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, today, 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
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
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
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
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.
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.
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.
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
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
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.
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
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
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
Is there, I ask, can there be, a more interesting study than that of
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
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
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)
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
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
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
G is an ordinarily constructed alternator, supplying the primary P
of an induction coil, the secondary S of which charges the
condensers or jars C C. 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 p p of a second induction coil.
This primary p p has a small air gap a b.
The secondary s of this coil is provided with knobs or spheres K K
of the proper size and set at a distance suitable for the
A long arc is established between the terminals A B of the first
induction coil. M M are the mica plates.
Each time the arc is broken between A and B the jars are quickly
charged and discharged through the primary p p, producing a snapping
spark between the knobs K K. 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 a b until the arc is again
broken by the draught.
In this manner sudden impulses, at long intervals, are produced in
the primary p p, which in the secondary s give a corresponding
number of impulses of great intensity. If the secondary knobs or
spheres, K K, 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
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.
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
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
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 centimeters, 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
The coil consists of two spools of hard rubber R R, held apart at a
distance of 10 centimeters by bolts c and nuts n, likewise of hard
rubber. Each spool comprises a tube T of approximately 8 centimeters
inside diameter, and 3 millimeters thick, upon which are screwed two
flanges F F, 24 centimeters square, the space between the flanges
being about 3 centimeters. The secondary, S S, 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 T1 T1 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 P P 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 t t. The ends of the secondary T1 T1 are also led out
of the oil through rubber tubes t1 t1 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 centimeters 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.
One of the changes is that the adjustable knobs A and B (Fig. 4), of
the discharger are held in jaws of brass, J J, 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 N S, 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, M M, of sufficient thickness. s1 s1 and s2 s2
are screws for fastening the wires. On each side one of the screws
is for large and the other for small wires. L L are screws for
fixing in position the rods R R, 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 A B, in Fig. 2 (the knobs a b 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
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 c c (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 R R, with
planed grooves g g (Fig. 7) to fit the middle portion of the pieces
c c, serve to clamp the latter and hold them firmly in position by
means of two bolts C C (of which only one is shown) passing through
the ends of the strips.
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
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
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 note-worthy of
these discharge phenomena.
I have stretched across the room two ordinary cotton covered wires,
each about 7 meters in length. They are supported on insulating
cords at a distance of about 30 centimeters. 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.
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.
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 centimeters 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, R R, of hard rubber (Fig. 9),
upon which I have glued two very thin wires w w, 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 t t.
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
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.
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 centimeters and the other 30 centimeters 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
meter, 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 meter.
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 centimeters in diameter
and 20 centimeters 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.
tubes are then placed in a large glass vessel, at a distance of 10
to 15 centimeters 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
centimeters. 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
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
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 (d d, Fig. 11) of no more than a few
centimeters 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
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
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
centimeters 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.
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
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.
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 meter 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.
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
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
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 paraffin 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
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.
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 paraffin oil. A primary of a diameter something
like six millimeters 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.
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 centimeter 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.
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
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.
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
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
enclosed 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.