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INTRODUCTION.--THE
SCOPE OF THE TESLA
LECTURES. BEFORE proceeding to
study the three Tesla lectures here presented, the reader may find it of
some assistance to have his attention directed to the main points of
interest and significance therein. The first of these lectures was
delivered in New York, at Columbia College, before the American
Institute of Electrical Engineers, May 20, 1891. The urgent desire
expressed immediately from all parts of Europe for an opportunity to
witness the brilliant and unusual experiments with which the lecture was
accompanied, induced Mr. Tesla to go to England early in 1892, when he
appeared before the Institution of Electrical Engineers, and a day
later, by special request, before the Royal Institution. His reception
was of the most enthusiastic and flattering nature on both occasions. He
then went, by invitation, to France, and repeated his novel
demonstrations before the Societe Internationale des Electriciens, and
the Societe Frangaise de Physique. Mr. Tesla returned to America in the
fall of 1892, and in February, 1893, delivered his third lecture before
the Franklin Institute of Philadelphia, in fulfilment of a long standing
promise to Prof. Houston. The following week, at the request of
President James I. Ayer, of the National Electric Light Association, the
same lecture was re-delivered in St. Louis. It had been intended to
limit the invitations to members, but the appeals from residents in the
city were so numerous and pressing that it became necessary to secure a
very large hall. Hence it came about that the lecture was listened to by
an audience of over 5,000 people, and was in some parts of a more
popular nature than either of its predecessors. Despite this concession
to the need of the hour and occasion, Mr. Tesla did not hesitate to show
many new and brilliant experiments, and to advance the frontier of
discovery far beyond any point he had theretofore marked publicly.
We may now proceed to a running review of the lectures themselves. The
ground covered by them is so vast that only the leading ideas and
experiments can here be touched upon ; besides, it is preferable that
the lectures should be carefully gone over for their own sake, it being
more than likely that each student will discover a new beauty or
stimulus in them. Taking up the course of reasoning followed by Mr.
Tesla in his first lecture, it will be noted that he started out with
the recognition of the fact, which he has now experimentally
demonstrated, that for the production of light waves, primarily,
electrostatic effects must be brought into play, and continued study has
led him to the opinion that all electrical and magnetic effects may be
referred to electrostatic molecular forces. This opinion finds a
singular confirmation in one of the most striking experiments which he
describes, namely, the production of a veritable flame by the agitation
of electrostatically charged molecules. It is of the highest interest to
observe that this result points out a way of obtaining a flame which
consumes no material and in which no chemical action whatever takes
place. It also throws a light on the nature of the ordinary flame, which
Mr. Tesla believes to be due to electrostatic molecular actions, which,
if true, would lead directly to the idea that even chemical affinities
might be electrostatic in their nature and that, as has already been
suggested, molecular forces in general may be referable to one and the
same cause. This singular phenomenon accounts in a plausible manner for
the unexplained fact that buildings are frequently set on fire during
thunder storms without having been at all struck by *\v lightning. It
may also explain the total disappearance of ships at sea.
One of the striking proofs of the correctness of the ideas advanced by
Mr. Tesla is the fact that, notwithstanding the employment of the most
powerful electromagnetic inductive effects, but .feeble luminosity is
obtainable, and this only in close proximity to the source of
disturbance; whereas, when the electrostatic effects are intensified,
the same initial energy suffices to excite luminosity at considerable
distances from the source. That there are only electrostatic effects
active seems to be clearly proved by Mr. Tesla's experiments with an
induction coil operated with alternating currents of very high
frequency. He shows how tubes may be made to glow brilliantly at
considerable distances from any object when placed in a powerful,
rapidly alternating, electrostatic field, and he describes many
interesting phenomena observed in such a field. His experiments open up
the possibility of lighting an apartment by simply creating in it such
an electrostatic field, and this, in a certain way, would appear to be
the ideal method of lighting a room, as it would allow the illuminating
device to be freely moved about. The power with which these exhausted
tubes, devoid of any electrodes, light up is certainly remarkable.
That the principle propounded by Mr. Tesla is a broad one is evident
from the many ways in which it may be practically applied. We need only
refer to the variety of the devices shown or described, all of which are
novel in character and will, without doubt, lead to further important
results at the hands of Mr. Tesla and other investigators. The
experiment, for instance, of lighting up a single filament or block of
refractory material with a single wire, is in itself sufficient to give
Mr. Tesla's work the stamp of originality, and the numerous other
experiments and effects which may be varied at will, are equally new and
interesting. Thus, the incandescent filament spinning in an unexhausted
globe, the well-known Crookes experiment on open circuit, and the many
others suggested, will not fail to interest the reader. Mr. Tesla has
made an exhaustive study of the various forms of the discharge presented
by an induction coil when operated with these rapidly alternating
currents, starting from the thread-like discharge and passing through
various stages to the true electric flame.
A point of great importance in the introduction of high tension
alternating current which Mr. Tesla brings out is the necessity of
carefully avoiding all gaseous matter in the high tension apparatus. He
shows that, at least with very rapidly alternating currents of high
potential, the discharge may work through almost any practicable
thickness of the best insulators, if air is present. In such cases the
air included within the apparatus is violently agitated and by molecular
bombardment the parts may be so greatly heated as to cause a rupture of
the. insulation. The practical outcome of this is, that, whereas with
steady currents, any kind of insulation may be used, with rapidly
alternating currents oils will probably be the best to employ, a fact
which has been observed, but not until now satisfactorily explained. The
recognition of the above fact is of special importance in the
construction of the costly commercial induction coils which are often
rendered useless in an unaccountable manner.
The truth of these views of Mr. Tesla is made evident by the interesting
experiments illustrative of the behavior of the air between charged
surfaces, the luminous streams formed by the charged molecules appearing
even when great thicknesses of tinbest insulators are interposed between
the charged surfaces. These luminous streams afford in themselves a very
interesting study for the experimenter. With these rapidly alternating
currents they become far more powerful and produce beautiful light
effects when they issue from a wire, pinwheel or other object attached
to a terminal of the coil ; and it is interesting to note that they
issue from a ball almost as freely as from a point, when the frequency
is very high.
From these experiments we also obtain a better idea of the importance of
taking into account the capacity and self-induction in the apparatus
employed and the possibilities offered by the use of condensers in
conjunction with alternate currents, the employment of currents of high
frequency, among other things, making it possible to reduce the
condenser to practicable dimensions. Another point of interest and
practical bearing is the fact, proved by Mr. Tesla, that for alternate
currents, especially those of high frequency, insulators are required
possessing a small specific inductive capacity, which at the same time
have a high insulating power.
Mr. Tesla also makes interesting and valuable suggestion in regard to
the economical utilization of iron in machines and transformers. He
shows how, by maintaining by continuous magnetization a flow of lines
through the iron, the latter may be kept near its maximum permeability
and a higher output and economy may be secured in such apparatus. This
principle may prove of considerable commercial importance in the
development of alternating systems. Mr. Tesla's suggestion that the same
result can be secured by heating the iron by hysteresis and eddy
currents, and increasing the permeability in this manner, while it may
appear less practical, nevertheless opens another direction for
investigation and improvement.
The demonstration of the fact that with alternating currents of high
frequency, sufficient energy may be transmitted under practicable
conditions through the glass of an incandescent lamp by electrostatic or
electromagnetic induction may lead to a departure in the construction of
such devices. Another important i I experimental result achieved is the
operation of lamps, and even \ 1 .motors, with the discharges of
condensers, this method affording a means of converting direct or
alternating currents. In this connection Mr. Tesla advocates the
perfecting of apparatus capable of generating electricity of high
tension from heat energy, believing this to be a better way of obtaining
electrical energy for practical purposes, particularly for the
production of light.
While many were probably prepared to encounter curious phenomena of
impedance in the use of a condenser discharged disruptively, the
experiments shown were extremely interesting on account of their
paradoxical character. The burning of an incandescent lamp at any candle
power when connected across a heavy metal bar, the existence of nodes on
the bar and the possibility of exploring the bar by means of an ordinary
Garde w voltmeter, are all peculiar developments, but perhaps the most
interesting observation is the phenomenon of impedance observed in the
lamp with a straight filament, which remains dark while the bulb glows.
Mr. Tesla's manner of operating an induction coil by means of the
disruptive discharge, and thus obtaining enormous differences of
potential from comparatively small and inexpensive coils, will be
appreciated by experimenters and will find valuable application in
laboratories. Indeed, his many suggestions and hints in regard to the
construction and use of apparatus in these investigations will be highly
valued and will aid materially in future research.
The London lecture was delivered twice. In its first form, before the
Institution of Electrical Engineers, it was in some respects an
amplification of several points not specially enlarged upon in the JS T
ew York lecture, but brought forward many additional discoveries and new
investigations. Its repetition, in*.""*] another form, at the Royal
Institution, was due to Prof. Dewar, who with Lord Rayleigh, manifested
a most lively interest in Mr. Tesla's work, and whose kindness
illustrated once more the strong V } English love of scientific truth
and appreciation of its votaries. } As an indefatigable experimenter,
Mr. Tesla was certainly no*^ where more at home than in the haunts of
Faraday, and as the / guest of Faraday's successor. This Royal
Institution lecture W summed up the leading points of Mr. Tesla's work,
in the high / potential, high frequency field, and we may here avail
ourselves J of so valuable a summarization, in a simple form, of a
subject by no means easy of comprehension until it has been thoroughly
studied.
In these London lectures, among the many notable points made was first,
the difficulty of constructing the alternators to obtain, the very high
frequencies needed. To obtain the high frequencies it was necessary to
provide several hundred polar projections, which were necessarily small
and offered many drawbacks, and this the more as exceedingly high
peripheral speeds had to be resorted to. In some of the first machines
both armature and field had polar projections. These machines produced a
curious noise, especially when the armature was started from the state
of rest, the field being charged. The most efficient machine was found
to be one with a drum armature, the iron body of which consisted of very
thin wire annealed with special care. It was, of course, desirable to
avoid the employment of iron in the armature, and several machines of
this kind, with moving or stationary conductors were constructed, but
the results obtained were not quite satisfactory, on account of the
great mechanical and other difficulties encountered.
The study of the properties of the high frequency currents obtained from
these machines is very interesting, as nearly every experiment discloses
something new. Two coils traversed by such a current attract or repel
each other with a force which, owing to the imperfection of our sense of
touch, seems continuous. An interesting observation, already noted under
another form, is that a piece of iron, surrounded by a coil through
which the current is passing appears to be continuously magnetized.
This apparent continuity might be ascribed to the deficiency of the
sense of touch, but there is evidence that in currents of such high
frequencies one of the impulses preponderates over the other.
As might be expected, conductors traversed by such currents are rapidly
heated, owing to the increase of the resistance, and the heating effects
are relatively much greater in the iron. The hysteresis losses in iron
are so great that an iron core, even if finely subdivided, is heated in
an incredibly short time. To give an idea of this, an ordinary iron wire
*^g* inch in diameter inserted within a coil having 250 turns, with a
current estimated to be five amperes passing through the coil, becomes
within two seconds' time so hot as to scorch wood. Beyond a certain
frequency, an iron core, no matter how finely subdivided, exercises a
dampening effect, and it was easy to find a point at which the impedance
<>f a coil was not affected by the presence of a core consisting of a
bundle of very thin well annealed and varnished iron wires.
Experiments with a telephone, a conductor in a strong magnetic field, or
with a condenser or arc, seem to afford certain proof that sounds far
above the usually accepted limit of hearing would be perceived if
produced with sufficient power. The arc produced by these currents
possesses several interesting features. Usually it emits a note the
pitch of which corresponds to twice the frequency of the current, but if
the frequency be sufficiently high it becomes noiseless, the limit of
audition being determined principally by the linear dimensions of the
arc. A curious feature of the arc is its persistency, which is due
partly to the inability of the gaseous column to cool and increase
considerably in resistance, as is the case with low frequencies, and
partly to the tendency of such a high frequency machine to maintain a
constant current.
In connection with these machines the condenser affords a particularly
interesting study. Striking effects are produced by proper adjustments
of capacity and self-induction. It is easy to raise the electromotive
force of the machine to many times the original value by simply
adjusting the capacity of a condenser connected in the induced circuit.
If the condenser be at some distance from the machine, the difference of
potential on the terminals of the latter may be only a small fraction of
that on the condenser.
But the most interesting experiences are gained when the tension of the
currents from the machine is raised by means of an induction coil. In
consequence of the enormous rate of change obtainable in the primary
current, much higher potential differences are obtained than with coils
operated in the usual ways, and, owing to the high frequency, the
secondary discharge possesses many striking peculiarities. Both the
electrodes behave generally alike, though it appears from some
observations that one current impulse preponderates over the other, as
before mentioned.
The physiological effects of the high tension discharge are found to be
so small that the shock of the coil can be supported without any
inconvenience, except perhaps a small burn produced by the discharge
upon approaching the hand to one of the terminals. The decidedly smaller
physiological effects of these currents are thought to be due either to
a different distribution through the body or to the tissues acting as
condensers. But in the case of an induction coil with a great many turns
the harmlessness is principally due to the fact that but little energy
is available in the external circuit when the same is closed through the
experimenter's body, on account of the great impedance of the coil.
In varying the frequency and strength of the currents through the
primary of the coil, the character of the secondary discharge is greatly
varied, and no less than five distinct forms are observed : A weak,
sensitive thread discharge, a powerful naming discharge, and three forms
of brush or streaming discharges. Each of these possesses certain
noteworthy features, but the most interesting to study are the latter.
Under certain conditions the streams, which are presumably due to the
violent agitation of the air molecules, issue freely from all points of
the coil, even through a thick insulation. If there is the smallest air
space between the primary and secondary, they will form there and surely
injure the coil by slowly warming the insulation. As they form even with
ordinary frequencies when the potential is excessive, the air-space must
be most carefully avoided. These high frequency streamers differ in
aspect and properties from those produced by a static machine. The wind
produced by them is small and should altogether cease if still
considerably higher frequencies could be obtained. A peculiarity is that
they issue as freely from surfaces as from points. ( hving to this, a
metallic vane, mounted in one of the terminals of the coil so as to
rotate freely, and having one of its sides covered with insulation, is
spun rapidly around. Such a vane would not rotate with a steady
potential, but with a high frequency coil it will spin, even if it be
entirely covered with insulation, provided the insulation on one side be
either thicker or of a higher specific inductive capacity. A Crookes
electric radiometer is also spun around when connected to one of the
terminals of the coil, but only at very high exhaustion or at ordinary
pressures.
There is still another and more striking peculiarity of such a high
frequency streamer, namely, it is hot. The heat is easily perceptible
with frequencies of about 10,000, even if the potential is not
excessively high. The heating effect is, of course, due to the molecular
impacts and collisions. Could the frequency and potential be pushed far
enough, then a brush could be produced resembling in every particular a
flame and giving light and heat, jet without a chemical process taking
place.
The hot brush, when properly produced, resembles a jet of burning gas
escaping under great pressure, and it emits an extraordinary strong
smell of ozone. The great ozonizing action is ascribed to the fact that
the agitation of the molecules of the air is more violent in such a
brush than in the ordinary streamer of a static machine. But the most
powerful brush discharges were produced by employing currents of much
higher frequencies than it was possible to obtain by means of the
alternators. These currents were obtained by disruptively discharging a
condenser and setting up oscillations. In this manner currents of a
frequency of several hundred thousand were obtained.
Currents of this kind, Mr. Tesla pointed out, produce striking effects.
At these frequencies, the impedance of a copper bar is so great that a
potential difference of several hundred volts can be maintained between
two points of a short and thick bar, and it is possible to keep an
ordinary incandescent lamp burning at full candle power by attaching the
terminals of the lamp to two points of the bar no more than a few inches
apart, When the frequency is extremely high, nodes are found to exist on
such a bar, and it is easy to locate them by means of a lamp.
By converting the high tension discharges of a low frequency coil in
this manner, it was found practicable to keep a few lamps burning on the
ordinary circuit in the laboratory, and by bringing the undulation to a
low pitch, it was possible to operate small motors.
This plan likewise allows of converting high tension discharges of one
direction into low tension unidirectional currents, by adjusting the
circuit so that there are no oscillations. In passing the oscillating
discharges through the primary of a specially constructed coil, it is
easy to obtain enormous potential differences with only few turns of the
secondary.
Great difficulties were at first experienced in producing a successful
coil on this plan. It was found necessary to keep all air, or gaseous
matter in general, away from the charged surfaces, and oil immersion was
resorted to. The wires used were heavily covered with gutta-percha and
wound in oil, or the air was pumped out by means of a Sprengel pump. The
general arrangement was the following: An ordinary induction coil,
operated from a low frequency alternator, was used to charge Leyden
jars. The jars were made to discharge over a single or multiple gap
through the primary of the second coil. To insure the action of the gap,
the arc was blown out by a magnet or air blast. To adjust the potential
in the secondary a small oil condenser was used, or polished brass
spheres of different sizes were screwed on the terminals and their
distance adjusted.
When the conditions were carefully determined to suit each experiment,
magnificent effects were obtained. Two wires, stretched through the
room, each being connected to one of the terminals of the coil, emitted
streams so powerful that the light from them allowed distinguishing the
objects in the room ; the wires became luminous even though covered with
thick and most excellent insulation. When two straight wires, or two
concentric circles of wire, are connected to the terminals, and set at
the proper distance, a uniform luminous sheet is produced between them.
It was possible in this way to cover an ana of more than one meter
square completely with the streams. By attaching to one terminal a large
circle of wire and to the other terminal a small sphere, the streams are
focused upon the sphere, produce a strongly lighted spot upon the same,
and present the appearance of a luminous cone. A very thin wire glued
upon a plate of hard rubber of great thickness, on the opposite side of
which is fastened a tinfoil coating, is rendered intensely luminous when
the coating is connected to the other terminal of the coil. Such an
experiment can be performed also with low frequency currents, but much
less satisfactorily.
When the terminals of such a coil, even of a very small one, are
separated by a rubber or glass plate, the discharge spreads over the
plate in the form of streams, threads or brilliant sparks, and affords a
magnificent display, which cannot be equaled by the largest coil
operated in the usual ways. By a simple adjustment it is possible to
produce with the coil a succession of brilliant sparks, exactly as with
a Holtz machine.
Under certain conditions, when the frequency of the oscillation is very
great, white, phantom-like streams are seen to break forth from the
terminals of the coil. The chief interesting feature about them is, that
they stream freely against the outstretched hand or other conducting
object without producing any sensation, and the hand may be approached
very near to the terminal without a spark being induced to jump. This is
due presumably to the fact that a considerable portion of the energy is
carried away or dissipated in the streamers, and the difference of
potential between the terminal and the hand is diminished.
It is found in such experiments that 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 very high, and the arc of the discharge produces a sound
which is loud and smooth (which indicates both that oscillation takes
place and that the sparks succeed each other with great rapidity), then
the luminous streams formed are perfectly uniform. They are generally of
a purplish hue, but when the molecular vibration is increased by raising
the potential, they assume a white color.
The luminous intensity of the streams increases rapidly when the
potential is increased; and with frequencies of only a few hundred
thousand, could the coil be made to withstand a sufficiently high
potential difference, there is no doubt that the space around a wire
could be made to emit a strong light, merely by the agitation of the
molecules of the air at ordinary pressure.
Such discharges of very high frequency which render luminous the air at
ordinary pressure we have very likely occasion to witness in the aurora
borealis. From many of these experiments it seems reasonable to infer
that sudden cosmic disturbances, such as eruptions on the sun, set the
electrostatic charge of the earth in an extremely rapid vibration, and
produce the glow by the violent agitation of the air in the upper and
even in the lower strata. It is thought that if the frequency were low?
or even more so if the charge were not at all vibrating, the lower dense
strata would break down as in a lightning discharge. Indications of such
breaking down have been repeatedly observed, but they can be attributed
to the fundamental disturbances, which are few in number, for the
superimposed vibration would be so rapid as not to allow a disruptive
break.
The study of these discharge phenomena has led Mr. Tesla to the
recognition of some important facts. It was found, as already stated,
that uascous matter must be most carefully excluded from any dielectric
which is subjected to great, rapidly changing electrostatic stresses.
Since it is difficult to exclude the gas perfectly when solid insulators
are used, it is necessary to resort to liquid dielectrics. When a solid
dielectric is used, it matters little how thick and how good it is; if
air be present, streamers form, which gradually heat the dielectric and
impair its insulating power, and the discharge finally breaks through.
Under ordinary conditions the best insulators are those which possess
the highest specific inductive capacity, but such insulators are not the
best to employ when working with these high frequency currents, for in
most cases the higher specific inductive capacity is rather a
disadvantage. The prime quality of the insulating medium for these
currents is continuity. For this reason principally it is necessary to
employ liquid insulators, such as oils. If two metal plates, connected
to the terminals of the coil, are immersed in oil and set a distance
apart, the coil may be kept working for any length of time without a
break occurring, or without the oil being warmed, but if air bubbles are
introduced, they become luminous ; the air molecules, by their impact
against the oil, heat it, and after some time cause the insulation to
give way. If, instead of the oil, a solid plate of the best dielectric,
even several times thicker than the oil intervening between the metal
plates, is inserted between the latter, the air having free access to
the charged surfaces, the dielectric i variably is warmed and breaks
down.
The employment of oil is advisable or necessary even with low
frequencies, if the potentials are such that streamers form, but only in
such cases, as is evident from the theory of the action. If the
potentials are so low that streamers do not form, then it is even
disadvantageous to employ oil, for it may, principally by confining the
heat, be the cause of the breaking down of the insulation.
The exclusion of gaseous matter is not only desirable on account of the
safety of the apparatus, but also on account of economy, especially in a
condenser, in which considerable waste of power may occur merely owing
to the presence of air, if the electric density on the charged surfaces
is great.
In the course of these investigations a phenomenon of special scientific
interest was observed. It may be ranked among the brush phenomena, in
fact it is a kind of brush which forms at, or near, a single terminal in
high vacuum. In a bulb with a conducting electrode, even if the latter
be of aluminum, the brush has only a very short existence, but it can be
preserved for a considerable length of time in a bulb devoid of any
conducting electrode. To observe the phenomenon it is found best to
employ a large spherical bulb having in its centre a small bulb
supported on a tube sealed to the neck of the former. The large bulb
being exhausted to a high degree, and the inside of the small bulb being
connected to one of the terminals of the coil, under certain conditions
there appears a misty haze around the small bulb, which, after passing
through some stages, assumes the form of a brush, generally at right
angles to the tube supporting the small bulb. When the brush assumes
this form it may be brought to a state of extreme sensitiveness to
electrostatic and magnetic influence. The bulb hanging straight down,
and all objects being remote from it, the approach of the observer
within a few paces will cause the brush to fly to the opposite side, and
if he walks around the bulb it will always keep on the opposite side. It
may begin to spin around the terminal long before it reaches that
sensitive stage. When it begins to turn around, principally, but also
before, it is affected by a magnet, and at a certain stage it is
susceptible to magnetic influence to an astonishing degree. A small
permanent magnet, with its poles at a distance of no more than two
centimetres will affect it visibly at a distance of two metres, slowing
down or accelerating the rotation according to how it is held relatively
to the brush.
When the bulb hangs with the globe down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction, and on the (magnetic) equator the brush should not turn at
all. The rotation may be reversed by a magnet kept at some distance. The
brush rotates best, seemingly, when it is at right angles to the lines
of force of the earth. It, very likely rotates, when at its maximum
speed, in synchronism with the alternations, say, 10,000 times a second.
The rotation can be slowed down or accelerated by the approach or
recession of the observer, or any conducting body, but it cannot be
reversed by putting the bulb in any position. Very curious experiments
may be performed with the brush when in its most sensitive state. For
instance, the brush resting in one position, the experimenter may, by
selecting a proper position, approach the hand at a certain considerable
distance to the bulb, and he may cjuisi' the brush to pass oft bv merely
stiffening the muscles of the arm, the mere change of configuration of
the arm and the consequent imperceptible displacement being sufficient
to disturb the delicate balance. When it begins to rotate slowly, and
tinhands are held at a proper distance, it is impossible to make even
the slightest motion without producing a visible effect upon the brush.
A metal plate connected to the other terminal of the coil affects it at
a great distance, slowing down the rotation often to one turn a second.
Mr. Tesla hopes that this phenomenon will prove a valuable aid in the
investigation of the nature of the forces acting in an electrostatic or
magnetic field. If there is any motion which is measurable going on in
the space, such a brush would be apt to reveal it. It is, so to speak, a
beam of light, frictionless, devoid of inertia. On account of its
marvellous sensitiveness to electrostatic or magnetic disturbances it
may be the means of sending signals through submarine cables with any
speed, and even of transmitting intelligence to a .distance without
wires.
In operating an induction coil with these rapidly alternating currents,
it is astonishing to note, for the first time, the great importance of
the relation of capacity, self-induction, and frequency as bearing upon
the general result. The combined effect of these elements produces many
curious effects. For instance. two metal plates are connected to the
terminals and set at a small distance, so that an arc is formed between
them. This arc />/vvents a strong current from flowing through the coil.
If the artbe interrupted by the interposition of a glass plate, the
capacity of the condenser obtained counteracts the self-induction, and a
stronger current is made to pass. The effects of capacity are the most
striking, for in these experiments, since the self-induction and
frequency both are high, the critical capacity is very small, and need
be but slightly varied to produce a very considerable change. The
experimenter brings his body in contact with the terminals of the
secondary of the coil, or attaches to one or both terminals insulated
bodies of very small bulk, such as exhausted bulbs, and he produces a
considerable rise or fall of potential on the secondary, and greatly
affects the flow of the current through the primary coil.
In many of the phenomena observed, the presence of the air, or,
generally speaking, of a medium of a gaseous nature (using this term not
to imply specific properties, but in contradistinction to homogeneity or
perfect continuity) plays an important part, as it allows energy to be
dissipated by molecular impact or bombardment. The action is thus
explained: When an insulated body connected to a terminal of the coil is
suddenly charged to high potential, it acts inductively upon the
surrounding air, or whatever gaseous medium there might be. The
molecules or atoms which are near it are, of course, more attracted, and
move through a greater distance than the further ones. When the nearest
molecules strike the body they are repelled, and collisions occur at all
distances within the inductive distance. It is now clear that, if the
potential be steady, bat little loss of energy can be caused in this
way, for the molecules which are nearest to the body having had an
additional charge imparted to them by contact, are not attracted until
they have parted, if not with all, at least with most of the additional
charge, which can be accomplished only after a great many collisions.
This is inferred from the fact that with a steady potential there is but
little loss in dry air. When the potential, instead of being steady, is
alternating, the conditions are entirely different. In this case a
rhythmical bombardment occurs, no matter whether the molecules after
coming in contact with the body lose the imparted charge or not, and,
what is more, if the charge is not lost, the impacts are all the more
violent. Still, if the frequency of the impulses be very small, the loss
caused by the impacts and collisions would not be serious unless the
potential was excessive. But when extremely high frequencies and more or
less high potentials are used, the loss may be very great, The total
energy lost per unit of time is proportionate to the product of the
number of impacts per second, or the frequency and the energy lost in
each impact. But the energy of an impact must be proportionate to the
square of the electric density of the body, on the assumption that the
charge imparted to the molecule is proportionate to that density. It is
concluded from this that the total energy lost must be proportionate to
the product of the frequency and the square of the electric density; but
this law needs experimental confirmation. Assuming the preceding
considerations to be true, then, by rapidly alternating the potential of
a body immersed in an insulating gaseous medium, any amount of energy
may be dissipated into space. Most of that energy, then, is not
dissipated in the form of long ether waves, propagated to considerable
distance, as is thought most generally, but is consumed in impact and
collisional losses that is, heat vibrations on the surface and in the
vicinity of the body. To reduce the dissipation it is necessary to work
with a small electric density the smaller, the higher the frequency.
The behavior of a gaseous medium to such rapid alternations of potential
makes it appear plausible that electrostatic disturbances of the earth,
produced by cosmic events, may have great influence upon the
meteorological condition^. When such disturbances occur both the
frequency of the vibrations of the charge and the potential are in all
probability excessive, and the energy converted into heat may be
considerable. Since the density must be unevenly distributed, either in
consequence of the irregularity of the earth's surface, or on account of
the condition of the atmosphere in various places, the effect produced
would accordingly vary from place to place. Considerable variations in
the temperature and pressure of the atmosphere may in this manner be
caused at any point of the surface of the earth. The variations may be
gradual or very sudden, according to the nature of the original
disturbance, and may produce rain and storms, or locally modify the
weather in any way.
From many experiences gathered in the course of these investigations it
appears certain that in lightning discharges the air is an element of
importance. For instance, during a storm a stream may form on a nail or
pointed projection of a building. If lightning strikes somewhere in the
neighborhood* the harmless static discharge may, in consequence of the
oscillations set up, assume the character of a high-frequency streamer,
and the nail or projection may be brought to a high temperature by the
violent impact of the air molecules. Thus, it is thought, a building may
be set on fire without the lightning striking it. In like manner small
metallic objects may be fused and volatilized as frequently occurs in
lightning discharges merely because they are surrounded by air. Were
they immersed in a practically continuous medium, such as oil, they
would probably be safe, as the energy would have to spend itself
elsewhere.
An instructive experience having a bearing on this subject is the
following: A glass tube of an inch or so in diameter and several inches
long is taken, and a platnium wire sealed into it, the wire running
through the center of the tube from end to end. The tube is exhausted to
a moderate degree. If a steady current is passed through the wire it is
heated uniformly in all parts and the gas in the tube is of no
consequence. But if high frequency discharges are directed through the
wire, it is heated more on the ends than in the middle portion, and if
the frequency, or rate of charge, is high enough, the wire might as well
be cut in the middle as not, for most of the heating on the ends is due
to the rarefied gas. Here the gas might only act as a conductor of no
impedance, diverting the current from the wire as the impedance of the
latter is enormously increased, and merely heating the ends of the wire
by reason of their resistance to the passage of the discharge. But it is
not at all necessary that the gas in the tube should he conducting ; it
might be at an extremely low pressure, still the ends of the wire would
be heated ; however, as is ascertained by experience, only the two ends
would in such case not be electrically connected through the gaseous
medium. Now, what with these frequencies and potentials occurs in an
exhausted tube, occurs in the lightning discharge at ordinary pressure.
From the facility with which any amount of energy may be carried off
through a gas, Mr. Tesla infers that the best w T ay to render harmless
a lightning discharge is to afford it in some way a passage through a
volume of gas.
The recognition of some of the above facts has a bearing upon
far-reaching scientific investigations in which extremely high
frequencies and potentials are used. In such cases the air is an
important factor to be considered. So, for instance, if two wires are
attached to the terminals of the coil, and the streamers issue from'
them, there is dissipation of energy in the form of heat and light, and
the wires behave like a condenser of larger capacity. If the wires be
immersed in oil, the dissipation of energy is prevented, or at least
reduced, and the apparent capacity is diminished. The action of the air
would seem to make it very difficult to tell, from the measured or
computed capacity of a condenser in which the air is acted upon, its
actual capacity or vibration period, especially if the condenser is of
very small surface and is charged to a very high potential. As many
important results are dependant upon the correctness of the estimation
of the vibration period, this subject demands the most careful scrutiny
of investigators.
In Leyden jars the loss due to the presence of air is comparatively
small, principally on account of the great surface of the coatings and
the small external action, but if there are streamers on the top, the
loss may be considerable, and the period of vibration is affected. In a
resonator, the density is small, but the frequency is extreme, and may
introduce a considerable error. It appears certain, at any rate, that
the periods of vibration of a charged body in a gaseous and in a
continuous medium, such as oil, are different, on account of the action
of the former, as explained.
Another fact recognized, which is of some consequence, is, that in
similar investigations the general considerations of static screening
are not applicable when a gaseous medium is present. This is evident
from the following experiment : A short and wide glass tube is taken and
covered with a substantial coating of bronze powder, barely allowing the
light to shine a little through. The tube is highly exhausted and
suspended on a metallic clasp from the end of a wire. When the wire is
connected with one of the terminals of the coil, the gas inside of the
tube is lighted in spite of the metal coating. Here the metal evidently
does not screen the gas inside as it ought to, even if it be very thin
and poorly conducting. Yet, in a condition of rest the metal coating,
however thin, screens the inside perfectly.
One of the most interesting results arrived at in pursuing these
experiments, is the demonstration of the fact that a gaseous medium,
upon which vibration is impressed by rapid changes of electrostatic
potential, is rigid. In illustration of this result an experiment made
by Mr. Tesla may by cited : A glass tube about one inch in diameter and
three feet long, with outside condenser coatings on the ends, was
exhausted to a certain point, when, the tube being suspended freely from
a wire connecting the upper coating to one of the terminals of the coil,
the discharge appeared in the form of a luminous thread passing through
the axis of the tube. Usually the thread was sharply defined in the
upper part of the tube and lost itself in the lower part. When a magnet
or the finger was quickly passed near the upper part of the luminous
thread, it was brought out of position by magnetic or electrostatic
influence, and a transversal vibration like that of a suspended cord,
with one or more distinct nodes, was set up, which lasted for a few
minutes and gradually died out. By suspending from the lower condenser
coating metal plates of different sizes, the speed of the vibration was
varied. This vibration would seem to show beyond doubt that the thread
possessed rigidity, at least to transversal displacements.
Many experiments were tried to demonstrate this property in air at
ordinary pressure. Though no positive evidence has been obtained, it is
thought, nevertheless, that a high frequency brush or streamer, if the
frequency could be pushed far enough, would be decidedly rigid. A small
sphere might then be moved within it quite freely, but if tin*own
against it the sphere would rebound. An ordinary flame cannot possess
rigidity to a marked degree because the vibration is directionless ; but
an electric arc, it is believed, must possess that property more or
less. A luminous band excited in a bulb by repeated discharges of a
Leyden jar must also possess rigidity, and if deformed and suddenly
released should vibrate.
From like considerations other conclusions of interest are readied. The
most probable medium filling the space is one consisting of independent
carriers immersed in an insulating fluid. If through' this medium
enormous electrostatic stresses are assumed to act, which vary rapidly
in intensity, it would allow the motion of a body through it, yet it
would be rigid and elastic, although the fluid itself might be devoid of
these properties. Furthermore, on the assumption that the independent
carriers are of any configuration such that the fluid resistance to
motion in one direction is greater than in another, a stress of that
nature would cause the carriers to arrange themselves in groups, since
they would turn to each other their sides of the greatest electric
density, in which position the fluid resistance to approach would be
smaller than to receding. If in a medium of the above characteristics a
brush would be formed by a steady potential, an exchange of the carriers
would go on continually, and there would be less carriers per unit of
volume in the brush than in the space at some distance from the
electrode, this corresponding to rarefaction. If the potential were
rapidly changing, the result would be very different ; the higher the
freqency of the pulses, the slower would be the exchange of the carriers
; finally, the motion of translation through measurable space would
cease, and, with a sufficiently high frequency and intensity of the
stress, the carriers would be drawn towards the electrode, and
compression would result.
An interesting feature of these high frequency currents is that they
allow of operating all kinds of devices by connecting the device with
only one leading wire to the electric source. In fact, under certain
conditions it may be more economical to supply the electrical energy
witli one lead than with two.
An experiment of special interest shown by Mr. Tesla, is the running, by
the use of only one insulated line, of a motor operating on the
principle of the rotating magnetic field enunciated by Mr. Tesla. A
simple form of such a motor is obtained by winding upon a laminated iron
core a primary and close to it a secondary coil, closing the ends of the
latter and placing a freely movable metal disc within the influence of
the moving field. The secondary coil may, however, be omitted. When one
of the ends of the primary coil of the motor is connected to one of the
terminals of the high frequency coil arid the other end to an insulated
metal plate, which, it should be stated, is not absolutely necessary for
the success of the experiment, the disc is set in rotation.
Experiments of this kind seem to bring it within possibility to operate
a motor at any point of the earth's surface from a central source,
without any connection to the same except through the earth. If, by
means of powerful machinery, rapid variations of the earth's potential
were produced, a grounded wire reaching up to some height would be
traversed by a current which could be increased by connecting the free
end of the wire to a body of some size. The current might be converted
to low tension and used to operate a motor or other device. The
experiment, which would be one of great scientific interest, would
probably best succeed on a ship at sea. In this manner, even if it were
not possible to operate machinery, intelligence might be transmitted
quite certainly.
In the course of this experimental study special attention was devoted
to the heating effects produced by these currents, which are not only
striking, but open up the possibility of producing a more efficient
illuminant. It is sufficient to attach to the coil terminal a thin wire
or filament, to have the temperature of the latter perceptibly raised.
If the wire or filament be enclosed in a bulb, the heating effect is
increased by preventing the circulation of the air. If the air in the
bulb be strongly compressed, the displacements are smaller, the impacts
less violent, and the heating effect is diminished. On the contrary, if
the air in the bulb be exhausted, an inclosed lamp filament is brought
to incandescence, and any amount of light may thus be produced.
The heating of the inclosed lamp filament depends on so many things of a
different nature, that it is difficult to give a generally applicable
rule under which the maximum heating occurs. As regards the size of the
bull), it is ascertained that at ordinary or only slightly differing
atmospheric pressures, when air is a good insulator, the filament is
heated more in a small bulb, because of the better confinement of heat
in this case. At lower pressures, when air becomes conducting, the
heating effect is greater in a large bull), but at excessively high
degrees of exhaustion there seems to be, beyond a certain and rather
small size of the vessel, no perceptible difference in the heating.
The shape of the vessel is also of some importance, and it has been
found of advantage for reasons of economy to employ a spherical bulb
with the electrode mounted in its centre, where the rebounding molecules
collide.
It is desirable on account of economy that all the energy supplied to
the bulb from the source should reach without loss the body to be
heated. The loss in conveying the energy from the source to the body may
be reduced by employing thin wires heavily coated with insulation, and
by the use of electrostatic screens. It is to be remarked, that the
screen, cannot be connected to the ground as under ordinary conditions.
In the bulb itself a large portion of the energy' supplied may be lost
by molecular bombardment against the wire connecting the body to be
heated with the source. Considerable improvement was effected by
covering the glass stem containing the wire with a closely fitting
conducting tube. This tube is made to project a little above the glass,
and prevents the cracking of the latter near the heated body. The
effectiveness of the conducting tube is limited to very high degrees of
exhaustion. It diminishes the energy lost in bombardment for two
reasons; first, the charge given up by the atoms spreads over a greater
area, and hence the electric density at any point is small, and the
atoms are repelled with less energy than if they would strike against a
good insulator; secondly, as the tube is electrified by the atoms which
first come in contact with it, the progress of the following atoms
against the tube is more or less checked by the repulsion which the
electrified tube must exert upon the similarly electrified atoms. This,
it is thought, explains why the discharge through a bulb is established
with much greater facility when an insulator, than when a conductor, is
present.
During the investigations a great many bulbs of different construction,
with electrodes of different material, were experimented upon, and a
number of observations of interest were made. Mr. Tesla has found that
the deterioration of the electrode is the less, the higher the
frequency. This was to be expected, as then the heating is effected by
many small impacts, instead by fewer and more violent ones, which
quickly shatter the structure. The deterioration is also smaller when
the vibration is harmonic. Thus an electrode, maintained at a certain
degree of heat, lasts much longer with currents obtained from an
alternator, than with those obtained by means of a disruptive discharge.
One of the most durable electrodes was obtained from strongly compressed
carborundum, which is a kind of carbon recently produced by Mr. E. G.
Acheson, of Monongahela City, Pa. From experience, it is inferred, that
to be most durable, the electrode should be in the form of a sphere with
a highly polished surface.
In some bulbs refractory bodies were mounted in a carbon cup and put
under the molecular impact. It was observed in such experiments that the
carbon cup was heated at first, until a higher temperature was reached;
then most of the bombardment was directed against the refractory body,
and the carbon was relieved. In general, when different bodies were
mounted in the bulb, the hardest fusible would be relieved, and would
remain at a considerably lower temperature. This was necessitated by the
fact that most of the energy supplied would find its way through the
body \vhioh was more easily fused or "evaporated."
Curiously enough it appeared in some of the experiments made, that a
body was fused in a bulb under the molecular impact by evolution" of
less light than when fused by the application of heat in ordinary ways.
This may be ascribed to a loosening of the structure of the body under
the violent impacts and changing stresses.
Some experiments seem to indicate that under certain conditions a body,
conducting or nonconducting, may, when bombarded, emit light, which to
all appearances is due to phosphorescence, but may in reality be caused
by the incandescence of an infinitesimal layer, the mean temperature of
the body being comparatively small. Such might be the case if each
single rhythmical impact were capable of instantaneously exciting the
retina, and the rhythm were just high enough to cause a continuous
impression in the eye. According to this view, a coil operated by
disruptive discharge would be eminently adapted to produce such a
result, and it is found by experience that its power of exciting
phosphorescence is extraordinarily great. It is capable of exciting
phosphorescence at comparatively low degrees of exhaustion, and also
projects shadows at pressures far greater than those at which the mean
free path is comparable to the dimensions of the vessel. The latter
observation is of some importance, inasmuch as it may modify the
generally accepted views in regard to the "radiant state" phenomena.
A thought which early and naturally suggested itself to JVI r. Tesla,
was to utilize the great inductive effects of high frequency currents to
produce light in a sealed glass vessel without the use of leading in
wires. Accordingly, many bulbs were constructed in which the energy
necessary to maintain a button or filament at high incandescence, was
supplied through the glass by either electrostatic or electrodynamic
induction. It was easy to regulate the intensity of the light emitted by
means of an externally applied condenser coating connected to an
insulated plate, or simply by means of a plate attached to the bulb
which at the same time performed the function of a shade.
A subject of experiment, which has been exhaustively treated in England
by Prof. J. J. Thomson, has been followed up independently by Mr. Tesla
from the beginning of this study, namely, to excite by electrodynamic
induction a luminous band in a closed tube or bulb. In observing the
behavior of gases, and the luminous phenomena obtained, the importance
of the electrostatic effects was noted and it appeared desirable to
produce enormous potential differences, alternating with extreme
rapidity. Experiments in this direction led to some of the most
interesting results arrived at in the course of these investigations. It
was found that by rapid alternations of a high electrostatic potential,
exhausted tubes could be lighted at considerable distances from a
conductor connected to a properly constructed coil, and that it was
practicable to establish with the coil an alternating electrostatic
field, acting through the whole room and lighting a tube wherever it was
placed within the four walls. Phosphorescent bulbs may be excited in
such a field, and it is easy to regulate the effect by connecting to the
bulb a small insulated metal plate. It was likewise possible to maintain
a filament or button mounted in a tube at bright incandescence, and, in
one experiment, a mica vane was spun by the incandescence of a platinum
wire.
Coming now to the lecture delivered in Philadelphia and St. Louis, it
may be remarked that to the superficial reader, Mr. Tesla's
introduction, dealing with the importance of the eye, might appear as a
digression, but the thoughtful reader will find therein much food for
meditation and speculation. Throughout his discourse one can trace Mr.
Tesla's effort to present in a popular way thoughts and views on the
electrical phenomena which have in recent years captivated the
scientific world, but of which the general public has even yet merely
received an inkling. Mr. Tesla also dwells rather extensively on his
well-known method of high-frequency conversion ; and the large amount of
detail information will be gratefully received by students and
experimenters in this virgin field. The employment of apt analogies in
explaining the fundamental principles involved makes it easy for all to
gain a clear idea of their nature. Again, the ease with which, thanks to
Mr. Tesla's efforts, these high-frequency currents may now be obtained
from circuits carrying almost any kind of current, cannot fail to result
in an extensive broadening of this field of research, which offers so
many possibilities. M r. Tesla, true philosopher as he is, does not
hesitate to point out defects in some of his methods, and indicates the
lines which to him seem the most promising. Particular stress is laid by
him upon the employment of a medium in which the discharge electrodes
should be immersed in order that this method of conversion may be
brought to the highest perfection. He has evidently taken pains to give
as much useful information as possible to those who wish to follow in
his path, as he shows in detail the circuit arrangements to be adopted
in all ordinary cases met with in practice, and although some of these
methods were described by him two years before, the additional
information is still timely and welcome.
In his experiments he dwells first on some phenomena produced by
electrostatic force, which he considers in the light of modern theories
to be the most important force in nature for us to investigate. At the
very outset he shows a strikingly novel experiment illustrating the
effect of a rapidly varying electrostatic force in a gaseous medium, by
touching with one hand one of the terminals of a 200,000 volt
transformer and bringing tinother hand to the opposite terminal. The
powerful streamers which issued from his hand and astonished his
audiences formed a capital illustration of some of the views advanced,
and afforded Mr. Tesla an opportunity of pointing out the true reasons
why, with these currents, such an amount of energy can be passed through
the body with impunity. He then showed by experiment the difference
between a steady and a rapidly varying force upon the dielectric. This
difference is most strikingly illustrated in the experiment in which a
bulb attached to the end of a wire in connection with one of the
terminals of the transformer is ruptured, although all extraneous bodies
are remote from the bulb. He next illustrates how mechanical motions are
produced by a varying electrostatic force acting through a gaseous
medium. The importance of the action of the air is particularly
illustrated by an interesting experiment.
Taking up another class of phenomena, namely, those of dynamic
electricity, Mr. Tesla produced in a number of experiments a variety of
effects by the employment of only a single wire with the evident intent
of impressing upon his audience the idea that electric vibration or
current can be transmitted witli ease, without any return circuit ; also
how currents so transmitted can be converted and used for many practical
purposes. A number of experiments are then shown, illustrating the
effects of frequency, self-induction and capacity; then a number of ways
of operating motive and other devices by the use of a single lead. A
number of novel impedance phenomena are also shown which cannot fail to
arouse interest.
Mr. Tesla next dwelt upon a subject which he thinks of great importance,
that is, electrical resonance, which he explained in a popular way. He
expressed his firm conviction that by observing proper conditions,
intelligence, and possibly even power, can be transmitted through the
medium or through the earth; and he considers this problem worthy of
serious and immediate consideration.
Coming now to the light phenomena in particular, lie illustrated the
four distinct kinds of these phenomena in an original way, which to many
must have been a revelation. Mr. Tesla attributes these light effects to
molecular or atomic impacts produced by a varying electrostatic stress
in a gaseous medium. Fie illustrated in a series of novel experiments
the effect of the gas surrounding the conductor and shows beyond a doubt
that with high frequency and high potential currents, the surrounding
gas is of paramount importance in the heating of the conductor. He
attributes the heating partially to a conduction current and partially
to bombardment, and demonstrates that in many cases the heating may be
practically due to the bombardment alone. He pointed out also that the
skin effect is largely modified by the presence of the gas or of an
atomic medium in general. He showed also some interesting experiments in
which the effect of convection is illustrated. Probably one of the most
curious experiments in this connection is that in which a thin platinum
wire stretched along the axis of an exhausted tube is brought to
incandescence at certain points corresponding to the position of the
striae, while at others it remains dark. This experiment throws an
interesting light upon the nature of the strife and may lead to
important revelations.
Mr. Tesla also demonstrated the dissipation of energy through an atomic
medium and dwelt upon the behavior of vacuous space in conveying heat,
and in this connection showed the curious behavior of an electrode
stream, from which he concludes that the molecules of a gas probably
cannot be acted upon directly at measurable distances.
Mr. Tesla summarized the chief results arrived at in pursuing his
investigations in a manner which will serve as a valuable guide to all
who may engage in this work. Perhaps most interest will centre on his
general statements regarding the phenomena of phosphorescence, the most
important fact revealed in this direction being that when exciting a
phosphorescent bulb a certain definite potential gives the most
economical result.
The lectures will now be presented in the order of their date of
delivery. |
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