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EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY

A Lecture Delivered before the Institution of Electrical Engineers, London

by

NIKOLA TESLA

With a Portrait and Biographical Sketch of the Author

NEW YORK

1892







Biographical Sketch of Nikola Tesla.


While a large portion of the European family has been surging westward
during the last three or four hundred years, settling the vast
continents of America, another, but smaller, portion has been doing
frontier work in the Old World, protecting the rear by beating back
the "unspeakable Turk" and reclaiming gradually the fair lands that
endure the curse of Mohammedan rule. For a long time the Slav
people--who, after the battle of Kosovopjolje, in which the Turks
defeated the Servians, retired to the confines of the present
Montenegro, Dalmatia, Herzegovina and Bosnia, and "Borderland" of
Austria--knew what it was to deal, as our Western pioneers did, with
foes ceaselessly fretting against their frontier; and the races of
these countries, through their strenuous struggle against the armies
of the Crescent, have developed notable qualities of bravery and
sagacity, while maintaining a patriotism and independence unsurpassed
in any other nation.

It was in this interesting border region, and from among these valiant
Eastern folk, that Nikola Tesla was born in the year 1857, and the
fact that he, to-day, finds himself in America and one of our foremost
electricians, is striking evidence of the extraordinary attractiveness
alike of electrical pursuits and of the country where electricity
enjoys its widest application. Mr. Tesla's native place was Smiljan,
Lika, where his father was an eloquent clergyman of the Greek Church,
in which, by the way, his family is still prominently represented. His
mother enjoyed great fame throughout the countryside for her skill and
originality in needlework, and doubtless transmitted her ingenuity to
Nikola; though it naturally took another and more masculine direction.

The boy was early put to his books, and upon his father's removal to
Gospic he spent four years in the public school, and later, three
years in the Real School, as it is called. His escapades were such as
most quick witted boys go through, although he varied the programme on
one occasion by getting imprisoned in a remote mountain chapel rarely
visited for service; and on another occasion by falling headlong into
a huge kettle of boiling milk, just drawn from the paternal herds. A
third curious episode was that connected with his efforts to fly when,
attempting to navigate the air with the aid of an old umbrella, he
had, as might be expected, a very bad fall, and was laid up for six
weeks.

About this period he began to take delight in arithmetic and physics.
One queer notion he had was to work out everything by three or the
power of three. He was now sent to an aunt at Cartstatt, Croatia, to
finish his studies in what is known as the Higher Real School. It was
there that, coming from the rural fastnesses, he saw a steam engine
for the first time with a pleasure that he remembers to this day. At
Cartstatt he was so diligent as to compress the four years' course
into three, and graduated in 1873. Returning home during an epidemic
of cholera, he was stricken down by the disease and suffered so
seriously from the consequences that his studies were interrupted for
fully two years. But the time was not wasted, for he had become
passionately fond of experimenting, and as much as his means and
leisure permitted devoted his energies to electrical study and
investigation. Up to this period it had been his father's intention to
make a priest of him, and the idea hung over the young physicist like
a very sword of Damocles. Finally he prevailed upon his worthy but
reluctant sire to send him to Gratz in Austria to finish his studies
at the Polytechnic School, and to prepare for work as professor of
mathematics and physics. At Gratz he saw and operated a Gramme machine
for the first time, and was so struck with the objections to the use
of commutators and brushes that he made up his mind there and then to
remedy that defect in dynamo-electric machines. In the second year of
his course he abandoned the intention of becoming a teacher and took
up the engineering curriculum. After three years of absence he
returned home, sadly, to see his father die; but, having resolved to
settle down in Austria, and recognizing the value of linguistic
acquirements, he went to Prague and then to Buda-Pesth with the view
of mastering the languages he deemed necessary. Up to this time he had
never realized the enormous sacrifices that his parents had made in
promoting his education, but he now began to feel the pinch and to
grow unfamiliar with the image of Francis Joseph I. There was
considerable lag between his dispatches and the corresponding
remittance from home; and when the mathematical expression for the
value of the lag assumed the shape of an eight laid flat on its back,
Mr. Tesla became a very fair example of high thinking and plain
living, but he made up his mind to the struggle and determined to go
through depending solely on his own resources. Not desiring the fame
of a faster, he cast about for a livelihood, and through the help of
friends he secured a berth as assistant in the engineering department
of the government telegraphs. The salary was five dollars a week. This
brought him into direct contact with practical electrical work and
ideas, but it is needless to say that his means did not admit of much
experimenting. By the time he had extracted several hundred thousand
square and cube roots for the public benefit, the limitations,
financial and otherwise, of the position had become painfully
apparent, and he concluded that the best thing to do was to make a
valuable invention. He proceeded at once to make inventions, but their
value was visible only to the eye of faith, and they brought no grist
to the mill. Just at this time the telephone made its appearance in
Hungary, and the success of that great invention determined his
career, hopeless as the profession had thus far seemed to him. He
associated himself at once with telephonic work, and made various
telephonic inventions, including an operative repeater; but it did not
take him long to discover that, being so remote from the scenes of
electrical activity, he was apt to spend time on aims and results
already reached by others, and to lose touch. Longing for new
opportunities and anxious for the development of which he felt himself
possible, if once he could place himself within the genial and direct
influences of the gulf streams of electrical thought, he broke away
from the ties and traditions of the past, and in 1881 made his way to
Paris. Arriving in that city, the ardent young Likan obtained
employment as an electrical engineer with one of the largest electric
lighting companies. The next year he went to Strasburg to install a
plant, and on returning to Paris sought to carry out a number of ideas
that had now ripened into inventions. About this time, however, the
remarkable progress of America in electrical industry attracted his
attention, and once again staking everything on a single throw, he
crossed the Atlantic.

Mr. Tesla buckled down to work as soon as he landed on these shores,
put his best thought and skill into it, and soon saw openings for his
talent. In a short while a proposition was made to him to start his
own company, and, accepting the terms, he at once worked up a
practical system of arc lighting, as well as a potential method of
dynamo regulation, which in one form is now known as the "third brush
regulation." He also devised a thermo-magnetic motor and other kindred
devices, about which little was published, owing to legal
complications. Early in 1887 the Tesla Electric Company of New York
was formed, and not long after that Mr. Tesla produced his admirable
and epoch-marking motors for multiphase alternating currents, in
which, going back to his ideas of long ago, he evolved machines having
neither commutator nor brushes. It will be remembered that about the
time that Mr. Tesla brought out his motors, and read his thoughtful
paper before the American Institute of Electrical Engineers, Professor
Ferraris, in Europe, published his discovery of principles analogous
to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
Tesla was an independent inventor of this rotary field motor, for
although anticipated in dates by Ferraris, he could not have known
about Ferraris' work as it had not been published. Professor Ferraris
stated himself, with becoming modesty, that he did not think Tesla
could have known of his (Ferraris') experiments at that time, and adds
that he thinks Tesla was an independent and original inventor of this
principle. With such an acknowledgment from Ferraris there can be
little doubt about Tesla's originality in this matter.

Mr. Tesla's work in this field was wonderfully timely, and its worth
was promptly appreciated in various quarters. The Tesla patents were
acquired by the Westinghouse Electric Company, who undertook to
develop his motor and to apply it to work of different kinds. Its use
in mining, and its employment in printing, ventilation, etc., was
described and illustrated in _The Electrical World_ some years ago.
The immense stimulus that the announcement of Mr. Tesla's work gave to
the study of alternating current motors would, in itself, be enough to
stamp him as a leader.

Mr. Tesla is only 35 years of age. He is tall and spare with a
clean-cut, thin, refined face, and eyes that recall all the stories
one has read of keenness of vision and phenomenal ability to see
through things. He is an omnivorous reader, who never forgets; and he
possesses the peculiar facility in languages that enables the least
educated native of eastern Europe to talk and write in at least half a
dozen tongues. A more congenial companion cannot be desired for the
hours when one "pours out heart affluence in discursive talk," and
when the conversation, dealing at first with things near at hand and
next to us, reaches out and rises to the greater questions of life,
duty and destiny.

In the year 1890 he severed his connection with the Westinghouse
Company, since which time he has devoted himself entirely to the study
of alternating currents of high frequencies and very high potentials,
with which study he is at present engaged. No comment is necessary on
his interesting achievements in this field; the famous London lecture
published in this volume is a proof in itself. His first lecture on
his researches in this new branch of electricity, which he may be said
to have created, was delivered before the American Institute of
Electrical Engineers on May 20, 1891, and remains one of the most
interesting papers read before that society. It will be found
reprinted in full in _The Electrical World_, July 11, 1891. Its
publication excited such interest abroad that he received numerous
requests from English and French electrical engineers and scientists
to repeat it in those countries, the result of which has been the
interesting lecture published in this volume.

The present lecture presupposes a knowledge of the former, but it may
be read and understood by any one even though he has not read the
earlier one. It forms a sort of continuation of the latter, and
includes chiefly the results of his researches since that time.





EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY


I cannot find words to express how deeply I feel the honor of
addressing some of the foremost thinkers of the present time, and so
many able scientific men, engineers and electricians, of the country
greatest in scientific achievements.

The results which I have the honor to present before such a gathering
I cannot call my own. There are among you not a few who can lay better
claim than myself on any feature of merit which this work may contain.
I need not mention many names which are world-known--names of those
among you who are recognized as the leaders in this enchanting
science; but one, at least, I must mention--a name which could not be
omitted in a demonstration of this kind. It is a name associated with
the most beautiful invention ever made: it is Crookes!

When I was at college, a good time ago, I read, in a translation (for
then I was not familiar with your magnificent language), the
description of his experiments on radiant matter. I read it only once
in my life--that time--yet every detail about that charming work I
can remember this day. Few are the books, let me say, which can make
such an impression upon the mind of a student.

But if, on the present occasion, I mention this name as one of many
your institution can boast of, it is because I have more than one
reason to do so. For what I have to tell you and to show you this
evening concerns, in a large measure, that same vague world which
Professor Crookes has so ably explored; and, more than this, when I
trace back the mental process which led me to these advances--which
even by myself cannot be considered trifling, since they are so
appreciated by you--I believe that their real origin, that which
started me to work in this direction, and brought me to them, after a
long period of constant thought, was that fascinating little book
which I read many years ago.

And now that I have made a feeble effort to express my homage and
acknowledge my indebtedness to him and others among you, I will make a
second effort, which I hope you will not find so feeble as the first,
to entertain you.

Give me leave to introduce the subject in a few words.

A short time ago I had the honor to bring before our American
Institute of Electrical Engineers[A] some results then arrived at by
me in a novel line of work. I need not assure you that the many
evidences which I have received that English scientific men and
engineers were interested in this work have been for me a great
reward and encouragement. I will not dwell upon the experiments
already described, except with the view of completing, or more clearly
expressing, some ideas advanced by me before, and also with the view
of rendering the study here presented self-contained, and my remarks
on the subject of this evening's lecture consistent.

[Footnote A: For Mr. Tesla's American lecture on this subject see THE
ELECTRICAL WORLD of July 11, 1891, and for a report of his French
lecture see THE ELECTRICAL WORLD of March 26, 1892.]

This investigation, then, it goes without saying, deals with
alternating currents, and, to be more precise, with alternating
currents of high potential and high frequency. Just in how much a very
high frequency is essential for the production of the results
presented is a question which even with my present experience, would
embarrass me to answer. Some of the experiments may be performed with
low frequencies; but very high frequencies are desirable, not only on
account of the many effects secured by their use, but also as a
convenient means of obtaining, in the induction apparatus employed,
the high potentials, which in their turn are necessary to the
demonstration of most of the experiments here contemplated.

Of the various branches of electrical investigation, perhaps the most
interesting and immediately the most promising is that dealing with
alternating currents. The progress in this branch of applied science
has been so great in recent years that it justifies the most sanguine
hopes. Hardly have we become familiar with one fact, when novel
experiences are met with and new avenues of research are opened. Even
at this hour possibilities not dreamed of before are, by the use of
these currents, partly realized. As in nature all is ebb and tide, all
is wave motion, so it seems that; in all branches of industry
alternating currents--electric wave motion--will have the sway.

One reason, perhaps, why this branch of science is being so rapidly
developed is to be found in the interest which is attached to its
experimental study. We wind a simple ring of iron with coils; we
establish the connections to the generator, and with wonder and
delight we note the effects of strange forces which we bring into
play, which allow us to transform, to transmit and direct energy at
will. We arrange the circuits properly, and we see the mass of iron
and wires behave as though it were endowed with life, spinning a heavy
armature, through invisible connections, with great speed and
power--with the energy possibly conveyed from a great distance. We
observe how the energy of an alternating current traversing the wire
manifests itself--not so much in the wire as in the surrounding
space--in the most surprising manner, taking the forms of heat, light,
mechanical energy, and, most surprising of all, even chemical
affinity. All these observations fascinate us, and fill us with an
intense desire to know more about the nature of these phenomena. Each
day we go to our work in the hope of discovering,--in the hope that
some one, no matter who, may find a solution of one of the pending
great problems,--and each succeeding day we return to our task with
renewed ardor; and even if we _are_ unsuccessful, our work has not
been in vain, for in these strivings, in these efforts, we have found
hours of untold pleasure, and we have directed our energies to the
benefit of mankind.

We may take--at random, if you choose--any of the many experiments
which may be performed with alternating currents; a few of which
only, and by no means the most striking, form the subject of this
evening's demonstration: they are all equally interesting, equally
inciting to thought.

Here is a simple glass tube from which the air has been partially
exhausted. I take hold of it; I bring my body in contact with a wire
conveying alternating currents of high potential, and the tube in my
hand is brilliantly lighted. In whatever position I may put it,
wherever I may move it in space, as far as I can reach, its soft,
pleasing light persists with undiminished brightness.

Here is an exhausted bulb suspended from a single wire. Standing on an
insulated support. I grasp it, and a platinum button mounted in it is
brought to vivid incandescence.

Here, attached to a leading wire, is another bulb, which, as I touch
its metallic socket, is filled with magnificent colors of
phosphorescent light.

Here still another, which by my fingers' touch casts a shadow--the
Crookes shadow, of the stem inside of it.

Here, again, insulated as I stand on this platform, I bring my body in
contact with one of the terminals of the secondary of this induction
coil--with the end of a wire many miles long--and you see streams of
light break forth from its distant end, which is set in violent
vibration.

Here, once more, I attach these two plates of wire gauze to the
terminals of the coil. I set them a distance apart, and I set the coil
to work. You may see a small spark pass between the plates. I insert a
thick plate of one of the best dielectrics between them, and instead
of rendering altogether impossible, as we are used to expect, I _aid_
the passage of the discharge, which, as I insert the plate, merely
changes in appearance and assumes the form of luminous streams.

Is there, I ask, can there be, a more interesting study than that of
alternating currents?

In all these investigations, in all these experiments, which are so
very, very interesting, for many years past--ever since the greatest
experimenter who lectured in this hall discovered its principle--we
have had a steady companion, an appliance familiar to every one, a
plaything once, a thing of momentous importance now--the induction
coil. There is no dearer appliance to the electrician. From the ablest
among you, I dare say, down to the inexperienced student, to your
lecturer, we all have passed many delightful hours in experimenting
with the induction coil. We have watched its play, and thought and
pondered over the beautiful phenomena which it disclosed to our
ravished eyes. So well known is this apparatus, so familiar are these
phenomena to every one, that my courage nearly fails me when I think
that I have ventured to address so able an audience, that I have
ventured to entertain you with that same old subject. Here in reality
is the same apparatus, and here are the same phenomena, only the
apparatus is operated somewhat differently, the phenomena are
presented in a different aspect. Some of the results we find as
expected, others surprise us, but all captivate our attention, for in
scientific investigation each novel result achieved may be the centre
of a new departure, each novel fact learned may lead to important
developments.

Usually in operating an induction coil we have set up a vibration of
moderate frequency in the primary, either by means of an interrupter
or break, or by the use of an alternator. Earlier English
investigators, to mention only Spottiswoode and J.E.H. Gordon, have
used a rapid break in connection with the coil. Our knowledge and
experience of to-day enables us to see clearly why these coils under
the conditions of the tests did not disclose any remarkable phenomena,
and why able experimenters failed to perceive many of the curious
effects which have since been observed.

In the experiments such as performed this evening, we operate the coil
either from a specially constructed alternator capable of giving many
thousands of reversals of current per second, or, by disruptively
discharging a condenser through the primary, we set up a vibration in
the secondary circuit of a frequency of many hundred thousand or
millions per second, if we so desire; and in using either of these
means we enter a field as yet unexplored.

It is impossible to pursue an investigation in any novel line without
finally making some interesting observation or learning some useful
fact. That this statement is applicable to the subject of this lecture
the many curious and unexpected phenomena which we observe afford a
convincing proof. By way of illustration, take for instance the most
obvious phenomena, those of the discharge of the induction coil.

Here is a coil which is operated by currents vibrating with extreme
rapidity, obtained by disruptively discharging a Leyden jar. It would
not surprise a student were the lecturer to say that the secondary of
this coil consists of a small length of comparatively stout wire; it
would not surprise him were the lecturer to state that, in spite of
this, the coil is capable of giving any potential which the best
insulation of the turns is able to withstand: but although he may be
prepared, and even be indifferent as to the anticipated result, yet
the aspect of the discharge of the coil will surprise and interest
him. Every one is familiar with the discharge of an ordinary coil; it
need not be reproduced here. But, by way of contrast, here is a form
of discharge of a coil, the primary current of which is vibrating
several hundred thousand times per second. The discharge of an
ordinary coil appears as a simple line or band of light. The discharge
of this coil appears in the form of powerful brushes and luminous
streams issuing from all points of the two straight wires attached to
the terminals of the secondary. (Fig. 1.)

[Illustration: FIG. 1.--DISCHARGE BETWEEN TWO WIRES WITH FREQUENCIES
OF A FEW HUNDRED THOUSAND PER SECOND.]

Now compare this phenomenon which you have just witnessed with the
discharge of a Holtz or Wimshurst machine--that other interesting
appliance so dear to the experimenter. What a difference there is
between these phenomena! And yet, had I made the necessary
arrangements--which could have been made easily, were it not that they
would interfere with other experiments--I could have produced with
this coil sparks which, had I the coil hidden from your view and only
two knobs exposed, even the keenest observer among you would find it
difficult, if not impossible, to distinguish from those of an
influence or friction machine. This may be done in many ways--for
instance, by operating the induction coil which charges the condenser
from an alternating-current machine of very low frequency, and
preferably adjusting the discharge circuit so that there are no
oscillations set up in it. We then obtain in the secondary circuit, if
the knobs are of the required size and properly set, a more or less
rapid succession of sparks of great intensity and small quantity,
which possess the same brilliancy, and are accompanied by the same
sharp crackling sound, as those obtained from a friction or influence
machine.

Another way is to pass through two primary circuits, having a common
secondary, two currents of a slightly different period, which produce
in the secondary circuit sparks occurring at comparatively long
intervals. But, even with the means at hand this evening, I may
succeed in imitating the spark of a Holtz machine. For this purpose I
establish between the terminals of the coil which charges the
condenser a long, unsteady arc, which is periodically interrupted by
the upward current of air produced by it. To increase the current of
air I place on each side of the arc, and close to it, a large plate of
mica. The condenser charged from this coil discharges into the primary
circuit of a second coil through a small air gap, which is necessary
to produce a sudden rush of current through the primary. The scheme of
connections in the present experiment is indicated in Fig. 2.

[Illustration: FIG. 2.--IMITATING THE SPARK OF A HOLTZ MACHINE.]

G is an ordinarily constructed alternator, supplying the primary P of
an induction coil, the secondary S of which charges the condensers or
jars CC. The terminals of the secondary are connected to the inside
coatings of the jars, the outer coatings being connected to the ends
of the primary pp of a second induction coil. This primary pp has a
small air gap ab.

The secondary s of this coil is provided with knobs or spheres KK of
the proper size and set at a distance suitable for the experiment.

A long arc is established between the terminals AB of the first
induction coil. MM are the mica plates.

Each time the arc is broken between A and B the jars are quickly
charged and discharged through the primary pp, producing a snapping
spark between the knobs KK. Upon the arc forming between A and B the
potential falls, and the jars cannot be charged to such high potential
as to break through the air gap ab until the arc is again broken by
the draught.

In this manner sudden impulses, at long intervals, are produced in the
primary pp, which in the secondary s give a corresponding number of
impulses of great intensity. If the secondary knobs or spheres, KK,
are of the proper size, the sparks show much resemblance to those of a
Holtz machine.

But these two effects, which to the eye appear so very different, are
only two of the many discharge phenomena. We only need to change the
conditions of the test, and again we make other observations of
interest.

When, instead of operating the induction coil as in the last two
experiments, we operate it from a high frequency alternator, as in the
next experiment, a systematic study of the phenomena is rendered much
more easy. In such case, in varying the strength and frequency of the
currents through the primary, we may observe five distinct forms of
discharge, which I have described in my former paper on the subject[A]
before the American Institute of Electrical Engineers, May 20, 1891.

[Footnote A: See THE ELECTRICAL WORLD, July 11, 1891.]

It would take too much time, and it would lead us too far from the
subject presented this evening, to reproduce all these forms, but it
seems to me desirable to show you one of them. It is a brush
discharge, which is interesting in more than one respect. Viewed from
a near position it resembles much a jet of gas escaping under great
pressure. We know that the phenomenon is due to the agitation of the
molecules near the terminal, and we anticipate that some heat must be
developed by the impact of the molecules against the terminal or
against each other. Indeed, we find that the brush is hot, and only a
little thought leads us to the conclusion that, could we but reach
sufficiently high frequencies, we could produce a brush which would
give intense light and heat, and which would resemble in every
particular an ordinary flame, save, perhaps, that both phenomena might
not be due to the same agent--save, perhaps, that chemical affinity
might not be _electrical_ in its nature.

As the production of heat and light is here due to the impact of the
molecules, or atoms of air, or something else besides, and, as we can
augment the energy simply by raising the potential, we might, even
with frequencies obtained from a dynamo machine, intensify the action
to such a degree as to bring the terminal to melting heat. But with
such low frequencies we would have to deal always with something of
the nature of an electric current. If I approach a conducting object
to the brush, a thin little spark passes, yet, even with the
frequencies used this evening, the tendency to spark is not very
great. So, for instance, if I hold a metallic sphere at some distance
above the terminal you may see the whole space between the terminal
and sphere illuminated by the streams without the spark passing; and
with the much higher frequencies obtainable by the disruptive
discharge of a condenser, were it not for the sudden impulses, which
are comparatively few in number, sparking would not occur even at very
small distances. However, with incomparably higher frequencies, which
we may yet find means to produce efficiently, and provided that
electric impulses of such high frequencies could be transmitted
through a conductor, the electrical characteristics of the brush
discharge would completely vanish--no spark would pass, no shock would
be felt--yet we would still have to deal with an _electric_
phenomenon, but in the broad, modern interpretation of the word. In my
first paper before referred to I have pointed out the curious
properties of the brush, and described the best manner of producing
it, but I have thought it worth while to endeavor to express myself
more clearly in regard to this phenomenon, because of its absorbing
interest.

When a coil is operated with currents of very high frequency,
beautiful brush effects may be produced, even if the coil be of
comparatively small dimensions. The experimenter may vary them in
many ways, and, if it were nothing else, they afford a pleasing sight.
What adds to their interest is that they may be produced with one
single terminal as well as with two--in fact, often better with one
than with two.

But of all the discharge phenomena observed, the most pleasing to the
eye, and the most instructive, are those observed with a coil which is
operated by means of the disruptive discharge of a condenser. The
power of the brushes, the abundance of the sparks, when the conditions
are patiently adjusted, is often amazing. With even a very small coil,
if it be so well insulated as to stand a difference of potential of
several thousand volts per turn, the sparks may be so abundant that
the whole coil may appear a complete mass of fire.

Curiously enough the sparks, when the terminals of the coil are set at
a considerable distance, seem to dart in every possible direction as
though the terminals were perfectly independent of each other. As the
sparks would soon destroy the insulation it is necessary to prevent
them. This is best done by immersing the coil in a good liquid
insulator, such as boiled-out oil. Immersion in a liquid may be
considered almost an absolute necessity for the continued and
successful working of such a coil.

It is of course out of the question, in an experimental lecture, with
only a few minutes at disposal for the performance of each experiment,
to show these discharge phenomena to advantage, as to produce each
phenomenon at its best a very careful adjustment is required. But even
if imperfectly produced, as they are likely to be this evening, they
are sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of
completeness, give a short description of the coil and other apparatus
used in the experiments with the disruptive discharge this evening.

[Illustration: FIG. 3.--DISRUPTIVE DISCHARGE COIL.]

It is contained in a box B (Fig. 3) of thick boards of hard wood,
covered on the outside with zinc sheet Z, which is carefully soldered
all around. It might be advisable, in a strictly scientific
investigation, when accuracy is of great importance, to do away with
the metal cover, as it might introduce many errors, principally on
account of its complex action upon the coil, as a condenser of very
small capacity and as an electrostatic and electromagnetic screen.
When the coil is used for such experiments as are here contemplated,
the employment of the metal cover offers some practical advantages,
but these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the
space between should, of course, not be too small, certainly not less
than, say, five centimetres, but much more if possible; especially the
two sides of the zinc box, which are at right angles to the axis of
the coil, should be sufficiently remote from the latter, as otherwise
they might impair its action and be a source of loss.

The coil consists of two spools of hard rubber RR, held apart at a
distance of 10 centimetres by bolts c and nuts n, likewise of hard
rubber. Each spool comprises a tube T of approximately 8 centimetres
inside diameter, and 3 millimetres thick, upon which are screwed two
flanges FF, 24 centimetres square, the space between the flanges being
about 3 centimetres. The secondary, SS, of the best gutta
percha-covered wire, has 26 layers, 10 turns in each, giving for each
half a total of 260 turns. The two halves are wound oppositely and
connected in series, the connection between both being made over the
primary. This disposition, besides being convenient, has the advantage
that when the coil is well balanced--that is, when both of its
terminals T_1 T_1 are connected to bodies or devices of equal
capacity--there is not much danger of breaking through to the primary,
and the insulation between the primary and the secondary need not be
thick. In using the coil it is advisable to attach to _both_ terminals
devices of nearly equal capacity, as, when the capacity of the
terminals is not equal, sparks will be apt to pass to the primary. To
avoid this, the middle point of the secondary may be connected to the
primary, but this is not always practicable.

The primary PP is wound in two parts, and oppositely, upon a wooden
spool W, and the four ends are led out of the oil through hard rubber
tubes tt. The ends of the secondary T_1 T_1 are also led out of the
oil through rubber tubes t_1 t_1 of great thickness. The primary and
secondary layers are insulated by cotton cloth, the thickness of the
insulation, of course, bearing some proportion to the difference of
potential between the turns of the different layers. Each half of the
primary has four layers, 24 turns in each, this giving a total of 96
turns. When both the parts are connected in series, this gives a
ratio of conversion of about 1:2.7, and with the primaries in
multiple, 1:5.4; but in operating with very rapidly alternating
currents this ratio does not convey even an approximate idea of the
ratio of the E.M.Fs. in the primary and secondary circuits. The coil
is held in position in the oil on wooden supports, there being about 5
centimetres thickness of oil all round. Where the oil is not specially
needed, the space is filled with pieces of wood, and for this purpose
principally the wooden box B surrounding the whole is used.

The construction here shown is, of course, not the best on general
principles, but I believe it is a good and convenient one for the
production of effects in which an excessive potential and a very small
current are needed.

In connection with the coil I use either the ordinary form of
discharger or a modified form. In the former I have introduced two
changes which secure some advantages, and which are obvious. If they
are mentioned, it is only in the hope that some experimenter may find
them of use.

[Illustration: FIG. 4.--ARRANGEMENT OF IMPROVED DISCHARGER AND
MAGNET.]

One of the changes is that the adjustable knobs A and B (Fig. 4),
of the discharger are held in jaws of brass, JJ, by spring pressure,
this allowing of turning them successively into different positions,
and so doing away with the tedious process of frequent polishing up.

The other change consists in the employment of a strong electromagnet
NS, which is placed with its axis at right angles to the line joining
the knobs A and B, and produces a strong magnetic field between them.
The pole pieces of the magnet are movable and properly formed so as to
protrude between the brass knobs, in order to make the field as
intense as possible; but to prevent the discharge from jumping to the
magnet the pole pieces are protected by a layer of mica, MM, of
sufficient thickness. s_1 s_1 and s_2 s_2 are screws for fastening the
wires. On each side one of the screws is for large and the other for
small wires. LL are screws for fixing in position the rods RR, which
support the knobs.

In another arrangement with the magnet I take the discharge between
the rounded pole pieces themselves, which in such case are insulated
and preferably provided with polished brass caps.

The employment of an intense magnetic field is of advantage
principally when the induction coil or transformer which charges the
condenser is operated by currents of very low frequency. In such a
case the number of the fundamental discharges between the knobs may be
so small as to render the currents produced in the secondary
unsuitable for many experiments. The intense magnetic field then
serves to blow out the arc between the knobs as soon as it is formed,
and the fundamental discharges occur in quicker succession.

Instead of the magnet, a draught or blast of air may be employed with
some advantage. In this case the arc is preferably established between
the knobs AB, in Fig. 2 (the knobs ab being generally joined, or
entirely done away with), as in this disposition the arc is long and
unsteady, and is easily affected by the draught.

When a magnet is employed to break the arc, it is better to choose
the connection indicated diagrammatically in Fig. 5, as in this case
the currents forming the arc are much more powerful, and the magnetic
field exercises a greater influence. The use of the magnet permits,
however, of the arc being replaced by a vacuum tube, but I have
encountered great difficulties in working with an exhausted tube.

[Illustration: FIG. 5.--ARRANGEMENT WITH LOW-FREQUENCY ALTERNATOR AND
IMPROVED DISCHARGER.]

[Illustration: FIG. 6.--DISCHARGER WITH MULTIPLE GAPS.]

The other form of discharger used in these and similar experiments is
indicated in Figs. 6 and 7. It consists of a number of brass pieces cc
(Fig. 6), each of which comprises a spherical middle portion m with an
extension e below--which is merely used to fasten the piece in a lathe
when polishing up the discharging surface--and a column above, which
consists of a knurled flange f surmounted by a threaded stem l
carrying a nut n, by means of which a wire is fastened to the column.
The flange f conveniently serves for holding the brass piece when
fastening the wire, and also for turning it in any position when it
becomes necessary to present a fresh discharging surface. Two stout
strips of hard rubber RR, with planed grooves gg (Fig. 7) to fit the
middle portion of the pieces cc, serve to clamp the latter and hold
them firmly in position by means of two bolts CC (of which only one is
shown) passing through the ends of the strips.

[Illustration: FIG. 7.--DISCHARGER WITH MULTIPLE GAPS.]

In the use of this kind of discharger I have found three principal
advantages over the ordinary form. First, the dielectric strength of a
given total width of air space is greater when a great many small air
gaps are used instead of one, which permits of working with a smaller
length of air gap, and that means smaller loss and less deterioration
of the metal; secondly by reason of splitting the arc up into smaller
arcs, the polished surfaces are made to last much longer; and,
thirdly, the apparatus affords some gauge in the experiments. I
usually set the pieces by putting between them sheets of uniform
thickness at a certain very small distance which is known from the
experiments of Sir William Thomson to require a certain electromotive
force to be bridged by the spark.

It should, of course, be remembered that the sparking distance is much
diminished as the frequency is increased. By taking any number of
spaces the experimenter has a rough idea of the electromotive force,
and he finds it easier to repeat an experiment, as he has not the
trouble of setting the knobs again and again. With this kind of
discharger I have been able to maintain an oscillating motion without
any spark being visible with the naked eye between the knobs, and they
would not show a very appreciable rise in temperature. This form of
discharge also lends itself to many arrangements of condensers and
circuits which are often very convenient and time-saving. I have used
it preferably in a disposition similar to that indicated in Fig. 2,
when the currents forming the arc are small.

I may here mention that I have also used dischargers with single or
multiple air gaps, in which the discharge surfaces were rotated with
great speed. No particular advantage was, however, gained by this
method, except in cases where the currents from the condenser were
large and the keeping cool of the surfaces was necessary, and in cases
when, the discharge not being oscillating of itself, the arc as soon
as established was broken by the air current, thus starting the
vibration at intervals in rapid succession. I have also used
mechanical interrupters in many ways. To avoid the difficulties with
frictional contacts, the preferred plan adopted was to establish the
arc and rotate through it at great speed a rim of mica provided with
many holes and fastened to a steel plate. It is understood, of course,
that the employment of a magnet, air current, or other interrupter,
produces no effect worth noticing, unless the self-induction, capacity
and resistance are so related that there are oscillations set up upon
each interruption.

I will now endeavor to show you some of the most noteworthy of these
discharge phenomena.

I have stretched across the room two ordinary cotton covered wires,
each about 7 metres in length. They are supported on insulating cords
at a distance of about 30 centimetres. I attach now to each of the
terminals of the coil one of the wires and set the coil in action.
Upon turning the lights off in the room you see the wires strongly
illuminated by the streams issuing abundantly from their whole surface
in spite of the cotton covering, which may even be very thick. When
the experiment is performed under good conditions, the light from the
wires is sufficiently intense to allow distinguishing the objects in a
room. To produce the best result it is, of course, necessary to adjust
carefully the capacity of the jars, the arc between the knobs and the
length of the wires. My experience is that calculation of the length
of the wires leads, in such case, to no result whatever. The
experimenter will do best to take the wires at the start very long,
and then adjust by cutting off first long pieces, and then smaller and
smaller ones as he approaches the right length.

A convenient way is to use an oil condenser of very small capacity,
consisting of two small adjustable metal plates, in connection with
this and similar experiments. In such case I take wires rather short
and set at the beginning the condenser plates at maximum distance. If
the streams for the wires increase by approach of the plates, the
length of the wires is about right; if they diminish the wires are too
long for that frequency and potential. When a condenser is used in
connection with experiments with such a coil, it should be an oil
condenser by all means, as in using an air condenser considerable
energy might be wasted. The wires leading to the plates in the oil
should be very thin, heavily coated with some insulating compound, and
provided with a conducting covering--this preferably extending under
the surface of the oil. The conducting cover should not be too near
the terminals, or ends, of the wire, as a spark would be apt to jump
from the wire to it. The conducting coating is used to diminish the
air losses, in virtue of its action as an electrostatic screen. As to
the size of the vessel containing the oil, and the size of the plates,
the experimenter gains at once an idea from a rough trial. The size of
the plates _in oil_ is, however, calculable, as the dielectric losses
are very small.

In the preceding experiment it is of considerable interest to know
what relation the quantity of the light emitted bears to the frequency
and potential of the electric impulses. My opinion is that the heat as
well as light effects produced should be proportionate, under
otherwise equal conditions of test, to the product of frequency and
square of potential, but the experimental verification of the law,
whatever it may be, would be exceedingly difficult. One thing is
certain, at any rate, and that is, that in augmenting the potential
and frequency we rapidly intensify the streams; and, though it may be
very sanguine, it is surely not altogether hopeless to expect that we
may succeed in producing a practical illuminant on these lines. We
would then be simply using burners or flames, in which there would be
no chemical process, no consumption of material, but merely a transfer
of energy, and which would, in all probability emit more light and
less heat than ordinary flames.

[Illustration: FIG. 8.--EFFECT PRODUCED BY CONCENTRATING STREAMS.]

The luminous intensity of the streams is, of course, considerably
increased when they are focused upon a small surface. This may be
shown by the following experiment:

I attach to one of the terminals of the coil a wire w (Fig. 8), bent
in a circle of about 30 centimetres in diameter, and to the other
terminal I fasten a small brass sphere s, the surface of the wire
being preferably equal to the surface of the sphere, and the centre of
the latter being in a line at right angles to the plane of the wire
circle and passing through its centre. When the discharge is
established under proper conditions, a luminous hollow cone is formed,
and in the dark one-half of the brass sphere is strongly illuminated,
as shown in the cut.

By some artifice or other, it is easy to concentrate the streams upon
small surfaces and to produce very strong light effects. Two thin
wires may thus be rendered intensely luminous.

In order to intensify the streams the wires should be very thin and
short; but as in this case their capacity would be generally too small
for the coil--at least, for such a one as the present--it is necessary
to augment the capacity to the required value, while, at the same
time, the surface of the wires remains very small. This may be done in
many ways.

Here, for instance, I have two plates, RR, of hard rubber (Fig. 9),
upon which I have glued two very thin wires ww, so as to form a name.
The wires may be bare or covered with the best insulation--it is
immaterial for the success of the experiment. Well insulated wires, if
anything, are preferable. On the back of each plate, indicated by the
shaded portion, is a tinfoil coating tt. The plates are placed in line
at a sufficient distance to prevent a spark passing from one to the
other wire. The two tinfoil coatings I have joined by a conductor C,
and the two wires I presently connect to the terminals of the coil. It
is now easy, by varying the strength and frequency of the currents
through the primary, to find a point at which, the capacity of the
system is best suited to the conditions, and the wires become so
strongly luminous that, when the light in the room is turned off the
name formed by them appears in brilliant letters.

[Illustration: FIG. 9.--WIRES RENDERED INTENSELY LUMINOUS.]

It is perhaps preferable to perform this experiment with a coil
operated from an alternator of high frequency, as then, owing to the
harmonic rise and fall, the streams are very uniform, though they are
less abundant then when produced with such a coil as the present. This
experiment, however, may be performed with low frequencies, but much
less satisfactorily.

[Illustration: FIG. 10.--LUMINOUS DISCS.]

When two wires, attached to the terminals of the coil, are set at the
proper distance, the streams between them may be so intense as to
produce a continuous luminous sheet. To show this phenomenon I have
here two circles, C and c (Fig. 10), of rather stout wire, one being
about 80 centimetres and the other 30 centimetres in diameter. To each
of the terminals of the coil I attach one of the circles. The
supporting wires are so bent that the circles may be placed in the
same plane, coinciding as nearly as possible. When the light in the
room is turned off and the coil set to work, you see the whole space
between the wires uniformly filled with streams, forming a luminous
disc, which could be seen from a considerable distance, such is the
intensity of the streams. The outer circle could have been much larger
than the present one; in fact, with this coil I have used much larger
circles, and I have been able to produce a strongly luminous sheet,
covering an area of more than one square metre, which is a remarkable
effect with this very small coil. To avoid uncertainty, the circle has
been taken smaller, and the area is now about 0.43 square metre.

The frequency of the vibration, and the quickness of succession of the
sparks between the knobs, affect to a marked degree the appearance of
the streams. When the frequency is very low, the air gives way in more
or less the same manner, as by a steady difference of potential, and
the streams consist of distinct threads, generally mingled with thin
sparks, which probably correspond to the successive discharges
occurring between the knobs. But when the frequency is extremely high,
and the arc of the discharge produces a very _loud_ but _smooth_
sound--showing both that oscillation takes place and that the sparks
succeed each other with great rapidity--then the luminous streams
formed are perfectly uniform. To reach this result very small coils
and jars of small capacity should be used. I take two tubes of thick
Bohemian glass, about 5 centimetres in diameter and 20 centimetres
long. In each of the tubes I slip a primary of very thick copper wire.
On the top of each tube I wind a secondary of much thinner
gutta-percha covered wire. The two secondaries I connect in series,
the primaries preferably in multiple arc. The tubes are then placed in
a large glass vessel, at a distance of 10 to 15 centimetres from each
other, on insulating supports, and the vessel is filled with boiled
out oil, the oil reaching about an inch above the tubes. The free ends
of the secondary are lifted out of the oil and placed parallel to each
other at a distance of about 10 centimetres. The ends which are
scraped should be dipped in the oil. Two four-pint jars joined in
series may be used to discharge through the primary. When the
necessary adjustments in the length and distance of the wires above
the oil and in the arc of discharge are made, a luminous sheet is
produced between the wires which is perfectly smooth and textureless,
like the ordinary discharge through a moderately exhausted tube.

I have purposely dwelt upon this apparently insignificant experiment.
In trials of this kind the experimenter arrives at the startling
conclusion that, to pass ordinary luminous discharges through gases,
no particular degree of exhaustion is needed, but that the gas may be
at ordinary or even greater pressure. To accomplish this, a very high
frequency is essential; a high potential is likewise required, but
this is a merely incidental necessity. These experiments teach us
that, in endeavoring to discover novel methods of producing light by
the agitation of atoms, or molecules, of a gas, we need not limit our
research to the vacuum tube, but may look forward quite seriously to
the possibility of obtaining the light effects without the use of any
vessel whatever, with air at ordinary pressure.

Such discharges of very high frequency, which render luminous the air
at ordinary pressures, we have probably often occasion to witness in
Nature. I have no doubt that if, as many believe, the aurora borealis
is produced by sudden cosmic disturbances, such as eruptions at the
sun's surface, which set the electrostatic charge of the earth in an
extremely rapid vibration, the red glow observed is not confined to
the upper rarefied strata of the air, but the discharge traverses, by
reason of its very high frequency, also the dense atmosphere in the
form of a _glow_, such as we ordinarily produce in a slightly
exhausted tube. If the frequency were very low, or even more so, if
the charge were not at all vibrating, the dense air would break down
as in a lightning discharge. Indications of such breaking down of the
lower dense strata of the air have been repeatedly observed at the
occurrence of this marvelous phenomenon; but if it does occur, it can
only be attributed to the fundamental disturbances, which are few in
number, for the vibration produced by them would be far too rapid to
allow a disruptive break. It is the original and irregular impulses
which affect the instruments; the superimposed vibrations probably
pass unnoticed.

When an ordinary low frequency discharge is passed through moderately
rarefied air, the air assumes a purplish hue. If by some means or
other we increase the intensity of the molecular, or atomic,
vibration, the gas changes to a white color. A similar change occurs
at ordinary pressures with electric impulses of very high frequency.
If the molecules of the air around a wire are moderately agitated, the
brush formed is reddish or violet; if the vibration is rendered
sufficiently intense, the streams become white. We may accomplish this
in various ways. In the experiment before shown with the two wires
across the room, I have endeavored to secure the result by pushing to
a high value both the frequency and potential: in the experiment with
the thin wires glued on the rubber plate I have concentrated the
action upon a very small surface--in other words, I have worked with a
great electric density.

A most curious form of discharge is observed with such a coil when the
frequency and potential are pushed to the extreme limit. To perform
the experiment, every part of the coil should be heavily insulated,
and only two small spheres--or, better still, two sharp-edged metal
discs (dd, Fig. 11) of no more than a few centimetres in
diameter--should be exposed to the air. The coil here used is immersed
in oil, and the ends of the secondary reaching out of the oil are
covered with an air-tight cover of hard rubber of great thickness.
All cracks, if there are any, should be carefully stopped up, so that
the brush discharge cannot form anywhere except on the small spheres
or plates which are exposed to the air. In this case, since there are
no large plates or other bodies of capacity attached to the terminals,
the coil is capable of an extremely rapid vibration. The potential may
be raised by increasing, as far as the experimenter judges proper, the
rate of change of the primary current. With a coil not widely
differing from the present, it is best to connect the two primaries in
multiple arc; but if the secondary should have a much greater number
of turns the primaries should preferably be used in series, as
otherwise the vibration might be too fast for the secondary. It occurs
under these conditions that misty white streams break forth from the
edges of the discs and spread out phantom-like into space. With this
coil, when fairly well produced, they are about 25 to 30 centimetres
long. When the hand is held against them no sensation is produced, and
a spark, causing a shock, jumps from the terminal only upon the hand
being brought much nearer. If the oscillation of the primary current
is rendered intermittent by some means or other, there is a
corresponding throbbing of the streams, and now the hand or other
conducting object may be brought in still greater proximity to the
terminal without a spark being caused to jump.

[Illustration: FIG. 11.--PHANTOM STREAMS.]

Among the many beautiful phenomena which may be produced with such a
coil I have here selected only those which appear to possess some
features of novelty, and lead us to some conclusions of interest. One
will not find it at all difficult to produce in the laboratory, by
means of it, many other phenomena which appeal to the eye even more
than these here shown, but present no particular feature of novelty.

Early experimenters describe the display of sparks produced by an
ordinary large induction coil upon an insulating plate separating the
terminals. Quite recently Siemens performed some experiments in which
fine effects were obtained, which were seen by many with interest. No
doubt large coils, even if operated with currents of low frequencies,
are capable of producing beautiful effects. But the largest coil ever
made could not, by far, equal the magnificent display of streams and
sparks obtained from such a disruptive discharge coil when properly
adjusted. To give an idea, a coil such as the present one will cover
easily a plate of 1 metre in diameter completely with the streams. The
best way to perform such experiments is to take a very thin rubber or
a glass plate and glue on one side of it a narrow ring of tinfoil of
very large diameter, and on the other a circular washer, the centre of
the latter coinciding with that of the ring, and the surfaces of both
being preferably equal, so as to keep the coil well balanced. The
washer and ring should be connected to the terminals by heavily
insulated thin wires. It is easy in observing the effect of the
capacity to produce a sheet of uniform streams, or a fine network of
thin silvery threads, or a mass of loud brilliant sparks, which
completely cover the plate.

Since I have advanced the idea of the conversion by means of the
disruptive discharge, in my paper before the American Institute of
Electrical Engineers at the beginning of the past year, the interest
excited in it has been considerable. It affords us a means for
producing any potentials by the aid of inexpensive coils operated from
ordinary systems of distribution, and--what is perhaps more
appreciated--it enables us to convert currents of any frequency into
currents of any other lower or higher frequency. But its chief value
will perhaps be found in the help which it will afford us in the
investigations of the phenomena of phosphorescence, which a disruptive
discharge coil is capable of exciting in innumerable cases where
ordinary coils, even the largest, would utterly fail.

Considering its probable uses for many practical purposes, and its
possible introduction into laboratories for scientific research, a few
additional remarks as to the construction of such a coil will perhaps
not be found superfluous.

It is, of course, absolutely necessary to employ in such a coil wires
provided with the best insulation.

Good coils may be produced by employing wires covered with several
layers of cotton, boiling the coil a long time in pure wax, and
cooling under moderate pressure. The advantage of such a coil is that
it can be easily handled, but it cannot probably give as satisfactory
results as a coil immersed in pure oil. Besides, it seems that the
presence of a large body of wax affects the coil disadvantageously,
whereas this does not seem to be the case with oil. Perhaps it is
because the dielectric losses in the liquid are smaller.

I have tried at first silk and cotton covered wires with oil
immersion, but I have been gradually led to use gutta-percha covered
wires, which proved most satisfactory. Gutta-percha insulation adds,
of course, to the capacity of the coil, and this, especially if the
coil be large, is a great disadvantage when extreme frequencies are
desired; but on the other hand, gutta-percha will withstand much more
than an equal thickness of oil, and this advantage should be secured
at any price. Once the coil has been immersed, it should never be
taken out of the oil for more than a few hours, else the gutta-percha
will crack up and the coil will not be worth half as much as before.
Gutta-percha is probably slowly attacked by the oil, but after an
immersion of eight to nine months I have found no ill effects.

I have obtained in commerce two kinds of gutta-percha wire: in one the
insulation sticks tightly to the metal, in the other it does not.
Unless a special method is followed to expel all air, it is much safer
to use the first kind. I wind the coil within an oil tank so that all
interstices are filled up with the oil. Between the layers I use cloth
boiled out thoroughly in oil, calculating the thickness according to
the difference of potential between the turns. There seems not to be a
very great difference whatever kind of oil is used; I use paraffine or
linseed oil.

To exclude more perfectly the air, an excellent way to proceed, and
easily practicable with small coils, is the following: Construct a box
of hard wood of very thick boards which have been for a long time
boiled in oil. The boards should be so joined as to safely withstand
the external air pressure. The coil being placed and fastened in
position within the box, the latter is closed with a strong lid, and
covered with closely fitting metal sheets, the joints of which are
soldered very carefully. On the top two small holes are drilled,
passing through the metal sheet and the wood, and in these holes two
small glass tubes are inserted and the joints made air-tight. One of
the tubes is connected to a vacuum pump, and the other with a vessel
containing a sufficient quantity of boiled-out oil. The latter tube
has a very small hole at the bottom, and is provided with a stopcock.
When a fairly good vacuum has been obtained, the stopcock is opened
and the oil slowly fed in. Proceeding in this manner, it is impossible
that any big bubbles, which are the principal danger, should remain
between the turns. The air is most completely excluded, probably
better than by boiling out, which, however, when gutta-percha coated
wires are used, is not practicable.

For the primaries I use ordinary line wire with a thick cotton
coating. Strands of very thin insulated wires properly interlaced
would, of course, be the best to employ for the primaries, but they
are not to be had.

In an experimental coil the size of the wires is not of great
importance. In the coil here used the primary is No. 12 and the
secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the
results aimed at would not be materially affected.

I have dwelt at some length upon the various forms of brush discharge
because, in studying them, we not only observe phenomena which please
our eye, but also afford us food for thought, and lead us to
conclusions of practical importance. In the use of alternating
currents of very high tension, too much precaution cannot be taken to
prevent the brush discharge. In a main conveying such currents, in an
induction coil or transformer, or in a condenser, the brush discharge
is a source of great danger to the insulation. In a condenser
especially the gaseous matter must be most carefully expelled, for in
it the charged surfaces are near each other, and if the potentials are
high, just as sure as a weight will fall if let go, so the insulation
will give way if a single gaseous bubble of some size be present,
whereas, if all gaseous matter were carefully excluded, the condenser
would safely withstand a much higher difference of potential. A main
conveying alternating currents of very high tension may be injured
merely by a blow hole or small crack in the insulation, the more so as
a blowhole is apt to contain gas at low pressure; and as it appears
almost impossible to completely obviate such little imperfections, I
am led to believe that in our future distribution of electrical energy
by currents of very high tension liquid insulation will be used. The
cost is a great drawback, but if we employ an oil as an insulator the
distribution of electrical energy with something like 100,000 volts,
and even more, become, at least with higher frequencies, so easy that
they could be hardly called engineering feats. With oil insulation
and alternate current motors transmissions of power can be effected
with safety and upon an industrial basis at distances of as much as a
thousand miles.

A peculiar property of oils, and liquid insulation in general, when
subjected to rapidly changing electric stresses, is to disperse any
gaseous bubbles which may be present, and diffuse them through its
mass, generally long before any injurious break can occur. This
feature may be easily observed with an ordinary induction coil by
taking the primary out, plugging up the end of the tube upon which the
secondary is wound, and filling it with some fairly transparent
insulator, such as paraffine oil. A primary of a diameter something
like six millimetres smaller than the inside of the tube may be
inserted in the oil. When the coil is set to work one may see, looking
from the top through the oil, many luminous points--air bubbles which
are caught by inserting the primary, and which are rendered luminous
in consequence of the violent bombardment. The occluded air, by its
impact against the oil, heats it; the oil begins to circulate,
carrying some of the air along with it, until the bubbles are
dispersed and the luminous points disappear. In this manner, unless
large bubbles are occluded in such way that circulation is rendered
impossible, a damaging break is averted, the only effect being a
moderate warming up of the oil. If, instead of the liquid, a solid
insulation, no matter how thick, were used, a breaking through and
injury of the apparatus would be inevitable.

The exclusion of gaseous matter from any apparatus in which the
dielectric is subjected to more or less rapidly changing electric
forces is, however, not only desirable in order to avoid a possible
injury of the apparatus, but also on account of economy. In a
condenser, for instance, as long as only a solid or only a liquid
dielectric is used, the loss is small; but if a gas under ordinary or
small pressure be present the loss may be very great. Whatever the
nature of the force acting in the dielectric may be, it seems that in
a solid or liquid the molecular displacement produced by the force is
small; hence the product of force and displacement is insignificant,
unless the force be very great; but in a gas the displacement, and
therefore this product, is considerable; the molecules are free to
move, they reach high speeds, and the energy of their impact is lost
in heat or otherwise. If the gas be strongly compressed, the
displacement due to the force is made smaller, and the losses are
reduced.

In most of the succeeding experiments I prefer, chiefly on account of
the regular and positive action, to employ the alternator before
referred to. This is one of the several machines constructed by me for
the purposes of these investigations. It has 384 pole projections, and
is capable of giving currents of a frequency of about 10,000 per
second. This machine has been illustrated and briefly described in my
first paper before the American Institute of Electrical Engineers, May
20, 1891, to which I have already referred. A more detailed
description, sufficient to enable any engineer to build a similar
machine, will be found in several electrical journals of that period.

The induction coils operated from the machine are rather small,
containing from 5,000 to 15,000 turns in the secondary. They are
immersed in boiled-out linseed oil, contained in wooden boxes covered
with zinc sheet.

I have found it advantageous to reverse the usual position of the
wires, and to wind, in these coils, the primaries on the top; this
allowing the use of a much bigger primary, which, of course, reduces
the danger of overheating and increases the output of the coil. I make
the primary on each side at least one centimetre shorter than the
secondary, to prevent the breaking through on the ends, which would
surely occur unless the insulation on the top of the secondary be very
thick, and this, of course, would be disadvantageous.

When the primary is made movable, which is necessary in some
experiments, and many times convenient for the purposes of adjustment,
I cover the secondary with wax, and turn it off in a lathe to a
diameter slightly smaller than the inside of the primary coil. The
latter I provide with a handle reaching out of the oil, which serves
to shift it in any position along the secondary.

I will now venture to make, in regard to the general manipulation of
induction coils, a few observations bearing upon points which have not
been fully appreciated in earlier experiments with such coils, and are
even now often overlooked.

The secondary of the coil possesses usually such a high self-induction
that the current through the wire is inappreciable, and may be so even
when the terminals are joined by a conductor of small resistance. If
capacity is added to the terminals, the self-induction is
counteracted, and a stronger current is made to flow through the
secondary, though its terminals are insulated from each other. To one
entirely unacquainted with the properties of alternating currents
nothing will look more puzzling. This feature was illustrated in the
experiment performed at the beginning with the top plates of wire
gauze attached to the terminals and the rubber plate. When the plates
of wire gauze were close together, and a small arc passed between
them, the arc _prevented_ a strong current from passing through the
secondary, because it did away with the capacity on the terminals;
when the rubber plate was inserted between, the capacity of the
condenser formed counteracted the self-induction of the secondary, a
stronger current passed now, the coil performed more work, and the
discharge was by far more powerful.

The first thing, then, in operating the induction coil is to combine
capacity with the secondary to overcome the self-induction. If the
frequencies and potentials are very high gaseous matter should be
carefully kept away from the charged surfaces. If Leyden jars are
used, they should be immersed in oil, as otherwise considerable
dissipation may occur if the jars are greatly strained. When high
frequencies are used, it is of equal importance to combine a condenser
with the primary. One may use a condenser connected to the ends of the
primary or to the terminals of the alternator, but the latter is not
to be recommended, as the machine might be injured. The best way is
undoubtedly to use the condenser in series with the primary and with
the alternator, and to adjust its capacity so as to annul the
self-induction of both the latter. The condenser should be adjustable
by very small steps, and for a finer adjustment a small oil condenser
with movable plates may be used conveniently.

I think it best at this juncture to bring before you a phenomenon,
observed by me some time ago, which to the purely scientific
investigator may perhaps appear more interesting than any of the
results which I have the privilege to present to you this evening.

It may be quite properly ranked among the brush phenomena--in fact, it
is a brush, formed at, or near, a single terminal in high vacuum.

In bulbs provided with a conducting terminal, though it be of
aluminium, the brush has but an ephemeral existence, and cannot,
unfortunately, be indefinitely preserved in its most sensitive state,
even in a bulb devoid of any conducting electrode. In studying the
phenomenon, by all means a bulb having no leading-in wire should be
used. I have found it best to use bulbs constructed as indicated in
Figs. 12 and 13.

In Fig. 12 the bulb comprises an incandescent lamp globe L, in the
neck of which is sealed a barometer tube b, the end of which is blown
out to form a small sphere s. This sphere should be sealed as closely
as possible in the centre of the large globe. Before sealing, a thin
tube t, of aluminium sheet, may be slipped in the barometer tube, but
it is not important to employ it.

The small hollow sphere s is filled with some conducting powder, and a
wire w is cemented in the neck for the purpose of connecting the
conducting powder with the generator.

[Illustration: FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.]

The construction shown in Fig. 13 was chosen in order to remove from
the brush any conducting body which might possibly affect it. The bulb
consists in this case of a lamp globe L, which has a neck n, provided
with a tube b and small sphere s, sealed to it, so that two entirely
independent compartments are formed, as indicated in the drawing. When
the bulb is in use, the neck n is provided with a tinfoil coating,
which is connected to the generator and acts inductively upon the
moderately rarefied and highly conducting gas inclosed in the neck.
From there the current passes through the tube b into the small sphere
s to act by induction upon the gas contained in the globe L.

It is of advantage to make the tube t very thick, the hole through it
very small, and to blow the sphere s very thin. It is of the greatest
importance that the sphere s be placed in the centre of the globe L.

[Illustration: FIG. 14.--FORMS AND PHASES OF THE ROTATING BRUSH.]

Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
Fig. 14 shows the brush as it first appears in a bulb provided with a
conducting terminal; but, as in such a bulb it very soon
disappears--often after a few minutes--I will confine myself to the
description of the phenomenon as seen in a bulb without conducting
electrode. It is observed under the following conditions:

When the globe L (Figs. 12 and 13) is exhausted to a very high
degree, generally the bulb is not excited upon connecting the wire w
(Fig. 12) or the tinfoil coating of the bulb (Fig. 13) to the terminal
of the induction coil. To excite it, it is usually sufficient to grasp
the globe L with the hand. An intense phosphorescence then spreads at
first over the globe, but soon gives place to a white, misty light.
Shortly afterward one may notice that the luminosity is unevenly
distributed in the globe, and after passing the current for some time
the bulb appears as in Fig. 15. From this stage the phenomenon will
gradually pass to that indicated in Fig. 16, after some minutes,
hours, days or weeks, according as the bulb is worked. Warming the
bulb or increasing the potential hastens the transit.

[Illustration: FIG. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING
BRUSH.]

When the brush assumes the form indicated in Fig. 16, it maybe brought
to a state of extreme sensitiveness to electrostatic and magnetic
influence. The bulb hanging straight down from a wire, and all objects
being remote from it, the approach of the observer at a few paces from
the bulb will cause the brush to fly to the opposite side, and if he
walks around the bulb it will always keep on the opposite side. It may
begin to spin around the terminal long before it reaches that
sensitive stage. When it begins to turn around principally, but also
before, it is affected by a magnet, and at a certain stage it is
susceptible to magnetic influence to an astonishing degree. A small
permanent magnet, with its poles at a distance of no more than two
centimetres, will affect it visibly at a distance of two metres,
slowing down or accelerating the rotation according to how it is held
relatively to the brush. I think I have observed that at the stage
when it is most sensitive to magnetic, it is not most sensitive to
electrostatic, influence. My explanation is, that the electrostatic
attraction between the brush and the glass of the bulb, which retards
the rotation, grows much quicker than the magnetic influence when the
intensity of the stream is increased.

When the bulb hangs with the globe L down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction and on the equator the brush should not turn at all. The
rotation may be reversed by a magnet kept at some distance. The brush
rotates best, seemingly, when it is at right angles to the lines of
force of the earth. It very likely rotates, when at its maximum speed,
in synchronism with the alternations, say 10,000 times a second. The
rotation can be slowed down or accelerated by the approach or receding
of the observer, or any conducting body, but it cannot be reversed by
putting the bulb in any position. When it is in the state of the
highest sensitiveness and the potential or frequency be varied the
sensitiveness is rapidly diminished. Changing either of these but
little will generally stop the rotation. The sensitiveness is likewise
affected by the variations of temperature. To attain great
sensitiveness it is necessary to have the small sphere s in the centre
of the globe L, as otherwise the electrostatic action of the glass of
the globe will tend to stop the rotation. The sphere s should be small
and of uniform thickness; any dissymmetry of course has the effect to
diminish the sensitiveness.

The fact that the brush rotates in a definite direction in a permanent
magnetic field seems to show that in alternating currents of very high
frequency the positive and negative impulses are not equal, but that
one always preponderates over the other.

Of course, this rotation in one direction may be due to the action of
two elements of the same current upon each other, or to the action of
the field produced by one of the elements upon the other, as in a
series motor, without necessarily one impulse being stronger than the
other. The fact that the brush turns, as far as I could observe, in
any position, would speak for this view. In such case it would turn
at any point of the earth's surface. But, on the other hand, it is
then hard to explain why a permanent magnet should reverse the
rotation, and one must assume the preponderance of impulses of one
kind.

As to the causes of the formation of the brush or stream, I think it
is due to the electrostatic action of the globe and the dissymmetry of
the parts. If the small bulb s and the globe L were perfect concentric
spheres, and the glass throughout of the same thickness and quality, I
think the brush would not form, as the tendency to pass would be equal
on all sides. That the formation of the stream is due to an
irregularity is apparent from the fact that it has the tendency to
remain in one position, and rotation occurs most generally only when
it is brought out of this position by electrostatic or magnetic
influence. When in an extremely sensitive state it rests in one
position, most curious experiments may be performed with it. For
instance, the experimenter may, by selecting a proper position,
approach the hand at a certain considerable distance to the bulb, and
he may cause the brush to pass off by merely stiffening the muscles of
the arm. When it begins to rotate slowly, and the hands are held at a
proper distance, it is impossible to make even the slightest motion
without producing a visible effect upon the brush. A metal plate
connected to the other terminal of the coil affects it at a great
distance, slowing down the rotation often to one turn a second.

I am firmly convinced that such a brush, when we learn how to produce
it properly, will prove a valuable aid in the investigation of the
nature of the forces acting in an electrostatic or magnetic field. If
there is any motion which is measurable going on in the space, such a
brush ought to reveal it. It is, so to speak, a beam of light,
frictionless, devoid of inertia.

I think that it may find practical applications in telegraphy. With
such a brush it would be possible to send dispatches across the
Atlantic, for instance, with any speed, since its sensitiveness may be
so great that the slightest changes will affect it. If it were
possible to make the stream more intense and very narrow, its
deflections could be easily photographed.

I have been interested to find whether there is a rotation of the
stream itself, or whether there is simply a stress traveling around in
the bulb. For this purpose I mounted a light mica fan so that its
vanes were in the path of the brush. If the stream itself was rotating
the fan would be spun around. I could produce no distinct rotation of
the fan, although I tried the experiment repeatedly; but as the fan
exerted a noticeable influence on the stream, and the apparent
rotation of the latter was, in this case, never quite satisfactory,
the experiment did not appear to be conclusive.

I have been unable to produce the phenomenon with the disruptive
discharge coil, although every other of these phenomena can be well
produced by it--many, in fact, much better than with coils operated
from an alternator.

It may be possible to produce the brush by impulses of one direction,
or even by a steady potential, in which case it would be still more
sensitive to magnetic influence.

In operating an induction coil with rapidly alternating currents, we
realize with astonishment, for the first time, the great importance
of the relation of capacity, self-induction and frequency as regards
the general result. The effects of capacity are the most striking, for
in these experiments, since the self-induction and frequency both are
high, the critical capacity is very small, and need be but slightly
varied to produce a very considerable change. The experimenter may
bring his body in contact with the terminals of the secondary of the
coil, or attach to one or both terminals insulated bodies of very
small bulk, such as bulbs, and he may produce a considerable rise or
fall of potential, and greatly affect the flow of the current through
the primary. In the experiment before shown, in which a brush appears
at a wire attached to one terminal, and the wire is vibrated when the
experimenter brings his insulated body in contact with the other
terminal of the coil, the sudden rise of potential was made evident.

I may show you the behavior of the coil in another manner which
possesses a feature of some interest. I have here a little light fan
of aluminium sheet, fastened to a needle and arranged to rotate freely
in a metal piece screwed to one of the terminals of the coil. When the
coil is set to work, the molecules of the air are rhythmically
attracted and repelled. As the force with which they are repelled is
greater than that with which they are attracted, it results that there
is a repulsion exerted on the surfaces of the fan. If the fan were
made simply of a metal sheet, the repulsion would be equal on the
opposite sides, and would produce no effect. But if one of the
opposing surfaces is screened, or if, generally speaking, the
bombardment on this side is weakened in some way or other, there
remains the repulsion exerted upon the other, and the fan is set in
rotation. The screening is best effected by fastening upon one of the
opposing sides of the fan insulated conducting coatings, or, if the
fan is made in the shape of an ordinary propeller screw, by fastening
on one side, and close to it, an insulated metal plate. The static
screen may, however, be omitted, and simply a thickness of insulating
material fastened to one of the sides of the fan.

To show the behavior of the coil, the fan may be placed upon the
terminal and it will readily rotate when the coil is operated by
currents of very high frequency. With a steady potential, of course,
and even with alternating currents of very low frequency, it would not
turn, because of the very slow exchange of air and, consequently,
smaller bombardment; but in the latter case it might turn if the
potential were excessive. With a pin wheel, quite the opposite rule
holds good; it rotates best with a steady potential, and the effort is
the smaller the higher the frequency. Now, it is very easy to adjust
the conditions so that the potential is normally not sufficient to
turn the fan, but that by connecting the other terminal of the coil
with an insulated body it rises to a much greater value, so as to
rotate the fan, and it is likewise possible to stop the rotation by
connecting to the terminal a body of different size, thereby
diminishing the potential.

Instead of using the fan in this experiment, we may use the "electric"
radiometer with similar effect. But in this case it will be found that
the vanes will rotate only at high exhaustion or at ordinary
pressures; they will not rotate at moderate pressures, when the air is
highly conducting. This curious observation was made conjointly by
Professor Crookes and myself. I attribute the result to the high
conductivity of the air, the molecules of which then do not act as
independent carriers of electric charges, but act all together as a
single conducting body. In such case, of course, if there is any
repulsion at all of the molecules from the vanes, it must be very
small. It is possible, however, that the result is in part due to the
fact that the greater part of the discharge passes from the leading-in
wire through the highly conducting gas, instead of passing off from
the conducting vanes.

In trying the preceding experiment with the electric radiometer the
potential should not exceed a certain limit, as then the electrostatic
attraction between the vanes and the glass of the bulb may be so great
as to stop the rotation.

A most curious feature of alternate currents of high frequencies and
potentials is that they enable us to perform many experiments by the
use of one wire only. In many respects this feature is of great
interest.

In a type of alternate current motor invented by me some years ago I
produced rotation by inducing, by means of a single alternating
current passed through a motor circuit, in the mass or other circuits
of the motor, secondary currents, which, jointly with the primary or
inducing current, created a moving field of force. A simple but crude
form of such a motor is obtained by winding upon an iron core a
primary, and close to it a secondary coil, joining the ends of the
latter and placing a freely movable metal disc within the influence of
the field produced by both. The iron core is employed for obvious
reasons, but it is not essential to the operation. To improve the
motor, the iron core is made to encircle the armature. Again to
improve, the secondary coil is made to overlap partly the primary, so
that it cannot free itself from a strong inductive action of the
latter, repel its lines as it may. Once more to improve, the proper
difference of phase is obtained between the primary and secondary
currents by a condenser, self-induction, resistance or equivalent
windings.

I had discovered, however, that rotation is produced by means of a
single coil and core; my explanation of the phenomenon, and leading
thought in trying the experiment, being that there must be a true time
lag in the magnetization of the core. I remember the pleasure I had
when, in the writings of Professor Ayrton, which came later to my
hand, I found the idea of the time lag advocated. Whether there is a
true time lag, or whether the retardation is due to eddy currents
circulating in minute paths, must remain an open question, but the
fact is that a coil wound upon an iron core and traversed by an
alternating current creates a moving field of force, capable of
setting an armature in rotation. It is of some interest, in
conjunction with the historical Arago experiment, to mention that in
lag or phase motors I have produced rotation in the opposite direction
to the moving field, which means that in that experiment the magnet
may not rotate, or may even rotate in the opposite direction to the
moving disc. Here, then, is a motor (diagrammatically illustrated in
Fig. 17), comprising a coil and iron core, and a freely movable copper
disc in proximity to the latter.

[Illustration: FIG. 17.--SINGLE WIRE AND "NO-WIRE" MOTOR.]

To demonstrate a novel and interesting feature, I have, for a reason
which I will explain, selected this type of motor. When the ends of
the coil are connected to the terminals of an alternator the disc is
set in rotation. But it is not this experiment, now well known, which
I desire to perform. What I wish to show you is that this motor
rotates with _one single_ connection between it and the generator;
that is to say, one terminal of the motor is connected to one terminal
of the generator--in this case the secondary of a high-tension
induction coil--the other terminals of motor and generator being
insulated in space. To produce rotation it is generally (but not
absolutely) necessary to connect the free end of the motor coil to an
insulated body of some size. The experimenter's body is more than
sufficient. If he touches the free terminal with an object held in the
hand, a current passes through the coil and the copper disc is set in
rotation. If an exhausted tube is put in series with the coil, the
tube lights brilliantly, showing the passage of a strong current.
Instead of the experimenter's body, a small metal sheet suspended on a
cord may be used with the same result. In this case the plate acts as
a condenser in series with the coil. It counteracts the self-induction
of the latter and allows a strong current to pass. In such a
combination, the greater the self-induction of the coil the smaller
need be the plate, and this means that a lower frequency, or
eventually a lower potential, is required to operate the motor. A
single coil wound upon a core has a high self-induction; for this
reason principally, this type of motor was chosen to perform the
experiment. Were a secondary closed coil wound upon the core, it would
tend to diminish the self-induction, and then it would be necessary to
employ a much higher frequency and potential. Neither would be
advisable, for a higher potential would endanger the insulation of the
small primary coil, and a higher frequency would result in a
materially diminished torque.

It should be remarked that when such a motor with a closed secondary
is used, it is not at all easy to obtain rotation with excessive
frequencies, as the secondary cuts off almost completely the lines of
the primary--and this, of course, the more, the higher the
frequency--and allows the passage of but a minute current. In such a
case, unless the secondary is closed through a condenser, it is almost
essential, in order to produce rotation, to make the primary and
secondary coils overlap each other more or less.

But there is an additional feature of interest about this motor,
namely, it is not necessary to have even a single connection between
the motor and generator, except, perhaps, through the ground: for not
only is an insulated plate capable of giving off energy into space,
but it is likewise capable of deriving it from an alternating
electrostatic field, though in the latter case the available energy is
much smaller. In this instance one of the motor terminals is connected
to the insulated plate or body located within the alternating
electrostatic field, and the other terminal preferably to the ground.

It is quite possible, however, that such "no-wire" motors, as they
might be called, could be operated by conduction through the rarefied
air at considerable distances. Alternate currents, especially of high
frequencies, pass with astonishing freedom through even slightly
rarefied gases. The upper strata of the air are rarefied. To reach a
number of miles out into space requires the overcoming of difficulties
of a merely mechanical nature. There is no doubt that with the
enormous potentials obtainable by the use of high frequencies and oil
insulation luminous discharges might be passed through many miles of
rarefied air, and that, by thus directing the energy of many hundreds
or thousands of horse-power, motors or lamps might be operated at
considerable distances from stationary sources. But such schemes are
mentioned merely as possibilities. We shall have no need to transmit
power in this way. We shall have no need to _transmit_ power at all.
Ere many generations pass, our machinery will be driven by a power
obtainable at any point of the universe. This idea is not novel. Men
have been led to it long ago by instinct or reason. It has been
expressed in many ways, and in many places, in the history of old and
new. We find it in the delightful myth of Antheus, who derives power
from the earth; we find it among the subtile speculations of one of
your splendid mathematicians, and in many hints and statements of
thinkers of the present time. Throughout space there is energy. Is
this energy static or kinetic? If static our hopes are in vain; if
kinetic--and this we know it is, for certain--then it is a mere
question of time when men will succeed in attaching their machinery to
the very wheelwork of nature. Of all, living or dead, Crookes came
nearest to doing it. His radiometer will turn in the light of day and
in the darkness of the night; it will turn everywhere where there is
heat, and heat is everywhere. But, unfortunately, this beautiful
little machine, while it goes down to posterity as the most
interesting, must likewise be put on record as the most inefficient
machine ever invented!

The preceding experiment is only one of many equally interesting
experiments which may be performed by the use of only one wire with
alternate currents of high potential and frequency. We may connect an
insulated line to a source of such currents, we may pass an
inappreciable current over the line, and on any point of the same we
are able to obtain a heavy current, capable of fusing a thick copper
wire. Or we may, by the help of some artifice, decompose a solution in
any electrolytic cell by connecting only one pole of the cell to the
line or source of energy. Or we may, by attaching to the line, or only
bringing into its vicinity, light up an incandescent lamp, an
exhausted tube, or a phosphorescent bulb.

However impracticable this plan of working may appear in many cases,
it certainly seems practicable, and even recommendable, in the
production of light. A perfected lamp would require but little energy,
and if wires were used at all we ought to be able to supply that
energy without a return wire.

It is now a fact that a body may be rendered incandescent or
phosphorescent by bringing it either in single contact or merely in
the vicinity of a source of electric impulses of the proper character,
and that in this manner a quantity of light sufficient to afford a
practical illuminant may be produced. It is, therefore, to say the
least, worth while to attempt to determine the best conditions and to
invent the best appliances for attaining this object.

Some experiences have already been gained in this direction, and I
will dwell on them briefly, in the hope that they might prove useful.

The heating of a conducting body inclosed in a bulb, and connected to
a source of rapidly alternating electric impulses, is dependent on so
many things of a different nature, that it would be difficult to give
a generally applicable rule under which the maximum heating occurs. As
regards the size of the vessel, I have lately found that at ordinary
or only slightly differing atmospheric pressures, when air is a good
insulator, and hence practically the same amount of energy by a
certain potential and frequency is given off from the body, whether
the bulb be small or large, the body is brought to a higher
temperature if inclosed in a small bulb, because of the better
confinement of heat in this case.

At lower pressures, when air becomes more or less conducting, or if
the air be sufficiently warmed as to become conducting, the body is
rendered more intensely incandescent in a large bulb, obviously
because, under otherwise equal conditions of test, more energy may be
given off from the body when the bulb is large.

At very high degrees of exhaustion, when the matter in the bulb
becomes "radiant," a large bulb has still an advantage, but a
comparatively slight one, over the small bulb.

Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.

These observations were the result of a number of experiments, of
which one, showing the effect of the size of the bulb at a high degree
of exhaustion, may be described and shown here, as it presents a
feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4
inches diameter were taken, and in the centre of each was mounted an
equal length of an ordinary incandescent lamp filament of uniform
thickness. In each bulb the piece of filament was fastened to the
leading-in wire of platinum, contained in a glass stem sealed in the
bulb; care being taken, of course, to make everything as nearly alike
as possible. On each glass stem in the inside of the bulb was slipped
a highly polished tube made of aluminium sheet, which fitted the stem
and was held on it by spring pressure. The function of this aluminium
tube will be explained subsequently. In each bulb an equal length of
filament protruded above the metal tube. It is sufficient to say now
that under these conditions equal lengths of filament of the same
thickness--in other words, bodies of equal bulk--were brought to
incandescence. The three bulbs were sealed to a glass tube, which was
connected to a Sprengel pump. When a high vacuum had been reached, the
glass tube carrying the bulbs was sealed off. A current was then
turned on successively on each bulb, and it was found that the
filaments came to about the same brightness, and, if anything, the
smallest bulb, which was placed midway between the two larger ones,
may have been slightly brighter. This result was expected, for when
either of the bulbs was connected to the coil the luminosity spread
through the other two, hence the three bulbs constituted really one
vessel. When all the three bulbs were connected in multiple arc to the
coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only
came to redness. The bulbs were then sealed off and separately tried.
The brightness of the filaments was now such as would have been
expected on the supposition that the energy given off was
proportionate to the surface of the bulb, this surface in each case
representing one of the coatings of a condenser. Accordingly, time
was less difference between the largest and the middle sized than
between the latter and the smallest bulb.

An interesting observation was made in this experiment. The three
bulbs were suspended from a straight bare wire connected to a terminal
of the coil, the largest bulb being placed at the end of the wire, at
some distance from it the smallest bulb, and an equal distance from
the latter the middle-sized one. The carbons glowed then in both the
larger bulbs about as expected, but the smallest did not get its share
by far. This observation led me to exchange the position of the bulbs,
and I then observed that whichever of the bulbs was in the middle it
was by far less bright than it was in any other position. This
mystifying result was, of course, found to be due to the electrostatic
action between the bulbs. When they were placed at a considerable
distance, or when they were attached to the corners of an equilateral
triangle of copper wire, they glowed about in the order determined by
their surfaces.

As to the shape of the vessel, it is also of some importance,
especially at high degrees of exhaustion. Of all the possible
constructions, it seems that a spherical globe with the refractory
body mounted in its centre is the best to employ. In experience it has
been demonstrated that in such a globe a refractory body of a given
bulk is more easily brought to incandescence than when otherwise
shaped bulbs are used. There is also an advantage in giving to the
incandescent body the shape of a sphere, for self-evident reasons. In
any case the body should be mounted in the centre, where the atoms
rebounding from the glass collide. This object is best attained in
the spherical bulb; but it is also attained in a cylindrical vessel
with one or two straight filaments coinciding with its axis, and
possibly also in parabolical or spherical bulbs with the refractory
body or bodies placed in the focus or foci of the same; though the
latter is not probable, as the electrified atoms should in all cases
rebound normally from the surface they strike, unless the speed were
excessive, in which case they _would_ probably follow the general law
of reflection. No matter what shape the vessel may have, if the
exhaustion be low, a filament mounted in the globe is brought to the
same degree of incandescence in all parts; but if the exhaustion be
high and the bulb be spherical or pear-shaped, as usual, focal points
form and the filament is heated to a higher degree at or near such
points.

To illustrate the effect, I have here two small bulbs which are alike,
only one is exhausted to a low and the other to a very high degree.
When connected to the coil, the filament in the former glows uniformly
throughout all its length; whereas in the latter, that portion of the
filament which is in the centre of the bulb glows far more intensely
than the rest. A curious point is that the phenomenon occurs even if
two filaments are mounted in a bulb, each being connected to one
terminal of the coil, and, what is still more curious, if they be very
near together, provided the vacuum be very high. I noted in
experiments with such bulbs that the filaments would give way usually
at a certain point, and in the first trials I attributed it to a
defect in the carbon. But when the phenomenon occurred many times in
succession I recognized its real cause.

In order to bring a refractory body inclosed in a bulb to
incandescence, it is desirable, on account of economy, that all the
energy supplied to the bulb from the source should reach without loss
the body to be heated; from there, and from nowhere else, it should be
radiated. It is, of course, out of the question to reach this
theoretical result, but it is possible by a proper construction of the
illuminating device to approximate it more or less.

For many reasons, the refractory body is placed in the centre of the
bulb, and it is usually supported on a glass stem containing the
leading-in wire. As the potential of this wire is alternated, the
rarefied gas surrounding the stem is acted upon inductively, and the
glass stem is violently bombarded and heated. In this manner by far
the greater portion of the energy supplied to the bulb--especially
when exceedingly high frequencies are used--may be lost for the
purpose contemplated. To obviate this loss, or at least to reduce it
to a minimum, I usually screen the rarefied gas surrounding the stem
from the inductive action of the leading-in wire by providing the stem
with a tube or coating of conducting material. It seems beyond doubt
that the best among metals to employ for this purpose is aluminium, on
account of its many remarkable properties. Its only fault is that it
is easily fusible, and, therefore, its distance from the incandescing
body should be properly estimated. Usually, a thin tube, of a diameter
somewhat smaller than that of the glass stem, is made of the finest
aluminium sheet, and slipped on the stem. The tube is conveniently
prepared by wrapping around a rod fastened in a lathe a piece of
aluminium sheet of the proper size, grasping the sheet firmly with
clean chamois leather or blotting paper, and spinning the rod very
fast. The sheet is wound tightly around the rod, and a highly polished
tube of one or three layers of the sheet is obtained. When slipped on
the stem, the pressure is generally sufficient to prevent it from
slipping off, but, for safety, the lower edge of the sheet may be
turned inside. The upper inside corner of the sheet--that is, the one
which is nearest to the refractory incandescent body--should be cut
out diagonally, as it often happens that, in consequence of the
intense heat, this corner turns toward the inside and comes very near
to, or in contact with, the wire, or filament, supporting the
refractory body. The greater part of the energy supplied to the bulb
is then used up in heating the metal tube, and the bulb is rendered
useless for the purpose. The aluminium sheet should project above the
glass stem more or less--one inch or so--or else, if the glass be too
close to the incandescing body, it may be strongly heated and become
more or less conducting, whereupon it may be ruptured, or may, by its
conductivity, establish a good electrical connection between the metal
tube and the leading-in wire, in which case, again, most of the energy
will be lost in heating the former. Perhaps the best way is to make
the top of the glass tube, for about an inch, of a much smaller
diameter. To still further reduce the danger arising from the heating
of the glass stem, and also with the view of preventing an electrical
connection between the metal tube and the electrode, I preferably wrap
the stem with several layers of thin mica, which extends at least as
far as the metal tube. In some bulbs I have also used an outside
insulating cover.

The preceding remarks are only made to aid the experimenter in the
first trials, for the difficulties which he encounters he may soon
find means to overcome in his own way.

To illustrate the effect of the screen, and the advantage of using it,
I have here two bulbs of the same size, with their stems, leading-in
wires and incandescent lamp filaments tied to the latter, as nearly
alike as possible. The stem of one bulb is provided with an aluminium
tube, the stem of the other has none. Originally the two bulbs were
joined by a tube which was connected to a Sprengel pump. When a high
vacuum had been reached, first the connecting tube, and then the
bulbs, were sealed off; they are therefore of the same degree of
exhaustion. When they are separately connected to the coil giving a
certain potential, the carbon filament in the bulb provided with the
aluminium screen is rendered highly incandescent, while the filament
in the other bulb may, with the same potential, not even come to
redness, although in reality the latter bulb takes generally more
energy than the former. When they are both connected together to the
terminal, the difference is even more apparent, showing the importance
of the screening. The metal tube placed on the stem containing the
leading-in wire performs really two distinct functions: First: it acts
more or less as an electrostatic screen, thus economizing the energy
supplied to the bulb; and, second, to whatever extent it may fail to
act electrostatically, it acts mechanically, preventing the
bombardment, and consequently intense heating and possible
deterioration of the slender support of the refractory incandescent
body, or of the glass stem containing the leading-in wire. I say
_slender_ support, for it is evident that in order to confine the heat
more completely to the incandescing body its support should be very
thin, so as to carry away the smallest possible amount of heat by
conduction. Of all the supports used I have found an ordinary
incandescent lamp filament to be the best, principally because among
conductors it can withstand the highest degrees of heat.

The effectiveness of the metal tube as an electrostatic screen depends
largely on the degree of exhaustion.

At excessively high degrees of exhaustion--which are reached by using
great care and special means in connection with the Sprengel
pump--when the matter in the globe is in the ultra-radiant state, it
acts most perfectly. The shadow of the upper edge of the tube is then
sharply defined upon the bulb.

At a somewhat lower degree of exhaustion, which is about the ordinary
"non-striking" vacuum, and generally as long as the matter moves
predominantly in straight lines, the screen still does well. In
elucidation of the preceding remark it is necessary to state that what
is a "non-striking" vacuum for a coil operated, as ordinarily, by
impulses, or currents, of low-frequency, is not, by far, so when the
coil is operated by currents of very high frequency. In such case the
discharge may pass with great freedom through the rarefied gas through
which a low-frequency discharge may not pass, even though the
potential be much higher. At ordinary atmospheric pressures just the
reverse rule holds good: the higher the frequency, the less the spark
discharge is able to jump between the terminals, especially if they
are knobs or spheres of some size.

Finally, at very low degrees of exhaustion, when the gas is well
conducting, the metal tube not only does not act as an electrostatic
screen, but even is a drawback, aiding to a considerable extent the
dissipation of the energy laterally from the leading-in wire. This, of
course, is to be expected. In this case, namely, the metal tube is in
good electrical connection with the leading-in wire, and most of the
bombardment is directed upon the tube. As long as the electrical
connection is not good, the conducting tube is always of some
advantage, for although it may not greatly economize energy, still it
protects the support of the refractory button, and is a means for
concentrating more energy upon the same.

To whatever extent the aluminium tube performs the function of a
screen, its usefulness is therefore limited to very high degrees of
exhaustion when it is insulated from the electrode--that is, when the
gas as a whole is non-conducting, and the molecules, or atoms, act as
independent carriers of electric charges.

In addition to acting as a more or less effective screen, in the true
meaning of the word, the conducting tube or coating may also act, by
reason of its conductivity, as a sort of equalizer or dampener of the
bombardment against the stem. To be explicit, I assume the action as
follows: Suppose a rhythmical bombardment to occur against the
conducting tube by reason of its imperfect action as a screen, it
certainly must happen that some molecules, or atoms, strike the tube
sooner than others. Those which come first in contact with it give up
their superfluous charge, and the tube is electrified, the
electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment for two reasons: first,
the charge given up by the atoms spreads over a great area, and hence
the electric density at any point is small, and the atoms are repelled
with less energy than they would be if they would strike against a
good insulator: secondly, as the tube is electrified by the atoms
which first come in contact with it, the progress of the following
atoms against the tube is more or less checked by the repulsion which
the electrified tube must exert upon the similarly electrified atoms.
This repulsion may perhaps be sufficient to prevent a large portion of
the atoms from striking the tube, but at any rate it must diminish the
energy of their impact. It is clear that when the exhaustion is very
low, and the rarefied gas well conducting, neither of the above
effects can occur, and, on the other hand, the fewer the atoms, with
the greater freedom they move; in other words, the higher the degree
of exhaustion, up to a limit, the more telling will be both the
effects.

What I have just said may afford an explanation of the phenomenon
observed by Prof. Crookes, namely, that a discharge through a bulb is
established with much greater facility when an insulator than when a
conductor is present in the same. In my opinion, the conductor acts as
a dampener of the motion of the atoms in the two ways pointed out;
hence, to cause a visible discharge to pass through the bulb, a much
higher potential is needed if a conductor, especially of much surface,
be present.

For the sake of clearness of some of the remarks before made, I must
now refer to Figs. 18, 19 and 20, which illustrate various
arrangements with a type of bulb most generally used.

[Illustration: FIG. 18.--BULB WITH MICA TUBE AND ALUMINIUM SCREEN.]

[Illustration: FIG. 19.--IMPROVED BULB WITH SOCKET AND SCREEN.]

Fig. 18 is a section through a spherical bulb L, with the glass stem
s, containing the leading-in wire w; which has a lamp filament l
fastened to it, serving to support the refractory button m in the
centre. M is a sheet of thin mica wound in several layers around the
stem s, and a is the aluminium tube.

Fig. 19 illustrates such a bulb in a somewhat more advanced stage of
perfection. A metallic tube S is fastened by means of some cement to
the neck of the tube. In the tube is screwed a plug P, of insulating
material, in the centre of which is fastened a metallic terminal t,
for the connection to the leading-in wire w. This terminal must be
well insulated from the metal tube S, therefore, if the cement used is
conducting--and most generally it is sufficiently so--the space
between the plug P and the neck of the bulb should be filled with some
good insulating material, as mica powder.

[Illustration: FIG. 20.--BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.]

Fig. 20 shows a bulb made for experimental purposes. In this bulb the
aluminium tube is provided with an external connection, which serves
to investigate the effect of the tube under various conditions. It is
referred to chiefly to suggest a line of experiment followed.

Since the bombardment against the stem containing the leading-in wire
is due to the inductive action of the latter upon the rarefied gas, it
is of advantage to reduce this action as far as practicable by
employing a very thin wire, surrounded by a very thick insulation of
glass or other material, and by making the wire passing through the
rarefied gas as short as practicable. To combine these features I
employ a large tube T (Fig. 21), which protrudes into the bulb to some
distance, and carries on the top a very short glass stem s, into which
is sealed the leading-in wire w, and I protect the top of the glass
stem against the heat by a small, aluminium tube a and a layer of mica
underneath the same, as usual. The wire w, passing through the large
tube to the outside of the bulb, should be well insulated--with a
glass tube, for instance--and the space between ought to be filled out
with some excellent insulator. Among many insulating powders I have
tried, I have found that mica powder is the best to employ. If this
precaution is not taken, the tube T, protruding into the bulb, will
surely be cracked in consequence of the heating by the brushes which
are apt to form in the upper part of the tube, near the exhausted
globe, especially if the vacuum be excellent, and therefore the
potential necessary to operate the lamp very high.

Fig. 22 illustrates a similar arrangement, with a large tube T
protruding in to the part of the bulb containing the refractors button
m. In this case the wire leading from the outside into the bulb is
omitted, the energy required being supplied through condenser coatings
CC. The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge
might avoid passing through the wire w, which connects the inside
condenser coating to the incandescent button m. The molecular
bombardment against the glass stem in the bulb is a source of great
trouble. As illustration I will cite a phenomenon only too frequently
and unwillingly observed. A bulb, preferably a large one, may be
taken, and a good conducting body, such as a piece of carbon, may be
mounted in it upon a platinum wire sealed in the glass stem. The bulb
may be exhausted to a fairly high degree, nearly to the point when
phosphorescence begins to appear.

[Illustration: FIG. 21.--IMPROVED BULB WITH NON-CONDUCTING BUTTON.]

[Illustration: FIG. 22.--TYPE OF BULB WITHOUT LEADING-IN WIRE.]

When the bulb is connected with the coil, the piece of carbon, if
small, may become highly incandescent at first, but its brightness
immediately diminishes, and then the discharge may break through the
glass somewhere in the middle of the stem, in the form of bright
sparks, in spite of the fact that the platinum wire is in good
electrical connection with the rarefied gas through the piece of
carbon or metal at the top. The first sparks are singularly bright,
recalling those drawn from a clear surface of mercury. But, as they
heat the glass rapidly, they, of course, lose their brightness, and
cease when the glass at the ruptured place becomes incandescent, or
generally sufficiently hot to conduct. When observed for the first
time the phenomenon must appear very curious, and shows in a striking
manner how radically different alternate currents, or impulses, of
high frequency behave, as compared with steady currents, or currents
of low frequency. With such currents--namely, the latter--the
phenomenon would of course not occur. When frequencies such as are
obtained by mechanical means are used, I think that the rupture of the
glass is more or less the consequence of the bombardment, which warms
it up and impairs its insulating power; but with frequencies
obtainable with condensers I have no doubt that the glass may give way
without previous heating. Although this appears most singular at
first, it is in reality what we might expect to occur. The energy
supplied to the wire leading into the bulb is given off partly by
direct action through the carbon button, and partly by inductive
action through the glass surrounding the wire. The case is thus
analogous to that in which a condenser shunted by a conductor of low
resistance is connected to a source of alternating currents. As long
as the frequencies are low, the conductor gets the most, and the
condenser is perfectly safe: but when the frequency becomes excessive,
the _role_ of the conductor may become quite insignificant. In the
latter case the difference of potential at the terminals of the
condenser may become so great as to rupture the dielectric,
notwithstanding the fact that the terminals are joined by a conductor
of low resistance.

[Illustration: FIG. 23.--EFFECT PRODUCED BY A RUBY DROP.]

It is, of course, not necessary, when it is desired to produce the
incandescence of a body inclosed in a bulb by means of these currents,
that the body should be a conductor, for even a perfect non-conductor
may be quite as readily heated. For this purpose it is sufficient to
surround a conducting electrode with a non-conducting material, as,
for instance, in the bulb described before in Fig. 21, in which a thin
incandescent lamp filament is coated with a non-conductor, and
supports a button of the same material on the top. At the start the
bombardment goes on by inductive action through the non-conductor,
until the same is sufficiently heated to become conducting, when the
bombardment continues in the ordinary way.

A different arrangement used in some of the bulbs constructed is
illustrated in Fig. 23. In this instance a non-conductor m is
mounted in a piece of common arc light carbon so as to project some
small distance above the latter. The carbon piece is connected to the
leading-in wire passing through a glass stem, which is wrapped with
several layers of mica. An aluminium tube a is employed as usual for
screening. It is so arranged that it reaches very nearly as high as
the carbon and only the non-conductor m projects a little above it.
The bombardment goes at first against the upper surface of carbon, the
lower parts being protected by the aluminium tube. As soon, however,
as the non-conductor m is heated it is rendered good conducting, and
then it becomes the centre of the bombardment, being most exposed to
the same.

I have also constructed during these experiments many such single-wire
bulbs with or without internal electrode, in which the radiant matter
was projected against, or focused upon, the body to be rendered
incandescent. Fig. 24 illustrates one of the bulbs used. It consists
of a spherical globe L, provided with a long neck n, on the top, for
increasing the action in some cases by the application of an external
conducting coating. The globe L is blown out on the bottom into a very
small bulb b, which serves to hold it firmly in a socket S of
insulating material into which it is cemented. A fine lamp filament f,
supported on a wire w, passes through the centre of the globe L. The
filament is rendered incandescent in the middle portion, where the
bombardment proceeding from the lower inside surface of the globe is
most intense. The lower portion of the globe, as far as the socket S
reaches, is rendered conducting, either by a tinfoil coating or
otherwise, and the external electrode is connected to a terminal of
the coil.

The arrangement diagrammatically indicated in Fig. 24 was found to be
an inferior one when it was desired to render incandescent a filament
or button supported in the centre of the globe, but it was convenient
when the object was to excite phosphorescence.

In many experiments in which bodies of a different kind were mounted
in the bulb as, for instance, indicated in Fig. 23, some observations
of interest were made.

It was found, among other things, that in such cases, no matter where
the bombardment began, just as soon as a high temperature was reached
there was generally one of the bodies which seemed to take most of the
bombardment upon itself, the other, or others, being thereby relieved.
This quality appeared to depend principally on the point of fusion,
and on the facility with which the body was "evaporated," or,
generally speaking, disintegrated--meaning by the latter term not only
the throwing off of atoms, but likewise of larger lumps. The
observation made was in accordance with generally accepted notions. In
a highly exhausted bulb electricity is carried off from the electrode
by independent carriers, which are partly the atoms, or molecules, of
the residual atmosphere, and partly the atoms, molecules, or lumps
thrown off from the electrode. If the electrode is composed of bodies
of different character, and if one of these is more easily
disintegrated than the others, most of the electricity supplied is
carried off from that body, which is then brought to a higher
temperature than the others, and this the more, as upon an increase of
the temperature the body is still more easily disintegrated.

It seems to me quite probable that a similar process takes place in
the bulb even with a homogeneous electrode, and I think it to be the
principal cause of the disintegration. There is bound to be some
irregularity, even if the surface is highly polished, which, of
course, is impossible with most of the refractory bodies employed as
electrodes. Assume that a point of the electrode gets hotter,
instantly most of the discharge passes through that point, and a
minute patch is probably fused and evaporated. It is now possible that
in consequence of the violent disintegration the spot attacked sinks
in temperature, or that a counter force is created, as in an arc; at
any rate, the local tearing off meets with the limitations incident to
the experiment, whereupon the same process occurs on another place. To
the eye the electrode appears uniformly brilliant, but there are upon
it points constantly shifting and wandering around, of a temperature
far above the mean, and this materially hastens the process of
deterioration. That some such thing occurs, at least when the
electrode is at a lower temperature, sufficient experimental evidence
can be obtained in the following manner: Exhaust a bulb to a very high
degree, so that with a fairly high potential the discharge cannot
pass--that is, not a _luminous_ one, for a weak invisible discharge
occurs always, in all probability. Now raise slowly and carefully the
potential, leaving the primary current on no more than for an instant.
At a certain point, two, three, or half a dozen phosphorescent spots
will appear on the globe. These places of the glass are evidently more
violently bombarded than others, this being due to the unevenly
distributed electric density, necessitated, of course, by sharp
projections, or, generally speaking, irregularities of the electrode.
But the luminous patches are constantly changing in position, which is
especially well observable if one manages to produce very few, and
this indicates that the configuration of the electrode is rapidly
changing.

From experiences of this kind I am led to infer that, in order to be
most durable, the refractory button in the bulb should be in the form
of a sphere with a highly polished surface. Such a small sphere could
be manufactured from a diamond or some other crystal, but a better way
would be to fuse, by the employment of extreme degrees of temperature,
some oxide--as, for instance, zirconia--into a small drop, and then
keep it in the bulb at a temperature somewhat below its point of
fusion.

Interesting and useful results can no doubt be reached in the
direction of extreme degrees of heat. How can such high temperatures
be arrived at? How are the highest degrees of heat reached in nature?
By the impact of stars, by high speeds and collisions. In a collision
any rate of heat generation may be attained. In a chemical process we
are limited. When oxygen and hydrogen combine, they fall,
metaphorically speaking, from a definite height. We cannot go very far
with a blast, nor by confining heat in a furnace, but in an exhausted
bulb we can concentrate any amount of energy upon a minute button.
Leaving practicability out of consideration, this, then, would be the
means which, in my opinion, would enable us to reach the highest
temperature. But a great difficulty when proceeding in this way is
encountered, namely, in most cases the body is carried off before it
can fuse and form a drop. This difficulty exists principally with an
oxide such as zirconia, because it cannot be compressed in so hard a
cake that it would not be carried off quickly. I endeavored repeatedly
to fuse zirconia, placing it in a cup or arc light carbon as indicated
in Fig. 23. It glowed with a most intense light, and the stream of the
particles projected out of the carbon cup was of a vivid white: but
whether it was compressed in a cake or made into a paste with carbon,
it was carried off before it could be fused. The carbon cup containing
the zirconia had to be mounted very low in the neck of a large bulb,
as the heating of the glass by the projected particles of the oxide
was so rapid that in the first trial the bulb was cracked almost in an
instant when the current was turned on. The heating of the glass by
the projected particles was found to be always greater when the carbon
cup contained a body which was rapidly carried off--I presume because
in such cases, with the same potential, higher speeds were reached,
and also because, per unit of time, more matter was projected--that
is, more particles would strike the glass.

The before mentioned difficulty did not exist, however, when the body
mounted in the carbon cup offered great resistance to deterioration.
For instance, when an oxide was first fused in an oxygen blast and
then mounted in the bulb, it melted very readily into a drop.

Generally during the process of fusion magnificent light effects were
noted, of which it would be difficult to give an adequate idea. Fig.
23 is intended to illustrate the effect observed with a ruby drop. At
first one may see a narrow funnel of white light projected against the
top of the globe, where it produces an irregularly outlined
phosphorescent patch. When the point of the ruby fuses the
phosphorescence becomes very powerful; but as the atoms are projected
with much greater speed from the surface of the drop, soon the glass
gets hot and "tired," and now only the outer edge of the patch glows.
In this manner an intensely phosphorescent, sharply defined line, _l_,
corresponding to the outline of the drop, is produced, which spreads
slowly over the globe as the drop gets larger. When the mass begins to
boil, small bubbles and cavities are formed, which cause dark colored
spots to sweep across the globe. The bulb may be turned downward
without fear of the drop falling off, as the mass possesses
considerable viscosity.

I may mention here another feature of some interest, which I believe
to have noted in the course of these experiments, though the
observations do not amount to a certitude. It _appeared_ that under
the molecular impact caused by the rapidly alternating potential the
body was fused and maintained in that state at a lower temperature in
a highly exhausted bulb than was the case at normal pressure and
application of heat in the ordinary way--that is, at least, judging
from the quantity of the light emitted. One of the experiments
performed may be mentioned here by way of illustration. A small piece
of pumice stone was stuck on a platinum wire, and first melted to it
in a gas burner. The wire was next placed between two pieces of
charcoal and a burner applied so as to produce an intense heat,
sufficient to melt down the pumice stone into a small glass-like
button. The platinum wire had to be taken of sufficient thickness to
prevent its melting in the fire. While in the charcoal fire, or when
held in a burner to get a better idea of the degree of heat, the
button glowed with great brilliancy. The wire with the button was then
mounted in a bulb, and upon exhausting the same to a high degree, the
current was turned on slowly so as to prevent the cracking of the
button. The button was heated to the point of fusion, and when it
melted it did not, apparently, glow with the same brilliancy as
before, and this would indicate a lower temperature. Leaving out of
consideration the observer's possible, and even probable, error, the
question is, can a body under these conditions be brought from a solid
to a liquid state with evolution of _less_ light?

When the potential of a body is rapidly alternated it is certain that
the structure is jarred. When the potential is very high, although the
vibrations may be few--say 20,000 per second--the effect upon the
structure may be considerable. Suppose, for example, that a ruby is
melted into a drop by a steady application of energy. When it forms a
drop it will emit visible and invisible waves, which will be in a
definite ratio, and to the eye the drop will appear to be of a certain
brilliancy. Next, suppose we diminish to any degree we choose the
energy steadily supplied, and, instead, supply energy which rises and
falls according to a certain law. Now, when the drop is formed, there
will be emitted from it three different kinds of vibrations--the
ordinary visible, and two kinds of invisible waves: that is, the
ordinary dark waves of all lengths, and, in addition, waves of a well
defined character. The latter would not exist by a steady supply of
the energy; still they help to jar and loosen the structure. If this
really be the case, then the ruby drop will emit relatively less
visible and more invisible waves than before. Thus it would seem that
when a platinum wire, for instance, is fused by currents alternating
with extreme rapidity, it emits at the point of fusion less light and
more invisible radiation than it does when melted by a steady current,
though the total energy used up in the process of fusion is the same
in both cases. Or, to cite another example, a lamp filament is not
capable of withstanding as long with currents of extreme frequency as
it does with steady currents, assuming that it be worked at the same
luminous intensity. This means that for rapidly alternating currents
the filament should be shorter and thicker. The higher the
frequency--that is, the greater the departure from the steady
flow--the worse it would be for the filament. But if the truth of this
remark were demonstrated, it would be erroneous to conclude that such
a refractory button as used in these bulbs would be deteriorated
quicker by currents of extremely high frequency than by steady or low
frequency currents. From experience I may say that just the opposite
holds good: the button withstands the bombardment better with currents
of very high frequency. But this is due to the fact that a high
frequency discharge passes through a rarefied gas with much greater
freedom than a steady or low frequency discharge, and this will say
that with the former we can work with a lower potential or with a less
violent impact. As long, then, as the gas is of no consequence, a
steady or low frequency current is better; but as soon as the action
of the gas is desired and important, high frequencies are preferable.

In the course of these experiments a great many trials were made with
all kinds of carbon buttons. Electrodes made of ordinary carbon
buttons were decidedly more durable when the buttons were obtained by
the application of enormous pressure. Electrodes prepared by
depositing carbon in well known ways did not show up well; they
blackened the globe very quickly. From many experiences I conclude
that lamp filaments obtained in this manner can be advantageously used
only with low potentials and low frequency currents. Some kinds of
carbon withstand so well that, in order to bring them to the point of
fusion, it is necessary to employ very small buttons. In this case the
observation is rendered very difficult on account of the intense heat
produced. Nevertheless there can be no doubt that all kinds of carbon
are fused under the molecular bombardment, but the liquid state must
be one of great instability. Of all the bodies tried there were two
which withstood best--diamond and carborundum. These two showed up
about equally, but the latter was preferable, for many reasons. As it
is more than likely that this body is not yet generally known, I will
venture to call your attention to it.

It has been recently produced by Mr. E.G. Acheson, of Monongahela
City, Pa., U.S.A. It is intended to replace ordinary diamond powder
for polishing precious stones, etc., and I have been informed that it
accomplishes this object quite successfully. I do not know why the
name "carborundum" has been given to it, unless there is something in
the process of its manufacture which justifies this selection. Through
the kindness of the inventor, I obtained a short while ago some
samples which I desired to test in regard to their qualities of
phosphorescence and capability of withstanding high degrees of heat.

Carborundum can be obtained in two forms--in the form of "crystals"
and of powder. The former appear to the naked eye dark colored, but
are very brilliant; the latter is of nearly the same color as ordinary
diamond powder, but very much finer. When viewed under a microscope
the samples of crystals given to me did not appear to have any
definite form, but rather resembled pieces of broken up egg coal of
fine quality. The majority were opaque, but there were some which were
transparent and colored. The crystals are a kind of carbon containing
some impurities; they are extremely hard, and withstand for a long
time even an oxygen blast. When the blast is directed against them
they at first form a cake of some compactness, probably in consequence
of the fusion of impurities they contain. The mass withstands for a
very long time the blast without further fusion; but a slow carrying
off, or burning, occurs, and, finally, a small quantity of a
glass-like residue is left, which, I suppose, is melted alumina. When
compressed strongly they conduct very well, but not as well as
ordinary carbon. The powder, which is obtained from the crystals in
some way, is practically non-conducting. It affords a magnificent
polishing material for stones.

The time has been too short to make a satisfactory study of the
properties of this product, but enough experience has been gained in a
few weeks I have experimented upon it to say that it does possess some
remarkable properties in many respects. It withstands excessively high
degrees of heat, it is little deteriorated by molecular bombardment,
and it does not blacken the globe as ordinary carbon does. The only
difficulty which I have found in its use in connection with these
experiments was to find some binding material which would resist the
heat and the effect of the bombardment as successfully as carborundum
itself does.

I have here a number of bulbs which I have provided with buttons of
carborundum. To make such a button of carborundum crystals I proceed
in the following manner: I take an ordinary lamp filament and dip its
point in tar, or some other thick substance or paint which may be
readily carbonized. I next pass the point of the filament through the
crystals, and then hold it vertically over a hot plate. The tar
softens and forms a drop on the point of the filament, the crystals
adhering to the surface of the drop. By regulating the distance from
the plate the tar is slowly dried out and the button becomes solid. I
then once more dip the button in tar and hold it again over a plate
until the tar is evaporated, leaving only a hard mass which firmly
binds the crystals. When a larger button is required I repeat the
process several times, and I generally also cover the filament a
certain distance below the button with crystals. The button being
mounted in a bulb, when a good vacuum has been reached, first a weak
and then a strong discharge is passed through the bulb to carbonize
the tar and expel all gases, and later it is brought to a very intense
incandescence.

When the powder is used I have found it best to proceed as follows: I
make a thick paint of carborundum and tar, and pass a lamp filament
through the paint. Taking then most of the paint off by rubbing the
filament against a piece of chamois leather, I hold it over a hot
plate until the tar evaporates and the coating becomes firm. I repeat
this process as many times as it is necessary to obtain a certain
thickness of coating. On the point of the coated filament I form a
button in the same manner.

There is no doubt that such a button--properly prepared under great
pressure--of carborundum, especially of powder of the best quality,
will withstand the effect of the bombardment fully as well as anything
we know. The difficulty is that the binding material gives way, and
the carborundum is slowly thrown off after some time. As it does not
seem to blacken the globe in the least, it might be found useful for
coating the filaments of ordinary incandescent lamps, and I think that
it is even possible to produce thin threads or sticks of carborundum
which will replace the ordinary filaments in an incandescent lamp. A
carborundum coating seems to be more durable than other coatings, not
only because the carborundum can withstand high degrees of heat, but
also because it seems to unite with the carbon better than any other
material I have tried. A coating of zirconia or any other oxide, for
instance, is far more quickly destroyed. I prepared buttons of diamond
dust in the same manner as of carborundum, and these came in
durability nearest to those prepared of carborundum, but the binding
paste gave way much more quickly in the diamond buttons: this,
however, I attributed to the size and irregularity of the grains of
the diamond.

It was of interest to find whether carborundum possesses the quality
of phosphorescence. One is, of course, prepared to encounter two
difficulties: first, as regards the rough product, the "crystals,"
they are good conducting, and it is a fact that conductors do not
phosphoresce; second, the powder, being exceedingly fine, would not be
apt to exhibit very prominently this quality, since we know that when
crystals, even such as diamond or ruby, are finely powdered, they lose
the property of phosphorescence to a considerable degree.

The question presents itself here, can a conductor phosphoresce? What
is there in such a body as a metal, for instance, that would deprive
it of the quality of phosphorescence, unless it is that property which
characterizes it as a conductor? for it is a fact that most of the
phosphorescent bodies lose that quality when they are sufficiently
heated to become more or less conducting. Then, if a metal be in a
large measure, or perhaps entirely, deprived of that property, it
should be capable of phosphorescence. Therefore it is quite possible
that at some extremely high frequency, when behaving practically as a
non-conductor, a metal or any other conductor might exhibit the
quality of phosphorescence, even though it be entirely incapable of
phosphorescing under the impact of a low-frequency discharge. There
is, however, another possible way how a conductor might at least
_appear_ to phosphoresce.

Considerable doubt still exists as to what really is phosphorescence,
and as to whether the various phenomena comprised under this head are
due to the same causes. Suppose that in an exhausted bulb, under the
molecular impact, the surface of a piece of metal or other conductor
is rendered strongly luminous, but at the same time it is found that
it remains comparatively cool, would not this luminosity be called
phosphorescence? Now such a result, theoretically at least, is
possible, for it is a mere question of potential or speed. Assume the
potential of the electrode, and consequently the speed of the
projected atoms, to be sufficiently high, the surface of the metal
piece against which the atoms are projected would be rendered highly
incandescent, since the process of heat generation would be
incomparably faster than that of radiating or conducting away from the
surface of the collision. In the eye of the observer a single impact
of the atoms would cause an instantaneous flash, but if the impacts
were repeated with sufficient rapidity they would produce a
continuous impression upon his retina. To him then the surface of the
metal would appear continuously incandescent and of constant luminous
intensity, while in reality the light would be either intermittent or
at least changing periodically in intensity. The metal piece would
rise in temperature until equilibrium was attained--that is until the
energy continuously radiated would equal that intermittently supplied.
But the supplied energy might under such conditions not be sufficient
to bring the body to any more than a very moderate mean temperature,
especially if the frequency of the atomic impacts be very low--just
enough that the fluctuation of the intensity of the light emitted
could not be detected by the eye. The body would now, owing to the
manner in which the energy is supplied, emit a strong light, and yet
be at a comparatively very low mean temperature. How could the
observer call the luminosity thus produced? Even if the analysis of
the light would teach him something definite, still he would probably
rank it under the phenomena of phosphorescence. It is conceivable that
in such a way both conducting and non-conducting bodies may be
maintained at a certain luminous intensity, but the energy required
would very greatly vary with the nature and properties of the bodies.

These and some foregoing remarks of a speculative nature were made
merely to bring out curious features of alternate currents or electric
impulses. By their help we may cause a body to emit _more_ light,
while at a certain mean temperature, than it would emit if brought to
that temperature by a steady supply; and, again, we may bring a body
to the point of fusion, and cause it to emit _less_ light than when
fused by the application of energy in ordinary ways. It all depends on
how we supply the energy, and what kind of vibrations we set up: in
one case the vibrations are more, in the other less, adapted to affect
our sense of vision.

Some effects, which I had not observed before, obtained with
carborundum in the first trials, I attributed to phosphorescence, but
in subsequent experiments it appeared that it was devoid of that
quality. The crystals possess a noteworthy feature. In a bulb provided
with a single electrode in the shape of a small circular metal disc,
for instance, at a certain degree of exhaustion the electrode is
covered with a milky film, which is separated by a dark space from the
glow filling the bulb. When the metal disc is covered with carborundum
crystals, the film is far more intense, and snow-white. This I found
later to be merely an effect of the bright surface of the crystals,
for when an aluminium electrode was highly polished it exhibited more
or less the same phenomenon. I made a number of experiments with the
samples of crystals obtained, principally because it would have been
of special interest to find that they are capable of phosphorescence,
on account of their being conducting. I could not produce
phosphorescence distinctly, but I must remark that a decisive opinion
cannot be formed until other experimenters have gone over the same
ground.

The powder behaved in some experiments as though it contained alumina,
but it did not exhibit with sufficient distinctness the red of the
latter. Its dead color brightens considerably under the molecular
impact, but I am now convinced it does not phosphoresce. Still, the
tests with the powder are not conclusive, because powdered carborundum
probably does not behave like a phosphorescent sulphide, for example,
which could be finely powdered without impairing the phosphorescence,
but rather like powdered ruby or diamond, and therefore it would be
necessary, in order to make a decisive test, to obtain it in a large
lump and polish up the surface.

If the carborundum proves useful in connection with these and similar
experiments, its chief value will be found in the production of
coatings, thin conductors, buttons, or other electrodes capable of
withstanding extremely high degrees of heat.

The production of a small electrode capable of withstanding enormous
temperatures I regard as of the greatest importance in the manufacture
of light. It would enable us to obtain, by means of currents of very
high frequencies, certainly 20 times, if not more, the quantity of
light which is obtained in the present incandescent lamp by the same
expenditure of energy. This estimate may appear to many exaggerated,
but in reality I think it is far from being so. As this statement
might be misunderstood I think it necessary to expose clearly the
problem with which in this line of work we are confronted, and the
manner in which, in my opinion, a solution will be arrived at.

Any one who begins a study of the problem will be apt to think that
what is wanted in a lamp with an electrode is a very high degree of
incandescence of the electrode. There he will be mistaken. The high
incandescence of the button is a necessary evil, but what is really
wanted is the high incandescence of the gas surrounding the button. In
other words, the problem in such a lamp is to bring a mass of gas to
the highest possible incandescence. The higher the incandescence, the
quicker the mean vibration, the greater is the economy of the light
production. But to maintain a mass of gas at a high degree of
incandescence in a glass vessel, it will always be necessary to keep
the incandescent mass away from the glass; that is, to confine it as
much as possible to the central portion of the globe.

In one of the experiments this evening a brush was produced at the end
of a wire. This brush was a flame, a source of heat and light. It did
not emit much perceptible heat, nor did it glow with an intense light;
but is it the less a flame because it does not scorch my hand? Is it
the less a flame because it does not hurt my eye by its brilliancy?
The problem is precisely to produce in the bulb such a flame, much
smaller in size, but incomparably more powerful. Were there means at
hand for producing electric impulses of a sufficiently high frequency,
and for transmitting them, the bulb could be done away with, unless it
were used to protect the electrode, or to economize the energy by
confining the heat. But as such means are not at disposal, it becomes
necessary to place the terminal in a bulb and rarefy the air in the
same. This is done merely to enable the apparatus to perform the work
which it is not capable of performing at ordinary air pressure. In the
bulb we are able to intensify the action to any degree--so far that
the brush emits a powerful light.

The intensity of the light emitted depends principally on the
frequency and potential of the impulses, and on the electric density
of the surface of the electrode. It is of the greatest importance to
employ the smallest possible button, in order to push the density very
far. Under the violent impact of the molecules of the gas surrounding
it, the small electrode is of course brought to an extremely high
temperature, but around it is a mass of highly incandescent gas, a
flame photosphere, many hundred times the volume of the electrode.
With a diamond, carborundum or zirconia button the photosphere can be
as much as one thousand times the volume of the button. Without much
reflecting one would think that in pushing so far the incandescence of
the electrode it would be instantly volatilized. But after a careful
consideration he would find that, theoretically, it should not occur,
and in this fact--which, however, is experimentally demonstrated--lies
principally the future value of such a lamp.

At first, when the bombardment begins, most of the work is performed
on the surface of the button, but when a highly conducting photosphere
is formed the button is comparatively relieved. The higher the
incandescence of the photosphere the more it approaches in
conductivity to that of the electrode, and the more, therefore, the
solid and the gas form one conducting body. The consequence is that
the further is forced the incandescence the more work, comparatively,
is performed on the gas, and the less on the electrode. The formation
of a powerful photosphere is consequently the very means for
protecting the electrode. This protection, of course, is a relative
one, and it should not be thought that by pushing the incandescence
higher the electrode is actually less deteriorated. Still,
theoretically, with extreme frequencies, this result must be reached,
but probably at a temperature too high for most of the refractory
bodies known. Given, then, an electrode which can withstand to a very
high limit the effect of the bombardment and outward strain, it would
be safe no matter how much it is forced beyond that limit. In an
incandescent lamp quite different considerations apply. There the gas
is not at all concerned: the whole of the work is performed on the
filament; and the life of the lamp diminishes so rapidly with the
increase of the degree of incandescence that economical reasons compel
us to work it at a low incandescence. But if an incandescent lamp is
operated with currents of very high frequency, the action of the gas
cannot be neglected, and the rules for the most economical working
must be considerably modified.

In order to bring such a lamp with one or two electrodes to a great
perfection, it is necessary to employ impulses of very high frequency.
The high frequency secures, among others, two chief advantages, which
have a most important bearing upon the economy of the light
production. First, the deterioration of the electrode is reduced by
reason of the fact that we employ a great many small impacts, instead
of a few violent ones, which shatter quickly the structure; secondly,
the formation of a large photosphere is facilitated.

In order to reduce the deterioration of the electrode to the minimum,
it is desirable that the vibration be harmonic, for any suddenness
hastens the process of destruction. An electrode lasts much longer
when kept at incandescence by currents, or impulses, obtained from a
high-frequency alternator, which rise and fall more or less
harmonically, than by impulses obtained from a disruptive discharge
coil. In the latter case there is no doubt that most of the damage is
done by the fundamental sudden discharges.

One of the elements of loss in such a lamp is the bombardment of the
globe. As the potential is very high, the molecules are projected with
great speed; they strike the glass, and usually excite a strong
phosphorescence. The effect produced is very pretty, but for
economical reasons it would be perhaps preferable to prevent, or at
least reduce to the minimum, the bombardment against the globe, as in
such case it is, as a rule, not the object to excite phosphorescence,
and as some loss of energy results from the bombardment. This loss in
the bulb is principally dependent on the potential of the impulses and
on the electric density on the surface of the electrode. In employing
very high frequencies the loss of energy by the bombardment is greatly
reduced, for, first, the potential needed to perform a given amount of
work is much smaller; and, secondly, by producing a highly conducting
photosphere around the electrode, the same result is obtained as
though the electrode were much larger, which is equivalent to a
smaller electric density. But be it by the diminution of the maximum
potential or of the density, the gain is effected in the same manner,
namely, by avoiding violent shocks, which strain the glass much beyond
its limit of elasticity. If the frequency could be brought high
enough, the loss due to the imperfect elasticity of the glass would be
entirely negligible. The loss due to bombardment of the globe may,
however, be reduced by using two electrodes instead of one. In such
case each of the electrodes may be connected to one of the terminals;
or else, if it is preferable to use only one wire, one electrode may
be connected to one terminal and the other to the ground or to an
insulated body of some surface, as, for instance, a shade on the lamp.
In the latter case, unless some judgment is used, one of the
electrodes might glow more intensely than the other.

But on the whole I find it preferable when using such high frequencies
to employ only one electrode and one connecting wire. I am convinced
that the illuminating device of the near future will not require for
its operation more than one lead, and, at any rate, it will have no
leading-in wire, since the energy required can be as well transmitted
through the glass. In experimental bulbs the leading-in wire is most
generally used on account of convenience, as in employing condenser
coatings in the manner indicated in Fig. 22, for example, there is
some difficulty in fitting the parts, but these difficulties would not
exist if a great many bulbs were manufactured; otherwise the energy
can be conveyed through the glass as well as through a wire, and with
these high frequencies the losses are very small. Such illuminating
devices will necessarily involve the use of very high potentials, and
this, in the eyes of practical men, might be an objectionable feature.
Yet, in reality, high potentials are not objectionable--certainly not
in the least as far as the safety of the devices is concerned.

There are two ways of rendering an electric appliance safe. One is to
use low potentials, the other is to determine the dimensions of the
apparatus so that it is safe no matter how high a potential is used.
Of the two the latter seems to me the better way, for then the safety
is absolute, unaffected by any possible combination of circumstances
which might render even a low-potential appliance dangerous to life
and property. But the practical conditions require not only the
judicious determination of the dimensions of the apparatus; they
likewise necessitate the employment of energy of the proper kind. It
is easy, for instance, to construct a transformer capable of giving,
when operated from an ordinary alternate current machine of low
tension, say 50,000 volts, which might be required to light a highly
exhausted phosphorescent tube, so that, in spite of the high
potential, it is perfectly safe, the shock from it producing no
inconvenience. Still, such a transformer would be expensive, and in
itself inefficient; and, besides, what energy was obtained from it
would not be economically used for the production of light. The
economy demands the employment of energy in the form of extremely
rapid vibrations. The problem of producing light has been likened to
that of maintaining a certain high-pitch note by means of a bell. It
should be said a _barely audible_ note; and even these words would not
express it, so wonderful is the sensitiveness of the eye. We may
deliver powerful blows at long intervals, waste a good deal of energy,
and still not get what we want; or we may keep up the note by
delivering frequent gentle taps, and get nearer to the object sought
by the expenditure of much less energy. In the production of light, as
far as the illuminating device is concerned, there can be only one
rule--that is, to use as high frequencies as can be obtained; but the
means for the production and conveyance of impulses of such character
impose, at present at least, great limitations. Once it is decided to
use very high frequencies, the return wire becomes unnecessary, and
all the appliances are simplified. By the use of obvious means the
same result is obtained as though the return wire were used. It is
sufficient for this purpose to bring in contact with the bulb, or
merely in the vicinity of the same, an insulated body of some surface.
The surface need, of course, be the smaller, the higher the frequency
and potential used, and necessarily, also, the higher the economy of
the lamp or other device.

This plan of working has been resorted to on several occasions this
evening. So, for instance, when the incandescence of a button was
produced by grasping the bulb with the hand, the body of the
experimenter merely served to intensify the action. The bulb used was
similar to that illustrated in Fig. 19, and the coil was excited to a
small potential, not sufficient to bring the button to incandescence
when the bulb was hanging from the wire; and incidentally, in order to
perform the experiment in a more suitable manner, the button was taken
so large that a perceptible time had to elapse before, upon grasping
the bulb, it could be rendered incandescent. The contact with the bulb
was, of course, quite unnecessary. It is easy, by using a rather large
bulb with an exceedingly small electrode, to adjust the conditions so
that the latter is brought to bright incandescence by the mere
approach of the experimenter within a few feet of the bulb, and that
the incandescence subsides upon his receding.

[Illustration: FIG. 24.--BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT
OF PROJECTED MATTER.]

In another experiment, when phosphorescence was excited, a similar
bulb was used. Here again, originally, the potential was not
sufficient to excite phosphorescence until the action was
intensified--in this case, however, to present a different feature, by
touching the socket with a metallic object held in the hand. The
electrode in the bulb was a carbon button so large that it could not
be brought to incandescence, and thereby spoil the effect produced by
phosphorescence.

[Illustration: FIG. 25.--IMPROVED EXPERIMENTAL BULB.]

Again, in another of the early experiments, a bulb was used as
illustrated in Fig. 12. In this instance, by touching the bulb with
one or two fingers, one or two shadows of the stem inside were
projected against the glass, the touch of the finger producing the
same result as the application of an external negative electrode under
ordinary circumstances.

In all these experiments the action was intensified by augmenting the
capacity at the end of the lead connected to the terminal. As a rule,
it is not necessary to resort to such means, and would be quite
unnecessary with still higher frequencies; but when it _is_ desired,
the bulb, or tube, can be easily adapted to the purpose.

[Illustration: FIG. 26.--IMPROVED BULB WITH INTENSIFYING REFLECTOR.]

In Fig. 24, for example, an experimental bulb L is shown, which is
provided with a neck n on the top for the application of an external
tinfoil coating, which may be connected to a body of larger surface.
Such a lamp as illustrated in Fig. 25 may also be lighted by
connecting the tinfoil coating on the neck n to the terminal, and the
leading-in wire w to an insulated plate. If the bulb stands in a
socket upright, as shown in the cut, a shade of conducting material
may be slipped in the neck n, and the action thus magnified.

A more perfected arrangement used in some of these bulbs is
illustrated in Fig. 26. In this case the construction of the bulb is
as shown and described before, when reference was made to Fig. 19. A
zinc sheet Z, with a tubular extension T, is slipped over the metallic
socket S. The bulb hangs downward from the terminal t, the zinc sheet
Z, performing the double office of intensifier and reflector. The
reflector is separated from the terminal t by an extension of the
insulating plug P.

[Illustration: FIG. 27.--PHOSPHORESCENT TUBE WITH INTENSIFYING
REFLECTOR.]

A similar disposition with a phosphorescent tube is illustrated in
Fig. 27. The tube T is prepared from two short tubes of a different
diameter, which are sealed on the ends. On the lower end is placed an
outside conducting coating C, which connects to the wire w. The wire
has a hook on the upper end for suspension, and passes through the
centre of the inside tube, which is filled with some good and tightly
packed insulator. On the outside of the upper end of the tube T is
another conducting coating C_1 upon which is slipped a metallic
reflector Z, which should be separated by a thick insulation from the
end of wire w.

The economical use of such a reflector or intensifier would require
that all energy supplied to an air condenser should be recoverable,
or, in other words, that there should not be any losses, neither in
the gaseous medium nor through its action elsewhere. This is far from
being so, but, fortunately, the losses may be reduced to anything
desired. A few remarks are necessary on this subject, in order to make
the experiences gathered in the course of these investigations
perfectly clear.

Suppose a small helix with many well insulated turns, as in experiment
Fig. 17, has one of its ends connected to one of the terminals of the
induction coil, and the other to a metal plate, or, for the sake of
simplicity, a sphere, insulated in space. When the coil is set to
work, the potential of the sphere is alternated, and the small helix
now behaves as though its free end were connected to the other
terminal of the induction coil. If an iron rod be held within the
small helix it is quickly brought to a high temperature, indicating
the passage of a strong current through the helix. How does the
insulated sphere act in this case? It can be a condenser, storing and
returning the energy supplied to it, or it can be a mere sink of
energy, and the conditions of the experiment determine whether it is
more one or the other. The sphere being charged to a high potential,
it acts inductively upon the surrounding air, or whatever gaseous
medium there might be. The molecules, or atoms, which are near the
sphere are of course more attracted, and move through a greater
distance than the farther ones. When the nearest molecules strike the
sphere they are repelled, and collisions occur at all distances within
the inductive action of the sphere. It is now clear that, if the
potential be steady, but little loss of energy can be caused in this
way, for the molecules which are nearest to the sphere, having had an
additional charge imparted to them by contact, are not attracted until
they have parted, if not with all, at least with most of the
additional charge, which can be accomplished only after a great many
collisions. From the fact that with a steady potential there is but
little loss in dry air, one must come to such a conclusion. When the
potential of the sphere, instead of being steady, is alternating, the
conditions are entirely different. In this case a rhythmical
bombardment occurs, no matter whether the molecules after coming in
contact with the sphere lose the imparted charge or not; what is more,
if the charge is not lost, the impacts are only the more violent.
Still if the frequency of the impulses be very small, the loss caused
by the impacts and collisions would not be serious unless the
potential were excessive. But when extremely high frequencies and more
or less high potentials are used, the loss may be very great. The
total energy lost per unit of time is proportionate to the product of
the number of impacts per second, or the frequency and the energy lost
in each impact. But the energy of an impact must be proportionate to
the square of the electric density of the sphere, since the charge
imparted to the molecule is proportionate to that density. I conclude
from this that the total energy lost must be proportionate to the
product of the frequency and the square of the electric density; but
this law needs experimental confirmation. Assuming the preceding
considerations to be true, then, by rapidly alternating the potential
of a body immersed in an insulating gaseous medium, any amount of
energy may be dissipated into space. Most of that energy then, I
believe, is not dissipated in the form of long ether waves, propagated
to considerable distance, as is thought most generally, but is
consumed--in the case of an insulated sphere, for example--in impact
and collisional losses--that is, heat vibrations--on the surface and
in the vicinity of the sphere. To reduce the dissipation it is
necessary to work with a small electric density--the smaller the
higher the frequency.

But since, on the assumption before made, the loss is diminished with
the square of the density, and since currents of very high frequencies
involve considerable waste when transmitted through conductors, it
follows that, on the whole, it is better to employ one wire than two.
Therefore, if motors, lamps, or devices of any kind are perfected,
capable of being advantageously operated by currents of extremely high
frequency, economical reasons will make it advisable to use only one
wire, especially if the distances are great.

When energy is absorbed in a condenser the same behaves as though its
capacity were increased. Absorption always exists more or less, but
generally it is small and of no consequence as long as the frequencies
are not very great. In using extremely high frequencies, and,
necessarily in such case, also high potentials, the absorption--or,
what is here meant more particularly by this term, the loss of energy
due to the presence of a gaseous medium--is an important factor to be
considered, as the energy absorbed in the air condenser may be any
fraction of the supplied energy. This would seem to make it very
difficult to tell from the measured or computed capacity of an air
condenser its actual capacity or vibration period, especially if the
condenser is of very small surface and is charged to a very high
potential. As many important results are dependent upon the
correctness of the estimation of the vibration period, this subject
demands the most careful scrutiny of other investigators. To reduce
the probable error as much as possible in experiments of the kind
alluded to, it is advisable to use spheres or plates of large surface,
so as to make the density exceedingly small. Otherwise, when it is
practicable, an oil condenser should be used in preference. In oil or
other liquid dielectrics there are seemingly no such losses as in
gaseous media. It being impossible to exclude entirely the gas in
condensers with solid dielectrics, such condensers should be immersed
in oil, for economical reasons if nothing else; they can then be
strained to the utmost and will remain cool. In Leyden jars the loss
due to air is comparatively small, as the tinfoil coatings are large,
close together, and the charged surfaces not directly exposed; but
when the potentials are very high, the loss may be more or less
considerable at, or near, the upper edge of the foil, where the air is
principally acted upon. If the jar be immersed in boiled-out oil, it
will be capable of performing four times the amount of work which it
can for any length of time when used in the ordinary way, and the loss
will be inappreciable.

It should not be thought that the loss in heat in an air condenser is
necessarily associated with the formation of _visible_ streams or
brushes. If a small electrode, inclosed in an unexhausted bulb, is
connected to one of the terminals of the coil, streams can be seen to
issue from the electrode and the air in the bulb is heated; if,
instead of a small electrode, a large sphere is inclosed in the bulb,
no streams are observed, still the air is heated.

Nor should it be thought that the temperature of an air condenser
would give even an approximate idea of the loss in heat incurred, as
in such case heat must be given off much more quickly, since there is,
in addition to the ordinary radiation, a very active carrying away of
heat by independent carriers going on, and since not only the
apparatus, but the air at some distance from it is heated in
consequence of the collisions which must occur.

Owing to this, in experiments with such a coil, a rise of temperature
can be distinctly observed only when the body connected to the coil is
very small. But with apparatus on a larger scale, even a body of
considerable bulk would be heated, as, for instance, the body of a
person; and I think that skilled physicians might make observations of
utility in such experiments, which, if the apparatus were judiciously
designed, would not present the slightest danger.

A question of some interest, principally to meteorologists, presents
itself here. How does the earth behave? The earth is an air condenser,
but is it a perfect or a very imperfect one--a mere sink of energy?
There can be little doubt that to such small disturbance as might be
caused in an experiment the earth behaves as an almost perfect
condenser. But it might be different when its charge is set in
vibration by some sudden disturbance occurring in the heavens. In such
case, as before stated, probably only little of the energy of the
vibrations set up would be lost into space in the form of long ether
radiations, but most of the energy, I think, would spend itself in
molecular impacts and collisions, and pass off into space in the form
of short heat, and possibly light, waves. As both the frequency of the
vibrations of the charge and the potential are in all probability
excessive, the energy converted into heat may be considerable. Since
the density must be unevenly distributed, either in consequence of the
irregularity of the earth's surface, or on account of the condition of
the atmosphere in various places, the effect produced would
accordingly vary from place to place. Considerable variations in the
temperature and pressure of the atmosphere may in this manner be
caused at any point of the surface of the earth. The variations may be
gradual or very sudden, according to the nature of the general
disturbance, and may produce rain and storms, or locally modify the
weather in any way.

From the remarks before made one may see what an important factor of
loss the air in the neighborhood of a charged surface becomes when the
electric density is great and the frequency of the impulses excessive.
But the action as explained implies that the air is insulating--that
is, that it is composed of independent carriers immersed in an
insulating medium. This is the case only when the air is at something
like ordinary or greater, or at extremely small, pressure. When the
air is slightly rarefied and conducting, then true conduction losses
occur also. In such case, of course, considerable energy may be
dissipated into space even with a steady potential, or with impulses
of low frequency, if the density is very great.

When the gas is at very low pressure, an electrode is heated more
because higher speeds can be reached. If the gas around the electrode
is strongly compressed, the displacements, and consequently the
speeds, are very small, and the heating is insignificant. But if in
such case the frequency could be sufficiently increased, the electrode
would be brought to a high temperature as well as if the gas were at
very low pressure; in fact, exhausting the bulb is only necessary
because we cannot produce (and possibly not convey) currents of the
required frequency.

Returning to the subject of electrode lamps, it is obviously of
advantage in such a lamp to confine as much as possible the heat to
the electrode by preventing the circulation of the gas in the bulb. If
a very small bulb be taken, it would confine the heat better than a
large one, but it might not be of sufficient capacity to be operated
from the coil, or, if so, the glass might get too hot. A simple way to
improve in this direction is to employ a globe of the required size,
but to place a small bulb, the diameter of which is properly
estimated, over the refractory button contained in the globe. This
arrangement is illustrated in Fig. 28.

[Illustration: FIG. 28.--LAMP WITH AUXILIARY BULB FOR CONFINING THE
ACTION TO THE CENTRE.]

The globe L has in this case a large neck n, allowing the small bulb b
to slip through. Otherwise the construction is the same as shown in
Fig. 18, for example. The small bulb is conveniently supported upon
the stem s, carrying the refractory button m. It is separated from the
aluminium tube a by several layers of mica M, in order to prevent the
cracking of the neck by the rapid heating of the aluminium tube upon a
sudden turning on of the current. The inside bulb should be as small
as possible when it is desired to obtain light only by incandescence
of the electrode. If it is desired to produce phosphorescence, the
bulb should be larger, else it would be apt to get too hot, and the
phosphorescence would cease. In this arrangement usually only the
small bulb shows phosphorescence, as there is practically no
bombardment against the outer globe. In some of these bulbs
constructed as illustrated in Fig. 28 the small tube was coated with
phosphorescent paint, and beautiful effects were obtained. Instead of
making the inside bulb large, in order to avoid undue heating, it
answers the purpose to make the electrode m larger. In this case the
bombardment is weakened by reason of the smaller electric density.

Many bulbs were constructed on the plan illustrated in Fig. 29. Here a
small bulb b, containing the refractory button m, upon being exhausted
to a very high degree was sealed in a large globe L, which was then
moderately exhausted and sealed off. The principal advantage of this
construction was that it allowed of reaching extremely high vacua,
and, at the same time use a large bulb. It was found, in the course of
experiences with bulbs such as illustrated in Fig. 29, that it was
well to make the stem s near the seal at e very thick, and the
leading-in wire w thin, as it occurred sometimes that the stem at e
was heated and the bulb was cracked. Often the outer globe L was
exhausted only just enough to allow the discharge to pass through, and
the space between the bulbs appeared crimson, producing a curious
effect. In some cases, when the exhaustion in globe L was very low,
and the air good conducting, it was found necessary, in order to bring
the button m to high incandescence, to place, preferably on the upper
part of the neck of the globe, a tinfoil coating which was connected
to an insulated body, to the ground, or to the other terminal of the
coil, as the highly conducting air weakened the effect somewhat,
probably by being acted upon inductively from the wire w, where it
entered the bulb at e. Another difficulty--which, however, is always
present when the refractory button is mounted in a very small
bulb--existed in the construction illustrated in Fig. 29, namely, the
vacuum in the bulb b would be impaired in a comparatively short time.

[Illustration: FIG. 29.--LAMP WITH INDEPENDENT AUXILIARY BULB.]

The chief idea in the two last described constructions was to confine
the heat to the central portion of the globe by preventing the
exchange of air. An advantage is secured, but owing to the heating of
the inside bulb and slow evaporation of the glass the vacuum is hard
to maintain, even if the construction illustrated in Fig. 28 be
chosen, in which both bulbs communicate.

But by far the better way--the ideal way--would be to reach
sufficiently high frequencies. The higher the frequency the slower
would be the exchange of the air, and I think that a frequency may be
reached at which there would be no exchange whatever of the air
molecules around the terminal. We would then produce a flame in which
there would be no carrying away of material, and a queer flame it
would be, for it would be rigid! With such high frequencies the
inertia of the particles would come into play. As the brush, or flame,
would gain rigidity in virtue of the inertia of the particles, the
exchange of the latter would be prevented. This would necessarily
occur, for, the number of the impulses being augmented, the potential
energy of each would diminish, so that finally only atomic vibrations
could be set up, and the motion of translation through measurable
space would cease. Thus an ordinary gas burner connected to a source
of rapidly alternating potential might have its efficiency augmented
to a certain limit, and this for two reasons--because of the
additional vibration imparted, and because of a slowing down of the
process of carrying off. But the renewal being rendered difficult, and
renewal being necessary to maintain the _burner_, a continued increase
of the frequency of the impulses, assuming they could be transmitted
to and impressed upon the flame, would result in the "extinction" of
the latter, meaning by this term only the cessation of the chemical
process.

I think, however, that in the case of an electrode immersed in a fluid
insulating medium, and surrounded by independent carriers of electric
charges, which can be acted upon inductively, a sufficiently high
frequency of the impulses would probably result in a gravitation of
the gas all around toward the electrode. For this it would be only
necessary to assume that the independent bodies are irregularly
shaped; they would then turn toward the electrode their side of the
greatest electric density, and this would be a position in which the
fluid resistance to approach would be smaller than that offered to the
receding.

The general opinion, I do not doubt, is that it is out of the question
to reach any such frequencies as might--assuming some of the views
before expressed to be true--produce any of the results which I have
pointed out as mere possibilities. This may be so, but in the course
of these investigations, from the observation of many phenomena I have
gained the conviction that these frequencies would be much lower than
one is apt to estimate at first. In a flame we set up light vibrations
by causing molecules, or atoms, to collide. But what is the ratio of
the frequency of the collisions and that of the vibrations set up?
Certainly it must be incomparably smaller than that of the knocks of
the bell and the sound vibrations, or that of the discharges and the
oscillations of the condenser. We may cause the molecules of the gas
to collide by the use of alternate electric impulses of high
frequency, and so we may imitate the process in a flame; and from
experiments with frequencies which we are now able to obtain, I think
that the result is producible with impulses which are transmissible
through a conductor.

In connection with thoughts of a similar nature, it appeared to me of
great interest to demonstrate the rigidity of a vibrating gaseous
column. Although with such low frequencies as, say 10,000 per second,
which I was able to obtain without difficulty from a specially
constructed alternator, the task looked discouraging at first, I made
a series of experiments. The trials with air at ordinary pressure led
to no result, but with air moderately rarefied I obtain what I think
to be an unmistakable experimental evidence of the property sought
for. As a result of this kind might lead able investigators to
conclusions of importance I will describe one of the experiments
performed.

It is well known that when a tube is slightly exhausted the discharge
may be passed through it in the form of a thin luminous thread. When
produced with currents of low frequency, obtained from a coil operated
as usual, this thread is inert. If a magnet be approached to it, the
part near the same is attracted or repelled, according to the
direction of the lines of force of the magnet. It occurred to me that
if such a thread would be produced with currents of very high
frequency, it should be more or less rigid, and as it was visible it
could be easily studied. Accordingly I prepared a tube about 1 inch in
diameter and 1 metre long, with outside coating at each end. The tube
was exhausted to a point at which by a little working the thread
discharge could be obtained. It must be remarked here that the general
aspect of the tube, and the degree of exhaustion, are quite different
than when ordinary low frequency currents are used. As it was found
preferable to work with one terminal, the tube prepared was suspended
from the end of a wire connected to the terminal, the tinfoil coating
being connected to the wire, and to the lower coating sometimes a
small insulated plate was attached. When the thread was formed it
extended through the upper part of the tube and lost itself in the
lower end. If it possessed rigidity it resembled, not exactly an
elastic cord stretched tight between two supports, but a cord
suspended from a height with a small weight attached at the end. When
the finger or a magnet was approached to the upper end of the luminous
thread, it could be brought locally out of position by electrostatic
or magnetic action; and when the disturbing object was very quickly
removed, an analogous result was produced, as though a suspended cord
would be displaced and quickly released near the point of suspension.
In doing this the luminous thread was set in vibration, and two very
sharply marked nodes, and a third indistinct one, were formed. The
vibration, once set up, continued for fully eight minutes, dying
gradually out. The speed of the vibration often varied perceptibly,
and it could be observed that the electrostatic attraction of the
glass affected the vibrating thread; but it was clear that the
electrostatic action was not the cause of the vibration, for the
thread was most generally stationary, and could always be set in
vibration by passing the finger quickly near the upper part of the
tube. With a magnet the thread could be split in two and both parts
vibrated. By approaching the hand to the lower coating of the tube, or
insulated plate if attached, the vibration was quickened; also, as far
as I could see, by raising the potential or frequency. Thus, either
increasing the frequency or passing a stronger discharge of the same
frequency corresponded to a tightening of the cord. I did not obtain
any experimental evidence with condenser discharges. A luminous band
excited in a bulb by repeated discharges of a Leyden jar must possess
rigidity, and if deformed and suddenly released should vibrate. But
probably the amount of vibrating matter is so small that in spite of
the extreme speed the inertia cannot prominently assert itself.
Besides, the observation in such a case is rendered extremely
difficult on account of the fundamental vibration.

The demonstration of the fact--which still needs better experimental
confirmation--that a vibrating gaseous column possesses rigidity,
might greatly modify the views of thinkers. When with low frequencies
and insignificant potentials indications of that property may be
noted, how must a gaseous medium behave under the influence of
enormous electrostatic stresses which may be active in the
interstellar space, and which may alternate with inconceivable
rapidity? The existence of such an electrostatic, rhythmically
throbbing force--of a vibrating electrostatic field--would show a
possible way how solids might have formed from the ultra-gaseous
uterus, and how transverse and all kinds of vibrations may be
transmitted through a gaseous medium filling all space. Then, ether
might be a true fluid, devoid of rigidity, and at rest, it being
merely necessary as a connecting link to enable interaction. What
determines the rigidity of a body? It must be the speed and the amount
of moving matter. In a gas the speed may be considerable, but the
density is exceedingly small; in a liquid the speed would be likely to
be small, though the density may be considerable; and in both cases
the inertia resistance offered to displacement is practically _nil_.
But place a gaseous (or liquid) column in an intense, rapidly
alternating electrostatic field, set the particles vibrating with
enormous speeds, then the inertia resistance asserts itself. A body
might move with more or less freedom through the vibrating mass, but
as a whole it would be rigid.

There is a subject which I must mention in connection with these
experiments: it is that of high vacua. This is a subject the study of
which is not only interesting, but useful, for it may lead to results
of great practical importance. In commercial apparatus, such as
incandescent lamps, operated from ordinary systems of distribution, a
much higher vacuum than obtained at present would not secure a very
great advantage. In such a case the work is performed on the filament
and the gas is little concerned; the improvement, therefore, would be
but trifling. But when we begin to use very high frequencies and
potentials, the action of the gas becomes all important, and the
degree of exhaustion materially modifies the results. As long as
ordinary coils, even very large ones, were used, the study of the
subject was limited, because just at a point when it became most
interesting it had to be interrupted on account of the "non-striking"
vacuum being reached. But presently we are able to obtain from a small
disruptive discharge coil potentials much higher than even the largest
coil was capable of giving, and, what is more, we can make the
potential alternate with great rapidity. Both of these results enable
us now to pass a luminous discharge through almost any vacua
obtainable, and the field of our investigations is greatly extended.
Think we as we may, of all the possible directions to develop a
practical illuminant, the line of high vacua seems to be the most
promising at present. But to reach extreme vacua the appliances must
be much more improved, and ultimate perfection will not be attained
until we shall have discarded the mechanical and perfected an
_electrical_ vacuum pump. Molecules and atoms can be thrown out of a
bulb under the action of an enormous potential: _this_ will be the
principle of the vacuum pump of the future. For the present, we must
secure the best results we can with mechanical appliances. In this
respect, it might not be out of the way to say a few words about the
method of, and apparatus for, producing excessively high degrees of
exhaustion of which I have availed myself in the course of these
investigations. It is very probable that other experimenters have used
similar arrangements; but as it is possible that there may be an item
of interest in their description, a few remarks, which will render
this investigation more complete, might be permitted.

[Illustration: FIG. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF
EXHAUSTION.]

The apparatus is illustrated in a drawing shown in Fig. 30. S
represents a Sprengel pump, which has been specially constructed to
better suit the work required. The stop-cock which is usually employed
has been omitted, and instead of it a hollow stopper s has been fitted
in the neck of the reservoir R. This stopper has a small hole h,
through which the mercury descends; the size of the outlet o being
properly determined with respect to the section of the fall tube t,
which is sealed to the reservoir instead of being connected to it in
the usual manner. This arrangement overcomes the imperfections and
troubles which often arise from the use of the stopcock on the
reservoir and the connection of the latter with the fall tube.

The pump is connected through a U-shaped tube t to a very large
reservoir R_1. Especial care was taken in fitting the grinding
surfaces of the stoppers p and p_1, and both of these and the mercury
caps above them were made exceptionally long. After the U-shaped tube
was fitted and put in place, it was heated, so as to soften and take
off the strain resulting from imperfect fitting. The U-shaped tube was
provided with a stopcock C, and two ground connections g and g_1--one
for a small bulb b, usually containing caustic potash, and the other
for the receiver r, to be exhausted.

The reservoir R_1 was connected by means of a rubber tube to a
slightly larger reservoir R_2, each of the two reservoirs being
provided with a stopcock C_1 and C_2, respectively. The reservoir R_2
could be raised and lowered by a wheel and rack, and the range of its
motion was so determined that when it was filled with mercury and the
stopcock C_2 closed, so as to form a Torricellian vacuum in it when
raised, it could be lifted so high that the mercury in reservoir R_1
would stand a little above stopcock C_1; and when this stopcock was
closed and the reservoir R_2 descended, so as to form a Torricellian
vacuum in reservoir R_1, it could be lowered so far as to completely
empty the latter, the mercury filling the reservoir R_2 up to a little
above stopcock C_2.

The capacity of the pump and of the connections was taken as small as
possible relatively to the volume of reservoir R_1, since, of course,
the degree of exhaustion depended upon the ratio of these quantities.

With this apparatus I combined the usual means indicated by former
experiments for the production of very high vacua. In most of the
experiments it was convenient to use caustic potash. I may venture to
say, in regard to its use, that much time is saved and a more perfect
action of the pump insured by fusing and boiling the potash as soon
as, or even before, the pump settles down. If this course is not
followed the sticks, as ordinarily employed, may give moisture off at
a certain very slow rate, and the pump may work for many hours without
reaching a very high vacuum. The potash was heated either by a spirit
lamp or by passing a discharge through it, or by passing a current
through a wire contained in it. The advantage in the latter case was
that the heating could be more rapidly repeated.

Generally the process of exhaustion was the following:--At the start,
the stop-cocks C and C_1 being open, and all other connections closed,
the reservoir R_2 was raised so far that the mercury filled the
reservoir R_1 and a part of the narrow connecting U-shaped tube. When
the pump was set to work, the mercury would, of course, quickly rise
in the tube, and reservoir R_2 was lowered, the experimenter keeping
the mercury at about the same level. The reservoir R_2 was balanced
by a long spring which facilitated the operation, and the friction of
the parts was generally sufficient to keep it almost in any position.
When the Sprengel pump had done its work, the reservoir R_2 was
further lowered and the mercury descended in R_1 and filled R_2,
whereupon stopcock C_2 was closed. The air adhering to the walls of
R_1 and that absorbed by the mercury was carried off, and to free the
mercury of all air the reservoir R_2 was for a long time worked up and
down. During this process some air, which would gather below stopcock
C_2, was expelled from R_2 by lowering it far enough and opening the
stopcock, closing the latter again before raising the reservoir. When
all the air had been expelled from the mercury, and no air would
gather in R_2 when it was lowered, the caustic potash was resorted to.
The reservoir R_2 was now again raised until the mercury in R_1 stood
above stopcock C_1. The caustic potash was fused and boiled, and the
moisture partly carried off by the pump and partly re-absorbed; and
this process of heating and cooling was repeated many times, and each
time, upon the moisture being absorbed or carried off, the reservoir
R_2 was for a long time raised and lowered. In this manner all the
moisture was carried off from the mercury, and both the reservoirs
were in proper condition to be used. The reservoir R_2 was then again
raised to the top, and the pump was kept working for a long time. When
the highest vacuum obtainable with the pump had been reached the
potash bulb was usually wrapped with cotton which was sprinkled with
ether so as to keep the potash at a very low temperature, then the
reservoir R_2 was lowered, and upon reservoir R_1 being emptied the
receiver r was quickly sealed up.

When a new bulb was put on, the mercury was always raised above
stopcock C_1 which was closed, so as to always keep the mercury and
both the reservoirs in fine condition, and the mercury was never
withdrawn from R_1 except when the pump had reached the highest degree
of exhaustion. It is necessary to observe this rule if it is desired
to use the apparatus to advantage.

By means of this arrangement I was able to proceed very quickly, and
when the apparatus was in perfect order it was possible to reach the
phosphorescent stage in a small bulb in less than 15 minutes, which is
certainly very quick work for a small laboratory arrangement requiring
all in all about 100 pounds of mercury. With ordinary small bulbs the
ratio of the capacity of the pump, receiver, and connections, and that
of reservoir R was about 1-20, and the degrees of exhaustion reached
were necessarily very high, though I am unable to make a precise and
reliable statement how far the exhaustion was carried.

What impresses the investigator most in the course of these
experiences is the behavior of gases when subjected to great rapidly
alternating electrostatic stresses. But he must remain in doubt as to
whether the effects observed are due wholly to the molecules, or
atoms, of the gas which chemical analysis discloses to us, or whether
there enters into play another medium of a gaseous nature, comprising
atoms, or molecules, immersed in a fluid pervading the space. Such a
medium surely must exist, and I am convinced that, for instance, even
if air were absent, the surface and neighborhood of a body in space
would be heated by rapidly alternating the potential of the body; but
no such heating of the surface or neighborhood could occur if all free
atoms were removed and only a homogeneous, incompressible, and elastic
fluid--such as ether is supposed to be--would remain, for then there
would be no impacts, no collisions. In such a case, as far as the body
itself is concerned, only frictional losses in the inside could occur.

It is a striking fact that the discharge through a gas is established
with ever increasing freedom as the frequency of the impulses is
augmented. It behaves in this respect quite contrarily to a metallic
conductor. In the latter the impedance enters prominently into play as
the frequency is increased, but the gas acts much as a series of
condensers would: the facility with which the discharge passes through
seems to depend on the rate of change of potential. If it act so, then
in a vacuum tube even of great length, and no matter how strong the
current, self-induction could not assert itself to any appreciable
degree. We have, then, as far as we can now see, in the gas a
conductor which is capable of transmitting electric impulses of any
frequency which we may be able to produce. Could the frequency be
brought high enough, then a queer system of electric distribution,
which would be likely to interest gas companies, might be realized:
metal pipes filled with gas--the metal being the insulator, the gas
the conductor--supplying phosphorescent bulbs, or perhaps devices as
yet uninvented. It is certainly possible to take a hollow core of
copper, rarefy the gas in the same, and by passing impulses of
sufficiently high frequency through a circuit around it, bring the gas
inside to a high degree of incandescence; but as to the nature of the
forces there would be considerable uncertainty, for it would be
doubtful whether with such impulses the copper core would act as a
static screen. Such paradoxes and apparent impossibilities we
encounter at every step in this line of work, and therein lies, to a
great extent, the claim of the study.

I have here a short and wide tube which is exhausted to a high degree
and covered with a substantial coating of bronze, the coating allowing
barely the light to shine through. A metallic clasp, with a hook for
suspending the tube, is fastened around the middle portion of the
latter, the clasp being in contact with the bronze coating. I now want
to light the gas inside by suspending the tube on a wire connected to
the coil. Any one who would try the experiment for the first time, not
having any previous experience, would probably take care to be quite
alone when making the trial, for fear that he might become the joke of
his assistants. Still, the bulb lights in spite of the metal coating,
and the light can be distinctly perceived through the latter. A long
tube covered with aluminium bronze lights when held in one hand--the
other touching the terminal of the coil--quite powerfully. It might be
objected that the coatings are not sufficiently conducting; still,
even if they were highly resistant, they ought to screen the gas. They
certainly screen it perfectly in a condition of rest, but not by far
perfectly when the charge is surging in the coating. But the loss of
energy which occurs within the tube, notwithstanding the screen, is
occasioned principally by the presence of the gas. Were we to take a
large hollow metallic sphere and fill it with a perfect incompressible
fluid dielectric, there would be no loss inside of the sphere, and
consequently the inside might be considered as perfectly screened,
though the potential be very rapidly alternating. Even were the sphere
filled with oil, the loss would be incomparably smaller than when the
fluid is replaced by a gas, for in the latter case the force produces
displacements; that means impact and collisions in the inside.

No matter what the pressure of the gas may be, it becomes an important
factor in the heating of a conductor when the electric density is
great and the frequency very high. That in the heating of conductors
by lightning discharges air is an element of great importance, is
almost as certain as an experimental fact. I may illustrate the action
of the air by the following experiment: I take a short tube which is
exhausted to a moderate degree and has a platinum wire running through
the middle from one end to the other. I pass a steady or low frequency
current through the wire, and it is heated uniformly in all parts. The
heating here is due to conduction, or frictional losses, and the gas
around the wire has--as far as we can see--no function to perform. But
now let me pass sudden discharges, or a high frequency current,
through the wire. Again the wire is heated, this time principally on
the ends and least in the middle portion; and if the frequency of the
impulses, or the rate of change, is high enough, the wire might as
well be cut in the middle as not, for practically all the heating is
due to the rarefied gas. Here the gas might only act as a conductor of
no impedance diverting the current from the wire as the impedance of
the latter is enormously increased, and merely heating the ends of the
wire by reason of their resistance to the passage of the discharge.
But it is not at all necessary that the gas in the tube should he
conducting; it might be at an extremely low pressure, still the ends
of the wire would be heated--as, however, is ascertained by
experience--only the two ends would in such, case not be electrically
connected through the gaseous medium. Now what with these frequencies
and potentials occurs in an exhausted tube occurs in the lightning
discharges at ordinary pressure. We only need remember one of the
facts arrived at in the course of these investigations, namely, that
to impulses of very high frequency the gas at ordinary pressure
behaves much in the same manner as though it were at moderately low
pressure. I think that in lightning discharges frequently wires or
conducting objects are volatilized merely because air is present and
that, were the conductor immersed in an insulating liquid, it would be
safe, for then the energy would have to spend itself somewhere else.
From the behavior of gases to sudden impulses of high potential I am
led to conclude that there can be no surer way of diverting a
lightning discharge than by affording it a passage through a volume of
gas, if such a thing can be done in a practical manner.

There are two more features upon which I think it necessary to dwell
in connection with these experiments--the "radiant state" and the
"non-striking vacuum."

Any one who has studied Crookes' work must have received the
impression that the "radiant state" is a property of the gas
inseparably connected with an extremely high degree of exhaustion. But
it should be remembered that the phenomena observed in an exhausted
vessel are limited to the character and capacity of the apparatus
which is made use of. I think that in a bulb a molecule, or atom, does
not precisely move in a straight line because it meets no obstacle,
but because the velocity imparted to it is sufficient to propel it in
a sensibly straight line. The mean free path is one thing, but the
velocity--the energy associated with the moving body--is another, and
under ordinary circumstances I believe that it is a mere question of
potential or speed. A disruptive discharge coil, when the potential is
pushed very far, excites phosphorescence and projects shadows, at
comparatively low degrees of exhaustion. In a lightning discharge,
matter moves in straight lines as ordinary pressure when the mean free
path is exceedingly small, and frequently images of wires or other
metallic objects have been produced by the particles thrown off in
straight lines.

[Illustration: FIG. 31.--BULB SHOWING RADIANT LIME STREAM AT LOW
EXHAUSTION.]

I have prepared a bulb to illustrate by an experiment the correctness
of these assertions. In a globe L (Fig. 31) I have mounted upon a lamp
filament f a piece of lime l. The lamp filament is connected with a
wire which leads into the bulb, and the general construction of the
latter is as indicated in Fig. 19, before described. The bulb being
suspended from a wire connected to the terminal of the coil, and the
latter being set to work, the lime piece l and the projecting parts of
the filament f are bombarded. The degree of exhaustion is just such
that with the potential the coil is capable of giving phosphorescence
of the glass is produced, but disappears as soon as the vacuum is
impaired. The lime containing moisture, and moisture being given off
as soon as heating occurs, the phosphorescence lasts only for a few
moments. When the lime has been sufficiently heated, enough moisture
has been given off to impair materially the vacuum of the bulb. As the
bombardment goes on, one point of the lime piece is more heated than
other points, and the result is that finally practically all the
discharge passes through that point which is intensely heated, and a
white stream of lime particles (Fig. 31) then breaks forth from that
point. This stream is composed of "radiant" matter, yet the degree of
exhaustion is low. But the particles move in straight lines because
the velocity imparted to them is great, and this is due to three
causes--to the great electric density, the high temperature of the
small point, and the fact that the particles of the lime are easily
torn and thrown off--far more easily than those of carbon. With
frequencies such as we are able to obtain, the particles are bodily
thrown off and projected to a considerable distance; but with
sufficiently high frequencies no such thing would occur: in such case
only a stress would spread or a vibration would be propagated through
the bulb. It would be out of the question to reach any such frequency
on the assumption that the atoms move with the speed of light; but I
believe that such a thing is impossible; for this an enormous
potential would be required. With potentials which we are able to
obtain, even with a disruptive discharge coil, the speed must be quite
insignificant.

As to the "non-striking vacuum," the point to be noted is that it can
occur only with low frequency impulses, and it is necessitated by the
impossibility of carrying off enough energy with such impulses in high
vacuum since the few atoms which are around the terminal upon coming
in contact with the same are repelled and kept at a distance for a
comparatively long period of time, and not enough work can be
performed to render the effect perceptible to the eye. If the
difference of potential between the terminals is raised, the
dielectric breaks down. But with very high frequency impulses there
is no necessity for such breaking down, since any amount of work can
be performed by continually agitating the atoms in the exhausted
vessel, provided the frequency is high enough. It is easy to
reach--even with frequencies obtained from an alternator as here
used--a stage at which the discharge does not pass between two
electrodes in a narrow tube, each of these being connected to one of
the terminals of the coil, but it is difficult to reach a point at
which a luminous discharge would not occur around each electrode.

A thought which naturally presents itself in connection with high
frequency currents, is to make use of their powerful electro-dynamic
inductive action to produce light effects in a sealed glass globe. The
leading-in wire is one of the defects of the present incandescent
lamp, and if no other improvement were made, that imperfection at
least should be done away with. Following this thought, I have carried
on experiments in various directions, of which some were indicated in
my former paper. I may here mention one or two more lines of
experiment which have been followed up.

Many bulbs were constructed as shown in Fig. 32 and Fig. 33.

In Fig. 32 a wide tube T was sealed to a smaller W-shaped tube U, of
phosphorescent glass. In the tube T was placed a coil C of aluminium
wire, the ends of which were provided with small spheres t and t_1 of
aluminium, and reached into the U tube. The tube T was slipped into a
socket containing a primary coil through which usually the discharges
of Leyden jars were directed, and the rarefied gas in the small U tube
was excited to strong luminosity by the high-tension currents induced
in the coil C. When Leyden jar discharges were used to induce currents
in the coil C, it was found necessary to pack the tube T tightly with
insulating powder, as a discharge would occur frequently between the
turns of the coil, especially when the primary was thick and the air
gap, through which the jars discharged, large, and no little trouble
was experienced in this way.

[Illustration: FIG. 32.--ELECTRO-DYNAMIC INDUCTION TUBE.]

[Illustration: FIG. 33--ELECTRO-DYNAMIC INDUCTION LAMP.]

In Fig. 33 is illustrated another form of the bulb constructed. In
this case a tube T is sealed to a globe L. The tube contains a coil C,
the ends of which pass through two small glass tubes t and t_1, which
are sealed to the tube T. Two refractory buttons m and m_1 are mounted
on lamp filaments which are fastened to the ends of the wires passing
through the glass tubes t and t_1. Generally in bulbs made on this
plan the globe L communicated with the tube T. For this purpose the
ends of the small tubes t and t_1 were just a trifle heated in the
burner, merely to hold the wires, but not to interfere with the
communication. The tube T, with the small tubes, wires through the
same, and the refractory buttons m and m_1, was first prepared, and
then sealed to globe L, whereupon the coil C was slipped in and the
connections made to its ends. The tube was then packed with insulating
powder, jamming the latter as tight as possible up to very nearly the
end, then it was closed and only a small hole left through which the
remainder of the powder was introduced, and finally the end of the
tube was closed. Usually in bulbs constructed as shown in Fig. 33 an
aluminium tube a was fastened to the upper end s of each of the tubes
t and t_1, in order to protect that end against the heat. The buttons
m and m_1 could be brought to any degree of incandescence by passing
the discharges of Leyden jars around the coil C. In such bulbs with
two buttons a very curious effect is produced by the formation of the
shadows of each of the two buttons.

Another line of experiment, which has been assiduously followed, was
to induce by electro-dynamic induction a current or luminous discharge
in an exhausted tube or bulb. This matter has received such able
treatment at the hands of Prof. J.J. Thomson that I could add but
little to what he has made known, even had I made it the special
subject of this lecture. Still, since experiences in this line have
gradually led me to the present views and results, a few words must be
devoted here to this subject.

It has occurred, no doubt, to many that as a vacuum tube is made
longer the electromotive force per unit length of the tube, necessary
to pass a luminous discharge through the latter, gets continually
smaller; therefore, if the exhausted tube be made long enough, even
with low frequencies a luminous discharge could be induced in such a
tube closed upon itself. Such a tube might be placed around a ball or
on a ceiling, and at once a simple appliance capable of giving
considerable light would be obtained. But this would be an appliance
hard to manufacture and extremely unmanageable. It would not do to
make the tube up of small lengths, because there would be with
ordinary frequencies considerable loss in the coatings, and besides,
if coatings were used, it would be better to supply the current
directly to the tube by connecting the coatings to a transformer. But
even if all objections of such nature were removed, still, with low
frequencies the light conversion itself would be inefficient, as I
have before stated. In using extremely high frequencies the length of
the secondary--in other words, the size of the vessel--can be reduced
as far as desired, and the efficiency of the light conversion is
increased, provided that means are invented for efficiently obtaining
such high frequencies. Thus one is led, from theoretical and practical
considerations, to the use of high frequencies, and this means high
electromotive forces and small currents in the primary. When he works
with condenser charges--and they are the only means up to the present
known for reaching these extreme frequencies--he gets to electromotive
forces of several thousands of volts per turn of the primary. He
cannot multiply the electro-dynamic inductive effect by taking more
turns in the primary, for he arrives at the conclusion that the best
way is to work with one single turn--though he must sometimes depart
from this rule--and he must get along with whatever inductive effect
he can obtain with one turn. But before he has long experimented with
the extreme frequencies required to set up in a small bulb an
electromotive force of several thousands of volts he realizes the
great importance of electrostatic effects, and these effects grow
relatively to the electro-dynamic in significance as the frequency is
increased.

Now, if anything is desirable in this case, it is to increase the
frequency, and this would make it still worse for the electro-dynamic
effects. On the other hand, it is easy to exalt the electrostatic
action as far as one likes by taking more turns on the secondary, or
combining self-induction and capacity to raise the potential. It
should also be remembered that, in reducing the current to the
smallest value and increasing the potential, the electric impulses of
high frequency can be more easily transmitted through a conductor.

These and similar thoughts determined me to devote more attention to
the electrostatic phenomena, and to endeavor to produce potentials as
high as possible, and alternating as fast as they could be made to
alternate. I then found that I could excite vacuum tubes at
considerable distance from a conductor connected to a properly
constructed coil, and that I could, by converting the oscillatory
current of a condenser to a higher potential, establish electrostatic
alternating fields which acted through the whole extent of a room,
lighting up a tube no matter where it was held in space. I thought I
recognized that I had made a step in advance, and I have persevered in
this line; but I wish to say that I share with all lovers of science
and progress the one and only desire--to reach a result of utility to
men in any direction to which thought or experiment may lead me. I
think that this departure is the right one, for I cannot see, from the
observation of the phenomena which manifest themselves as the
frequency is increased, what there would remain to act between two
circuits conveying, for instance, impulses of several hundred millions
per second, except electrostatic forces. Even with such trifling
frequencies the energy would be practically all potential, and my
conviction has grown strong that, to whatever kind of motion light may
be due, it is produced by tremendous electrostatic stresses vibrating
with extreme rapidity.

Of all these phenomena observed with currents, or electric impulses,
of high frequency, the most fascinating for an audience are certainly
those which are noted in an electrostatic field acting through
considerable distance, and the best an unskilled lecturer can do is
to begin and finish with the exhibition of these singular effects. I
take a tube in the hand and move it about, and it is lighted wherever
I may hold it; throughout space the invisible forces act. But I may
take another tube and it might not light, the vacuum being very high.
I excite it by means of a disruptive discharge coil, and now it will
light in the electrostatic field. I may put it away for a few weeks or
months, still it retains the faculty of being excited. What change
have I produced in the tube in the act of exciting it? If a motion
imparted to the atoms, it is difficult to perceive how it can persist
so long without being arrested by frictional losses; and if a strain
exerted in the dielectric, such as a simple electrification would
produce, it is easy to see how it may persist indefinitely, but very
difficult to understand why such a condition should aid the excitation
when we have to deal with potentials which are rapidly alternating.

Since I have exhibited these phenomena for the first time, I have
obtained some other interesting effects. For instance, I have produced
the incandescence of a button, filament, or wire enclosed in a tube.
To get to this result it was necessary to economize the energy which
is obtained from the field and direct most of it on the small body to
be rendered incandescent. At the beginning the task appeared
difficult, but the experiences gathered permitted me to reach the
result easily. In Fig. 34 and Fig. 35 two such tubes are illustrated
which are prepared for the occasion. In Fig. 34 a short tube T_1,
sealed to another long tube T, is provided with a stem s, with a
platinum wire sealed in the latter. A very thin lamp filament l is
fastened to this wire, and connection to the outside is made through a
thin copper wire w. The tube is provided with outside and inside
coatings, C and C_1 respectively, and is filled as far as the coatings
reach with conducting, and the space above with insulating powder.
These coatings are merely used to enable me to perform two experiments
with the tube--namely, to produce the effect desired either by direct
connection of the body of the experimenter or of another body to the
wire w, or by acting inductively through the glass. The stem s is
provided with an aluminium tube a, for purposes before explained, and
only a small part of the filament reaches out of this tube. By holding
the tube T_1 anywhere in the electrostatic field the filament is
rendered incandescent.

[Illustration: FIG. 34.--TUBE WITH FILAMENT RENDERED INCANDESCENT IN
AN ELECTROSTATIC FIELD.]

[Illustration: FIG. 35.--CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.]

A more interesting piece of apparatus is illustrated in Fig. 35. The
construction is the same as before, only instead of the lamp filament
a small platinum wire p, sealed in a stem s, and bent above it in a
circle, is connected to the copper wire w, which is joined to an
inside coating C. A small stem s_1 is provided with a needle, on the
point of which is arranged to rotate very freely a very light fan of
mica v. To prevent the fan from falling out, a thin stem of glass g is
bent properly and fastened to the aluminium tube. When the glass tube
is held anywhere in the electrostatic field the platinum wire becomes
incandescent, and the mica vanes are rotated very fast.

Intense phosphorescence may be excited in a bulb by merely connecting
it to a plate within the field, and the plate need not be any larger
than an ordinary lamp shade. The phosphorescence excited with these
currents is incomparably more powerful than with ordinary apparatus. A
small phosphorescent bulb, when attached to a wire connected to a
coil, emits sufficient light to allow reading ordinary print at a
distance of five to six paces. It was of interest to see how some of
the phosphorescent bulbs of Professor Crookes would behave with these
currents, and he has had the kindness to lend me a few for the
occasion. The effects produced are magnificent, especially by the
sulphide of calcium and sulphide of zinc. From the disruptive
discharge coil they glow intensely merely by holding them in the hand
and connecting the body to the terminal of the coil.

To whatever results investigations of this kind may lead, their chief
interest lies for the present in the possibilities they offer for the
production of an efficient illuminating device. In no branch of
electric industry is an advance more desired than in the manufacture
of light. Every thinker, when considering the barbarous methods
employed, the deplorable losses incurred in our best systems of light
production, must have asked himself, What is likely to be the light of
the future? Is it to be an incandescent solid, as in the present lamp,
or an incandescent gas, or a phosphorescent body, or something like a
burner, but incomparably more efficient?

There is little chance to perfect a gas burner; not, perhaps, because
human ingenuity has been bent upon that problem for centuries without
a radical departure having been made--though this argument is not
devoid of force-but because in a burner the higher vibrations can
never be reached except by passing through all the low ones. For how
is a flame produced unless by a fall of lifted weights? Such process
cannot be maintained without renewal, and renewal is repeated passing
from low to high vibrations. One way only seems to be open to improve
a burner, and that is by trying to reach higher degrees of
incandescence. Higher incandescence is equivalent to a quicker
vibration; that means more light from the same material, and that,
again, means more economy. In this direction some improvements have
been made, but the progress is hampered by many limitations.
Discarding, then, the burner, there remain the three ways first
mentioned, which are essentially electrical.

Suppose the light of the immediate future to be a solid rendered
incandescent by electricity. Would it not seem that it is better to
employ a small button than a frail filament? From many considerations
it certainly must be concluded that a button is capable of a higher
economy, assuming, of course, the difficulties connected with the
operation of such a lamp to be effectively overcome. But to light such
a lamp we require a high potential; and to get this economically we
must use high frequencies.

Such considerations apply even more to the production of light by the
incandescence of a gas, or by phosphorescence. In all cases we require
high frequencies and high potentials. These thoughts occurred to me a
long time ago.

Incidentally we gain, by the use of very high frequencies, many
advantages, such as a higher economy in the light production, the
possibility of working with one lead, the possibility of doing away
with the leading-in wire, etc.

The question is, how far can we go with frequencies? Ordinary
conductors rapidly lose the facility of transmitting electric impulses
when the frequency is greatly increased. Assume the means for the
production of impulses of very great frequency brought to the utmost
perfection, every one will naturally ask how to transmit them when the
necessity arises. In transmitting such impulses through conductors we
must remember that we have to deal with _pressure_ and _flow_, in the
ordinary interpretation of these terms. Let the pressure increase to
an enormous value, and let the flow correspondingly diminish, then
such impulses--variations merely of pressure, as it were--can no doubt
be transmitted through a wire even if their frequency be many hundreds
of millions per second. It would, of course, be out of question to
transmit such impulses through a wire immersed in a gaseous medium,
even if the wire were provided with a thick and excellent insulation
for most of the energy would be lost in molecular bombardment and
consequent heating. The end of the wire connected to the source would
be heated, and the remote end would receive but a trifling part of the
energy supplied. The prime necessity, then, if such electric impulses
are to be used, is to find means to reduce as much as possible the
dissipation.

The first thought is, employ the thinnest possible wire surrounded by
the thickest practicable insulation. The next thought is to employ
electrostatic screens. The insulation of the wire may be covered with
a thin conducting coating and the latter connected to the ground. But
this would not do, as then all the energy would pass through the
conducting coating to the ground and nothing would get to the end of
the wire. If a ground connection is made it can only be made through a
conductor offering an enormous impedance, or though a condenser of
extremely small capacity. This, however, does not do away with other
difficulties.

If the wave length of the impulses is much smaller than the length of
the wire, then corresponding short waves will be sent up in the
conducting coating, and it will be more or less the same as though the
coating were directly connected to earth. It is therefore necessary to
cut up the coating in sections much shorter than the wave length. Such
an arrangement does not still afford a perfect screen, but it is ten
thousand times better than none. I think it preferable to cut up the
conducting coating in small sections, even if the current waves be
much longer than the coating.

If a wire were provided with a perfect electrostatic screen, it would
be the same as though all objects were removed from it at infinite
distance. The capacity would then be reduced to the capacity of the
wire itself, which would be very small. It would then be possible to
send over the wire current vibrations of very high frequencies at
enormous distance without affecting greatly the character of the
vibrations. A perfect screen is of course out of the question, but I
believe that with a screen such as I have just described telephony
could be rendered practicable across the Atlantic. According to my
ideas, the gutta-percha covered wire should be provided with a third
conducting coating subdivided in sections. On the top of this should
be again placed a layer of gutta-percha and other insulation, and on
the top of the whole the armor. But such cables will not be
constructed, for ere long intelligence--transmitted without
wires--will throb through the earth like a pulse through a living
organism. The wonder is that, with the present state of knowledge and
the experiences gained, no attempt is being made to disturb the
electrostatic or magnetic condition of the earth, and transmit, if
nothing else, intelligence.

It has been my chief aim in presenting these results to point out
phenomena or features of novelty, and to advance ideas which I am
hopeful will serve as starting points of new departures. It has been
my chief desire this evening to entertain you with some novel
experiments. Your applause, so frequently and generously accorded, has
told me that I have succeeded.

In conclusion, let me thank you most heartily for your kindness and
attention, and assure you that the honor I have had in addressing such
a distinguished audience, the pleasure I have had in presenting these
results to a gathering of so many able men--and among them also some
of those in whose work for many years past I have found enlightenment
and constant pleasure--I shall never forget.



[Transcriber's note: Corrected the following typesetting errors:
     1) 'preceived' to 'perceived', page 16.
     2) 'disharging' to 'discharging', page 30.
     3) 'park' to 'spark', page 33.
     4) 'pssition' to 'position', page 50.
     5) 'to th opposite side' to 'to the opposite side', page 56.
     6) 's resses' to 'stresses', page 147.]




