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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.