted into circuit with the motor. this is important, as the motor may start suddenly while trying to ascertain the cause of the stoppage. [illustration: fig. 743.--starting panel. in installing any kind of motor starting rheostat, it is necessary to provide main line knife switch and fuses in addition to the starting box. the appearance of the installation can be much improved by mounting all of these upon one panel.] having closed the switch, if the motor fail to start, it will be advisable to remove the load if possible, as the failure may arise from an overload of the machine. this being effected and the motor not starting, the terminals of the latter should be tested by the means already described for voltage. if no voltage be generated, a broken circuit or a defective contact may be looked for in the main fuse, switch, or starting box. the resistance coils of the latter, through the heat developed, frequently break in positions out of sight. if a defective contact of this nature cannot readily be seen, the contact arm should be moved slowly over the contacts, as it is possible the broken coil may be cut out of circuit by this means. [illustration: figs. 744 to 746.--cutler-hammer motor starting rheostats with no voltage and overload release. fig. 744, starter with fixed button contact, fig. 745, with renewable button contact, and fig. 746, with contact segments. in construction the resistance is enclosed in a pressed steel box on which is mounted a marbleized slate panel carrying the starting lever, contacts and protective devices. by means of a calibrated scale, the overload release (shown in the lower left hand corner, figs. 744 and 745, and in the lower right hand corner fig. 746) can be set to break the circuit on any overload not exceeding 50 per cent. of the rated capacity of the motor. this calibrated scale can also be used for determining, with a fair degree of accuracy, the amount of current being consumed by the motor.] if a difference of pressure exist between the motor terminals, the field magnets will, if shunt or compound wound and in good order, be excited, which may be ascertained by means of a bar of iron. if no magnetism be present, it will of course, indicate a broken or bad connection, either between the terminals of the field coils, or one or more of the coils themselves. if the bar pull strongly, the position of the brushes upon the commutator in regard to the neutral points should be ascertained, and the rocker adjusted, if necessary, to bring them into their correct positions. if this fail to start the motor, the connecting leads from the motor terminals to the brushes and the brushes themselves should be carefully examined for broken or bad connections, and defective contact of the brushes with the commutator. in the latter case, it may arise from a dirty state of the commutator, or from the brushes not being fed properly. if due to these causes, pressing the brushes down upon the commutator with the fingers will probably start the motor. if the failure to start arise from none of these causes, it is probably due to the field coils acting in opposition, or to a short circuited armature. this latter remark applies more especially to motors provided with drum armatures. [illustration: fig. 747.--allen-bradley compression type resistance units. the contact resistance between the discs composing the resistance column is subject to variations of pressure thereby producing proportionate resistance changes in the column as a whole. in the complete resistance unit, the resistances column is encased in a drawn steel tube, which is lined with a highly refractory cement, for purpose of insulation, _affording the column both mechanical and electrical protection_ and excluding the air which effectually prevents any combustion should the column become red hot due to overload. the ends of the tube are closed by means of caps through which pass electrodes for making connections between the discs and exterior conductors. the steel tube, when necessary, is provided with ribs or fins for the dissipation of acquired heat.] [illustration: fig. 748.--allen-bradley type z automatic motor starter. the operation of this machine is as follows: when the main switch is closed, the motor circuit is made through the fuses, resistance unit, current relay, and the motor armature. at the same time, the solenoid circuit is closed (this is connected directly across the line, and takes a current which is a small fraction of an ampere), and the plunger of the solenoid is drawn up, which produces a pressure on the resistance unit, and increases the current in the motor circuit to the predetermined value at which the current relay is set. when this value is reached the current relay operates and opens the solenoid circuit, which reduces the magnetic pull and allows the solenoid plunger to drop back slightly. this action increases the resistance in the motor circuit, which decreases the current sufficiently to allow the relay to close again. similar cycles of operation are repeated as the motor accelerates, and each time the plunger is drawn a little farther into the solenoid, until the short circuiting contacts on the top are pushed together, which short circuits the current relay and resistance unit, making them inoperative, and completing the operation of starting the motor. it will be noted that in starting a motor with this device the current is always held down to a certain predetermined value, and it is impossible to overload the motor by too rapid starting. the current relay is calibrated in amperes, and may be set to suit existing conditions. the action of the starter being controlled by a current relay and not by an oil or air dash pot, the motor will start rapidly when under a light load, and slowly when more heavily loaded. the fuses or circuit breakers may be set at a value that will furnish protection to the motor under running conditions.] precautions with shunt motors.--with motors of this type, because of the large amount of self-induction in the shunt windings, it is important to note: 1, that in switching on the field magnet, the current may take an appreciable time to grow to its normal value, and 2, that in switching off, especially with quick break switches, high voltages are induced in the windings, which may break down the insulation. [illustration: fig. 749.--monitor starter giving automatic start with knife switch control; designed for use with printing presses. it consists of a set of solenoids connected in series and so interoperating as to cut resistance out of the armature circuit of the motor as the apparatus it is driving comes up to speed. this type is for small motors or where no need arises for speed regulation; there is, therefore, no adjustment of speed possible aside from an actual change in motor conditions. at full speed the motor is directly across the main supply line. fig. 750.--monitor automatic starter, equipped with relay for push button control.] ques. what provision is made so that the magnetizing current will have time to reach its normal value? ans. the field connections are generally separated from the actual starter, and taken to the main switch, so that wherever the main switch is closed, the current flows through the field coils, before the starting lever is moved. ques. how are the connections arranged to avoid excessive voltage in the windings due to self-induction? ans. generally the armature and field magnet circuits are placed in a closed circuit that is never opened. in other cases, in order that the rise of voltage may not injure the insulation when the shunt is opened, a special form of main switch is sometimes used which, before breaking from the supply, puts a non-inductive resistance across the shunt of the motor. this is known as a _flashing resistance_. [illustration: figs. 751 to 753.--monitor control switches. fig. 751, push button "start" and "stop" switch; fig. 752, safety lever control switch with "slow" and "fast" buttons for rotary printing presses. this device will upon pressure of the "start" button, set the machine in motion and bring it up to the predetermined speed, either as previously set by the starter limits or by the setting of the rheostat arm. the stop button projects some distance beyond any other portion of the device, in order that in case of emergency the operator may stop the machine merely by hitting the face of the switch with his open hand. the lever control switch, fig. 753, is similar in its action to the push button switch but combines two other features: locking point, and visual indication of the station from whence the press has been stopped. with the lever at the downward position, the press is locked and cannot be started from any other station. in order to make the press ready to start the lever must be raised to the central position. thus a man may safely enter the press without delay by setting his station to the locked position, knowing that it cannot be started except by some one coming to that station and realizing that the press has been purposely locked. also, by looking along the press, it is possible to tell from which station it has been locked, and proper action can be immediately taken. the safety control station is usually combined for use on large rotary presses with the "slow" and "fast" push buttons as shown in fig. 752. a pressure upon the fast or slow buttons will cause the press speed to be correspondingly accelerated or retarded, and this action will continue so long as the button is pressed. the press continues to run at the speed attained at the instant of releasing the button. any speed may, therefore, be selected or changed to suit momentary requirements. this gives complete control excepting reversal which is not required of such a press.] [illustration: fig. 754.--wiring diagram of the standard form of monitor controller. a set of solenoids are connected in series and so interoperating as to cut resistance out of the armature circuit of the motor as the apparatus it is driving comes up to speed. the controller is designed to be used on all classes of work. in its simplest form, a single copper and graphite contact, is controlled by two magnets, so proportioned as to cut out the entire starting resistance when the armature current falls to a predetermined value. in the larger sizes, the number of steps controlling the resistance is increased and arranged to produce the correct acceleration. in every case the regulation of the starting resistance is effected entirely by the current passing to the motor without the use of dash pot, mechanical governor or delicate cut outs. the time element is thus directly proportioned to the load and the motor brought up to speed in the shortest time consistent with the load, but always with safe limitation of the armature current. the distinction between the current controlled starter and the starter with dash pot governor should be noted. the starter here shown limits the starting current to a fixed value throughout the starting operation, which is an ideal condition and prevents blowing fuses in starting.] ques. how can shunt motors be controlled from a distant point? ans. the starter and switch are placed at the desired point and the two main wires and the field wires run from that point to the motor. this requires additional wire which increases the cost and line loss. regulation of motor speed.--motors are generally run on constant voltage circuits. under these conditions, the speed of series motors varies with the load and at light loads becomes excessive. shunt motors run at nearly constant speeds. for many purposes, particularly for traction, and for driving tools, it is desirable to have speed regulation, so that motors running on constant voltage circuits may be made to run at different speeds. the following two methods are generally used for regulating the speed of motors operated on constant voltage circuits: 1. by inserting resistance in the armature circuit of a shunt wound motor; 2. by varying the field strength of series motors by switching sections of the field coils in or out of circuit. ques. describe the first method. ans. this method is illustrated in fig. 756. when the main switch is closed, the field becomes excited, then by moving the lever p of the starting rheostat the various contacts (1, 2, 3, 4, 5), more or less of the rheostat resistance is cut out of the armature circuit, thus varying the speed correspondingly. this is the same as the method of starting a motor, that is, _by variation of resistance in armature circuit_, but it should be noted that when this method is used for speed regulation, _a speed regulating rheostat should be used instead of the ordinary starting box_, because the latter, not being designed for the purpose, _will overheat and probably burn out_. [illustration: fig. 755.--monitor printing press controller. it provides variable speed and other control features required in the operation of large rotary presses, such as those used for printing newspapers. from any one of various stations similar to the one illustrated in fig. 753, located at all desirable places about the press, the latter may be started, stopped, accelerated, slowed down or locked. it differs from other types of printing press controller in that the solenoid has an overall maximum pull of less than one inch and does not actuate the main line current directly but through pilot circuits, which in turn, operate flapper switches; there are no sliding contacts. at the control stations, the operator can distinguish the accelerating button from the retarding button by the sense of touch and obviously he can in the same manner ascertain the position of the lever. the position of the lever whether at start, stop or safety, can be readily observed at a distance. when the lever of either control station is placed at stop, the current is disconnected from the motor and a powerful dynamic brake brings the press to rest without delay and without shock or harmful strain. the start will always be made with all resistance in the armature circuit, and with full field, and should the current supply fail, the controller will release and open the circuit to the motor. this controller will give a speed range as low as 10% of normal speed by armature resistance and, by field control, any increase within the speed of the motor.] ques. describe the second method. ans. this method of regulating the speed of a series motor is shown in fig. 757. the current through the armature will flow through all the field windings when the position of the switch lever s, is on contact 4, and the strength of the field will be the maximum. by moving the arm to contact 3, 2, etc., sections of the field winding are cut out, thus reducing the strength of field and varying the speed. [illustration: fig. 756.--speed regulation of shunt motor by variable resistance in the armature circuit.] ques. how does the speed vary with respect to variation of field strength? ans. decreasing the field strength of a motor increases its speed, while increasing the field strength decreases the speed. under the conditions of maximum field strength, as with switch s on point 1, the torque will be greatest for any given current strength and the reverse voltage also greatest at any given speed. the current through the armature of the motor, to perform any given work, will thus be a minimum, as well as the speed at which the motor has to run, in order to develop sufficient reverse voltage to permit this current to flow. regulation of speed by varying the field strength is limited in range of action, since the field saturation point is soon reached, moreover, with too low a field strength, armature reaction produces excessive field distortion, sparking, etc. [illustration: fig. 757.--speed regulation of series motor by cutting out sections of the field winding. in this method the field winding is tapped at several points, dividing the coil into sections and the leads from these points are connected a multi-point switch of the type that would be used on a rheostat. by moving the lever s, to the left or right, the current will flow through one or more sections of the field winding, thus decreasing or increasing the ampere turns and thereby providing means of regulation.] [illustration: fig. 758.--speed regulation of a series motor by the method of short circuiting sections of the field winding. it will be seen that there are seven different positions for the contact springs on the barrel contacts. a. represents the armature and brushes, little a, b, and c, the divided field magnet coils, l the line connection, and g the earth connection. the diagram shows the connections for trolley car operation.] ques. how is the speed of shunt and compound motors varied with respect to the normal speed in the two methods? ans. the first method (variable resistance in armature circuit) reduces the speed _below_ the normal or rated speed of the machine, while the second method increases the speed _above_ the normal. in the first method the amount of speed reduction depends partly upon the amount of resistance introduced into the armature circuit, and partly upon the load. in the second method the amount of speed increase depends entirely upon the amount of resistance placed in the shunt winding circuit. eighty-five per cent. is about the maximum speed reduction obtainable by armature resistance but so great a reduction is seldom satisfactory since comparatively slight increases in the load will cause the motor to stall. shunt field regulation may be obtained up to any point for which the motor is suited, the only limitation in this case being the maximum speed at which the motor may be safely operated. it should be remembered, however, that speed increase by shunt field weakening increases the current in proportion to the increase in speed, and care should be taken not to overload the armature. note.--a compound motor may be made to run at constant speed, if the current in the series winding of the field be arranged to act in opposition to that of the shunt winding. in such case, an increase of load will weaken the fields and allow more current to flow through the armature without decreasing the speed of the armature, as would be necessary in a shunt motor. such motors, however, are not very often used, since an overload would weaken the fields too much and cause trouble. if the current in the series field act in the same direction as that in the shunt fields, the motor will slow up some when a heavy load comes on, but will take care of the load without much trouble. note.--motors have much the same faults as dynamos, but they make themselves manifest in a different way. an open field circuit will prevent the motor starting, and will cause the melting of fuses or burning out of the armature. a short circuit in the fields, if it cut out only a part of the winding, will cause the motor to run faster and very likely spark badly. if the brushes be not set exactly opposite each other, there will also be bad sparking. if they be not at the neutral point, the motor will spark badly. brushes should always be set at the point of least sparking. if it become necessary to open the field circuit, it should be done slowly, letting the arc gradually die out. a quick break of a circuit in connection with any dynamo, or motor is not advisable, as it is very likely to break down the insulation of the machine. the ordinary starting box for motors is wound with comparatively fine wire and will get very hot if left in circuit long. the movement of the arm from the first to the last point should not occupy more than thirty seconds and if the armature do not begin to move at the first point, the arm should be thrown back and the trouble located. [illustration: fig. 759.--cutler-hammer multiple switch starter with no voltage release; for use with large motors, or with motors of medium size where the starting conditions are severe or when more than fifteen seconds are required to accelerate the motor. in operation, the cutting out of each step of resistance is accomplished by a separate lever and the levers themselves are so interlocked as to prevent closing switches except in proper order, beginning with the lever on the left. the last switch (the one on right hand side) is held by an electro-magnet when closed, each of the other switches being held in the closed position by a latching device on the switch next to it. in front of each switch is placed a metal stop, so arranged as to prevent any switch being operated until the one next to it on the left has been closed. these metal stops constitute the interlocking mechanism and prevent the starting of the motor in any way except by closing the switches in regular rotation, thus insuring proper resistance in the circuit and protecting the motor from excessive starting currents. when the current is interrupted, the electro-magnet releases the last switch, which, on opening, releases the latch on the switch next to it, allowing that switch to open, and this in turn releases the next latch and so on, the switches opening automatically one after another. in starting the motor, each switch should be closed quickly and firmly, pausing a second or two before closing the next switch to give the motor time to accelerate.] [illustration: fig. 760.--cutler-hammer speed regulator with no voltage release, _regulation by armature resistance only_, reducing speed of motor _below normal_. no resistance in the armature circuit. no provision is made in regulators of this type for increasing the speed of the motor. the maximum speed obtainable when these regulators are used is, therefore, the normal speed at which the motor is designed to operate with no resistance in circuit. with all resistance in circuit and the motor taking normal current these regulators will reduce the speed of the motor 50 per cent. if the motor be taking less than normal current the percentage of speed reduction obtainable will be correspondingly less. the notched fan tail extension on the lower end of the lever engages with a magnetically operated pawl to hold the lever squarely on any contact so long as the no voltage release magnet is energized.] ques. how is a wide range of speed regulation secured? ans. by a combination of the two methods. regulation by armature resistance.--speed regulators for this method of regulation, are designed to carry the normal current on any contact without overheating and when all the resistance is in the circuit, they will reduce the speed of the motor about 50 per cent. provided the motor be taking the normal current. when operating without resistance in the armature circuit, shunt wound and compound wound motors will regulate to approximately constant speed regardless of load. this characteristic of inherent regulation is lost, however, when armature resistance is employed to reduce the speed of the motor, fluctuations in load resulting in fluctuations in speed, which become more noticeable as the amount of resistance inserted in the armature circuit is increased. accordingly, it becomes necessary to move the lever of the speed regulator forward or backward to again obtain the speed at which the machine was operating before the load changed. [illustration: fig. 761.--cutler-hammer compound starter with no voltage and overload release. this is a starting rheostat and field regulator combined. in operation, two levers are employed, both being mounted on the same hub post and one lying directly under the other. the upper lever only is provided with a handle, but when moving from the off position to the starting position (that is to say, from left to right) the lower, or starting, lever is carried along by the upper, or speed regulating, lever until it comes in contact with the no voltage release magnet where it is held fast by the attraction of the magnet, leaving the upper lever free to be moved backward over the field contacts, thus weakening the shunt field of the motor little by little until the desired speed is attained. during the operation of starting the motor, the field resistance is short circuited by an auxiliary contact (the slotted metal strip shown near center of rheostat) but as soon as the starting lever touches the no voltage release magnet or, in other words, when the motor has been accelerated to normal speed, this short circuit is removed, and the field resistance becomes effective for speed regulation. the motor is accelerated from rest to normal speed by moving both levers from left to right, while increases in speed above normal are obtained by moving the upper lever from right to left. only the lower, or starting lever comes into contact with the no voltage release magnet. this lever is provided with a strong spiral spring which tends always to throw the lever back to the off position. hence should the voltage fail, the no voltage release magnet releases the starting lever and this, in flying back to the off position, opens the armature circuit of the motor and carries the speed regulating lever with it to the off position. the upper, or speed regulating lever, not being influenced by the spring, though mounted on the same hub post as the starting lever, may be moved back and forth at will or left indefinitely in the position which gives the speed desired.] when the speed of a motor driving a constant torque machine is reduced by inserting resistance in the armature circuit there is no corresponding reduction in current consumed. the motor runs more slowly simply because a part of the energy impelling it is shunted into the resistance and there dissipated in the form of heat. hence, whether the motor be operating at full speed or half speed, the amount of current consumed is the same; the only difference being that in the one case all the energy taken from the line is expended in driving the motor while in the other case only one half is utilized for power, the other half being dissipated in the resistance. speed regulation by armature resistance only is therefore open to two objections: 1, the difficulty of maintaining constant speed under varying load conditions, and 2, the necessity of wasting energy to secure speed reduction. these objections are, in part, offset by the fact that speed reduction by armature resistance may be applied to any motor of standard design and requires nothing more than the simplest and least expensive speed regulating rheostat. in cases where the motor will be operated nearly always at full speed, the difference in first cost of the installation may justify the use of the armature resistance method of control. as a rule, speed regulation by shunt field resistance is preferable. [illustration: fig. 762.--cutler-hammer compound speed regulator with no voltage and overload release; _regulation by combined armature and shunt field resistance_, designed to both decrease and increase the speed of a motor. speed reduction is accomplished by inserting resistance in the armature circuit, the maximum amount of speed reduction obtainable with these controllers being 50 per cent. below normal. speed increase is obtained by inserting resistance in the shunt field circuit, the maximum amount of speed increase obtainable with these controllers being 25 per cent. above normal.] regulation by shunt field resistance.--since regulation by this method is for speeds above normal, a starter must be used to bring the motor up to its rated speed. usually the starter is combined with the regulator, as shown in fig. 761, the device being called a _compound starter_. [illustration: figs. 763 to 765.--holzer-cabot shunt wound motor; diagrams showing connections and positions of index point for forward and reverse rotation. location and setting.--the motor should be placed in as cool, clean and well ventilated a location as possible, away from acid or other fumes which would attack the metal parts or insulation, and should be easily accessible for cleaning and oiling. do not put it in some corner where care of motor will be neglected because of the trouble of getting at it. the motor should be set so that the shaft is level and parallel with the shaft it is to drive so that the belt will run in the middle of the pulleys. do not use a belt which is much too heavy or too light for the work it has to do, as it will materially reduce the output of the motor. the belt should be about one-half inch narrower than the pulley. rotation.--in order to reverse the direction of rotation, interchange leads a and b, and shift brush ring as shown in the diagram above. suspended motors.--motors with ring oil bearings may be used on the wall or ceiling by taking off end caps and revolving 90 or 180 degrees until the oil wells come directly below the bearings. starting.--before starting the motor see that the armature revolves freely, that the bearings are full of oil, and the oil rings are in place and free to turn. examine connections carefully to see that they are according to above diagram, after which proceed as follows: 1. close the main knife switch. this action should not allow any current to pass through the motor (see note 2); 2. move the lever of the starting rheostat quickly and squarely onto the first segment, and hold it there for about a second; 3. move the lever to the second segment and hold it there for about a second; 4. move the lever to the third segment and hold it there for about a second, and so on from one segment to the next until the lever has been moved over all the segments to the short circuit position, where it should be held firmly by the retaining magnet. if the motor do not start when the lever of the starting rheostat is on the third segment, open the main knife switch and look for the trouble. this may consist of any of the following: a. wrong connections; b. too great a load on the motor; c. the motor brushes not in proper position; d. an open circuit of some kind; e. a short circuit of some kind. note 1.--it is always advisable, in case of trouble, to make sure that the fields of the motor are magnetized. this test is easily made by first closing the main knife switch, then moving the lever of the starting rheostat to the first segment, and finally having an assistant place a screw driver or other piece of iron against the pole pieces of the motor. if the fields be magnetized, a heavy pull on the iron should result. note 2.--any possibility of arcing on the first contact of the starting rheostat when starting can be obviated by _first_ moving the lever onto the initial contact, holding it there, and then closing the main line switch, after which proceed as per paragraphs 3 and 4. to stop the motor.--open the main knife switch and let the starting rheostat take care of itself. the lever will not fly back immediately, but will hold until the motor has slowed down considerably. note.--the above directions apply only to starters of the sliding contact type. temperatures.--if located as instructed above, these motors will carry full load as indicated on the name plate on the motor with a temperature rise of not over 40 degrees centigrade, or 75 degrees fahrenheit above the surrounding air. this will feel hot to the hand but is far below the danger point. if the motor feel too hot, get a thermometer and measure the temperature. to do this, place the bulb of the thermometer for 10 minutes against the frame, cover with a cloth or piece of waste, and note temperature as compared with that of room. if the motor run in a small, enclosed space with no ventilation, the temperatures will be somewhat higher than those given above. oiling.--fill the oil wells to the overflow before starting and keep them full. use good "dynamo oil." be sure that the oil rings turn freely while the motor is running. if in a dirty place, draw off the old fluid and fill with new every two or three months. care of motor.--the motor must be kept clean. if the commutator become rough, smooth it up with no. 00 sandpaper moistened with oil. when fitting new brushes or changing them, always sandpaper them down until they fit the commutator perfectly, by passing to and fro beneath the brush a strip of sandpaper, having the rough side toward the brush. brushes must _always_ be renewed before the metal of the holder comes in contact with the commutator. don't use anything on commutator except good mineral machine oil, or kerosene, and this only in very small quantities applied with a cloth having no lint or threads.] [illustration: fig. 766.--sectional view showing principal parts of reliance adjustable speed motor: 1, lever fulcrum pin; 2, lever; 3, sliding thrust bearing box; 4, ball bearing; 5, armature shaft end nut; 6, cap; 7, commutator end yoke; 8, lever rod; 9, compression spring; 10, steel frame; 11, speed adjustment nut; 12, thrust collars and pins; 13, hand wheel rod; 14, hand wheel; 15, sleeve nut; 16, oil well cover; 17, bearing bushing; 18, sleeve; 19, oil ring; 20, pinion end yoke; 21, rocker arm; 22, brush holder stud; 23, brush; 24, commutator; 25, armature; 26, armature laminations; 27, armature coils; 28, armature end plate; 29, armature shaft; 30, leads; 31, axial position of commutating pole; 32, axial position of main field pole; 33, slide rail screws; 34, end yoke cap screws; 35, slide rails; 36, commutating coil; 37, commutating pole; 38, main field pole; 39, main field coil.] [illustration: fig. 767.--cutler-hammer reversible starter with no voltage release, adapted to start and operate motor at full speed in either direction, such for instance as motors driving auxiliary motions on lathes, planers and other machine tools which may rotate in either direction but always at constant speed. they are not designed to reduce the speed of the motor, but merely to start it and bring it smoothly up to full speed in either direction. two no voltage release latching devices are provided so that the lever will be held in the full speed position in either direction so long as the voltage of the line remains constant. on failure of voltage a strong centering spring attached to the hub-post of the lever throws the latter to the central, or off position. the shunt field circuit is not opened by starters of this type.] the weakening of the shunt field of a motor by the insertion of resistance in the shunt field circuit causes the armature to revolve more rapidly. one advantage of this method of control is that the motor will inherently regulate to approximately constant speed under widely varying load conditions. another advantage is found in the fact that all of the current taken from the line is utilized for power, the changes in speed being obtained not by dissipating a portion of the effective energy in the resistance (as in the case of the armature resistance method of control) but by weakening the reverse voltage by inserting resistance in the shunt field circuit. speed increase by shunt field weakening is limited, however, to about 10 to 15 per cent. above the normal speed in motors of standard construction. greater ranges of speed can be obtained from motors especially designed for shunt field control but should not be attempted with motors of standard design without first ascertaining from the manufacturer the maximum safe speed. combined armature and shunt field control.--regulation by combined armature and shunt field resistance is by far the easiest way of obtaining a wide range of speeds. rheostats embodying these methods are known as _compound speed regulators_, one form being shown in fig. 762. standard regulators can be obtained giving a wide range of speed variation, and special regulators may be constructed giving practically any desired range. selection of starters and regulators.--unsatisfactory operation of these devices is, in nearly all cases, due to lack of precaution in selecting the proper piece of apparatus for the work to be done. one of the commonest errors is to select a rheostat of insufficient capacity. if the current required to operate the motor at full speed with no resistance in circuit be greater than the rated capacity of the rheostat, overheating of the resistance will result. an increase in temperature even to a point where the hand cannot be held on the enclosing case need cause no apprehension, but should the resistance become red hot it indicates that the apparatus is being worked far beyond its capacity, and the load on the motor should be reduced or a regulator of greater capacity substituted. if the current required to operate the motor at full speed with no resistance in circuit be less than the rated capacity of the rheostat no overheating will occur, but it will not be possible to secure the full 50 per cent. speed reduction the rheostat is designed to give with all resistance in circuit. [illustration: fig. 768.--various sizes of watson commutator. the segments are punched from hard drawn copper strip and are insulated from each other and the core by amber mica, of hardness corresponding to that of the copper in order that the wear of mica and copper may be uniform. the segments are assembled in a ring under great pressure and are repeatedly heated and tightened, being finally secured and rigidly locked together.] in ordering a starter or regulator, the manufacturer should be furnished with the following information: 1. horse power of motor with which speed regulator will be used; 2. voltage of motor; 3. winding of motor, whether series, shunt, or compound wound; 4. nature of the machine which motor is to operate; 5. normal rated speed of motor to be used; 6. maximum speed at which it is desired to operate the motor; 7. minimum speed at which it is desired to operate the motor; 8. whether controller will ever be required to reverse direction of motor or to operate it in one direction only; 9. if reversible controller be desired, whether or not full range of speed control is required in both directions; 10. whether the regulator shall be equipped with any of the following devices: no voltage release, overload release, knife switch, fuses; 11. whether button contacts or renewable contact segments are preferred; 12. giving, also, if possible, the resistance of the shunt field cold, and the shunt field current at the maximum speed required. if this cannot be ascertained, give horse power, voltage, normal speed, maximum speed required, serial number of motor and name of manufacturer. [illustration: fig. 769.--organ blower speed regulator; diagram showing operation and method of installing. a cord running from the top of the organ bellows passes over two pulleys and is then made fast to the chain furnished with the regulator. this chain passes around a sheave which turns on a post projecting from the center of the slate panel. attached to the lower end of the chain is a weight, also furnished with the regulator. as the air is exhausted from the bellows the latter slowly collapses, drawing the rope down with it, and in so doing turns the sheave from left to right, thus cutting resistance out of circuit and increasing the speed of the motor which pumps air into the bellows. responding to the inrush of air, the bellows expands, relaxing the tension on the rope which is now pulled in the opposite direction by the weight, thus turning the sheave from right to left, cutting resistance into circuit once more and slowing down the motor. the speed of the motor is thus automatically regulated by the bellows, with the result that a practically uniform pressure is maintained at all times. in connection with an organ blower regulator it is necessary to install a separate starting rheostat. this is required for the reason that all organ bellows leak. during the intermissions in the musical part of the service, or at other times when the blower is not operating, the air gradually escapes and the bellows settles down, moving the rheostat arm to the right and cutting out resistance. with the motor at rest and the bellows empty all the blower regulator resistance would be short circuited and it is therefore necessary to avoid throwing the motor directly across the line when starting again. a starting rheostat with no voltage release is suitable for this purpose, and should be installed within easy reach of the organist, so that a moment or two before beginning to play he can move the lever of the starting box and get the motor into operation. where remote control is desirable a self starter can be substituted for the manually operated starting box, in which case the entire installation can be controlled by a push button, or single throw knife switch.] [illustration: fig. 770.--general electric type k7 controller with cover open showing construction. the mechanism consists of a long spindle, carrying a number of heavy brass or gun metal segments, making contact for a longer or shorter time with a corresponding number of spring contacts. the spindle is provided at its upper end with a handle, and the various contacts are made by turning it through an arc of about 150â°. for this method a moderate amount of resistance is employed. the first contact joins both motors and the full amount of resistance in series across the line, and as the motors are standing still, maximum current flows so that they exert their full torque. the moment they start to revolve, the current tends to fall, due to the generation of a reverse voltage; to prevent this and maintain a heavy current for some time, thus obtaining rapid acceleration, the resistance is arranged so that it can be gradually reduced, until at about the fourth notch the two motors are in series without resistance across the line. to increase still further the speed in the above type of controller, the series fields may be shunted, and then the next steps place the motors in parallel with the resistance.] speed regulation of traction motors.--the speed regulator for motors of this class is called a _controller_, and being located in an exposed place is enclosed in a metal casing. controllers are designed to be used for starting, stopping, reversing, and regulating the speed of motors where one or more of these operations have to be frequently repeated. the controller used with a single motor equipment is practically the same as any other single motor starting box, excepting that the resistance has sufficient carrying capacity to be left in the circuit some time. when the motor is to operate at full speed all the resistance is cut out. to reverse, a reversing notch is placed in the armature or field circuit, but not in both. ques. what provision is made to overcome the arc when the circuit is opened? ans. a magnetic field is used with such polarity that it blows out the arc. [illustration: fig. 771.--controller of the rauch and lang electric vehicles. it is of the flat radial type. two movable copper leaf contacts of ample size make all commutations necessary to obtain the various speeds. five speeds forward and reverse are provided.] magnetic blow out coils are used on all controllers designed for 500 volt circuits, and on types designed for lower voltages requiring more than 60 amperes normal capacity. the coils are wound with either copper wire or flat strips of sufficient capacity to carry full load current continuously without undue heating, and after being wound they are treated with an insulating compound making them moisture proof. ques. what provision is made to prevent reversal before bringing the controller lever to the "off" position? ans. controllers having separate reversing cylinders are fitted with mechanical interlocks making it necessary to place lever in off position before reversing. [illustration: figs. 772 to 782.--diagram of controller connections, illustrating the series parallel method of two motor control.] two motor regulation.--with a two motor equipment, the controller becomes more complicated because it must be arranged to switch the motors in series or in parallel, so as to secure economy at half and full speed. the various connections of series-parallel regulation are shown in figs. 772 to 782. from these diagrams it is seen that the motors are first operated in series until all the resistance is cut out by the controller (figs. 772 to 777). the next point on the controller puts the two motors in parallel with some resistance in the circuit (fig. 778), which resistance is gradually short circuited on the remaining controller points, until at full speed all the resistance is cut out, the two motors remaining in parallel (figs. 778 to 782). stopping a motor.--if it be desired to stop a motor, the main switch is opened. as the armature of the motor continues to operate, due to its inertia, it generates an electromotive force which sends a current through the shunt connected field circuit and helps to maintain the field excitation. when the speed of the motor has decreased sufficiently so as not to endanger the motor should the main switch be thrown, the current in the series magnet becomes weakened, and the spring throws back the starting box arm. it should be noted that in stopping a motor having a starting box provided with a no voltage release simply open the main switch and do not touch the lever because otherwise, the self induced voltage of the field circuit may puncture the field winding or the insulation of the adjoining wires in the starting box. hawkins practical library of electricity in handy pocket form price $1 each _they are not only the best, but the cheapest work published on electricity. each number being complete in itself. separate numbers sent postpaid to any address on receipt of price. they are guaranteed in every way or your money will be returned. complete catalog of series will be mailed free on request._ electrical guide, no. 1 containing the principles of elementary electricity, magnetism, induction, experiments, dynamos, electric machinery. electrical guide, no. 2 the construction of dynamos, motors, armatures, armature windings, installing of dynamos. electrical guide, no. 3 electrical instruments, testing, practical management of dynamos and motors. electrical guide, no. 4 distribution systems, wiring, wiring diagrams, sign flashers, storage batteries. electrical guide, no. 5 principles of alternating currents and alternators. electrical guide, no. 6 alternating current motors, transformers, converters, rectifiers. electrical guide, no. 7 alternating current systems, circuit breakers, measuring instruments. electrical guide, no. 8 alternating current switch boards, wiring, power stations, installation and operation. electrical guide, no. 9 telephone, telegraph, wireless, bells, lighting, railways. electrical guide, no. 10 modern practical applications of electricity and ready reference index of the 10 numbers. theo. audel & co. publishers 72 fifth avenue, new york 