A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN COILS OF
PRIMARY AND SECONDARY.
Mr. Tesla has applied his principle of magnetic shielding of parts to
the construction also of transformers, the shield being interposed
between the primary and secondary coils. In transformers of the ordinary
type it will be found that the wave of electromotive force of the
secondary very nearly coincides with that of the primary, being,
however, in opposite sign. At the same time the currents, both primary
and secondary, lag behind their respective electromotive forces; but as
this lag is practically or nearly the same in the case of each it
follows that the maximum and minimum of the primary and secondary
currents will nearly coincide, but differ in sign or direction, provided
the secondary be not loaded or if it contain devices having the property
of self-induction. On the other hand, the lag of the primary behind the
impressed electromotive force may be diminished by loading the secondary
with a non-inductive or dead resistance--such as incandescent
lamps--whereby the time interval between the maximum or minimum periods
of the primary and secondary currents is increased. This time interval,
however, is limited, and the results obtained by phase difference in the
operation of such devices as the Tesla alternating current motors can
only be approximately realized by such means of producing or securing
this difference, as above indicated, for it is desirable in such cases
that there should exist between the primary and secondary currents, or
those which, however produced, pass through the two circuits of the
motor, a difference of phase of ninety degrees; or, in other words, the
current in one circuit should be a maximum when that in the other
circuit is a minimum. To attain to this condition more perfectly, an
increased retardation of the secondary current is secured in the
following manner: Instead of bringing the primary and secondary coils or
circuits of a transformer into the closest possible relations, as has
hitherto been done, Mr. Tesla protects in a measure the secondary from
the inductive action or effect of the primary by surrounding either the
primary or the secondary with a comparatively thin magnetic shield or
screen. Under these modified conditions, as long as the primary current
has a small value, the shield protects the secondary; but as soon as the
primary current has reached a certain strength, which is arbitrarily
determined, the protecting magnetic shield becomes saturated and the
inductive action upon the secondary begins. It results, therefore, that
the secondary current begins to flow at a certain fraction of a period
later than it would without the interposed shield, and since this
retardation may be obtained without necessarily retarding the primary
current also, an additional lag is secured, and the time interval
between the maximum or minimum periods of the primary and secondary
currents is increased. Such a transformer may, by properly proportioning
its several elements and determining the proper relations between the
primary and secondary windings, the thickness of the magnetic shield,
and other conditions, be constructed to yield a constant current at all
loads.
[Illustration: FIG. 95.]
Fig. 95 is a cross-section of a transformer embodying this improvement.
Fig. 96 is a similar view of a modified form of transformer, showing
diagrammatically the manner of using the same.
A A is the main core of the transformer, composed of a ring of soft
annealed and insulated or oxidized iron wire. Upon this core is wound
the secondary circuit or coil B B. This latter is then covered with a
layer or layers of annealed and insulated iron wires C C, wound in a
direction at right angles to the secondary coil. Over the whole is then
wound the primary coil or wire D D. From the nature of this construction
it will be obvious that as long as the shield formed by the wires C is
below magnetic saturation the secondary coil or circuit is effectually
protected or shielded from the inductive influence of the primary,
although on open circuit it may exhibit some electromotive force. When
the strength of the primary reaches a certain value, the shield C,
becoming saturated, ceases to protect the secondary from inductive
action, and current is in consequence developed therein. For similar
reasons, when the primary current weakens, the weakening of the
secondary is retarded to the same or approximately the same extent.
[Illustration: FIG. 96.]
The specific construction of the transformer is largely immaterial. In
Fig. 96, for example, the core A is built up of thin insulated iron
plates or discs. The primary circuit D is wound next the core A. Over
this is applied the shield C, which in this case is made up of thin
strips or plates of iron properly insulated and surrounding the primary,
forming a closed magnetic circuit. The secondary B is wound over the
shield C. In Fig. 96, also, E is a source of alternating or rapidly
changing currents. The primary of the transformer is connected with the
circuit of the generator. F is a two-circuit alternating current motor,
one of the circuits being connected with the main circuit from the
source E, and the other being supplied with currents from the secondary
of the transformer.
PART II.
THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.
CHAPTER XXV.
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 Francaise 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 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 the best 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 experimental result achieved is the
operation of lamps, and even 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
Cardew 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 New
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 English love of scientific truth and
appreciation of its votaries. As an indefatigable experimenter, Mr.
Tesla was certainly nowhere more at home than in the haunts of Faraday,
and as the guest of Faraday's successor. This Royal Institution lecture
summed up the leading points of Mr. Tesla's work, in the high potential,
high frequency field, and we may here avail ourselves 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
1/16 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 of 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 flaming 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. Owing 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, yet 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 area 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 gaseous 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 invariably 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 cause the brush to pass off by 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
the hands are held at a proper distance, it is impossible to make even
the slightest motion without producing a visible effect upon the brush.
A metal plate connected to the other terminal of the coil affects it at
a great distance, slowing down the rotation often to one turn a second.
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 _prevents_ a strong current from flowing through the
coil. If the arc be 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, but 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 conditions. 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 platinum 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 be 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 way 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 thrown 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 reached. 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
frequency 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
with 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 and 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
bulb, 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 bulb, 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 which 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 Mr. 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. Mr. 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 the other 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 with 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, he 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. He 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 striae 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.
CHAPTER XXVI.
EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY HIGH FREQUENCY AND THEIR
APPLICATION TO METHODS OF ARTIFICIAL ILLUMINATION.[1]
[1] A lecture delivered before the American Institute of
Electrical Engineers, at Columbia College, N. Y.,
May 20, 1891.
There is no subject more captivating, more worthy of study, than nature.
To understand this great mechanism, to discover the forces which are
active, and the laws which govern them, is the highest aim of the
intellect of man.
Nature has stored up in the universe infinite energy. The eternal
recipient and transmitter of this infinite energy is the ether. The
recognition of the existence of ether, and of the functions it performs,
is one of the most important results of modern scientific research. The
mere abandoning of the idea of action at a distance, the assumption of a
medium pervading all space and connecting all gross matter, has freed
the minds of thinkers of an ever present doubt, and, by opening a new
horizon--new and unforeseen possibilities--has given fresh interest to
phenomena with which we are familiar of old. It has been a great step
towards the understanding of the forces of nature and their multifold
manifestations to our senses. It has been for the enlightened student of
physics what the understanding of the mechanism of the firearm or of the
steam engine is for the barbarian. Phenomena upon which we used to look
as wonders baffling explanation, we now see in a different light. The
spark of an induction coil, the glow of an incandescent lamp, the
manifestations of the mechanical forces of currents and magnets are no
longer beyond our grasp; instead of the incomprehensible, as before,
their observation suggests now in our minds a simple mechanism, and
although as to its precise nature all is still conjecture, yet we know
that the truth cannot be much longer hidden, and instinctively we feel
that the understanding is dawning upon us. We still admire these
beautiful phenomena, these strange forces, but we are helpless no
longer; we can in a certain measure explain them, account for them, and
we are hopeful of finally succeeding in unraveling the mystery which
surrounds them.
In how far we can understand the world around us is the ultimate thought
of every student of nature. The coarseness of our senses prevents us
from recognizing the ulterior construction of matter, and astronomy,
this grandest and most positive of natural sciences, can only teach us
something that happens, as it were, in our immediate neighborhood: of
the remoter portions of the boundless universe, with its numberless
stars and suns, we know nothing. But far beyond the limit of perception
of our senses the spirit still can guide us, and so we may hope that
even these unknown worlds--infinitely small and great--may in a measure
become known to us. Still, even if this knowledge should reach us, the
searching mind will find a barrier, perhaps forever unsurpassable, to
the _true_ recognition of that which _seems_ to be, the mere
_appearance_ of which is the only and slender basis of all our
philosophy.
Of all the forms of nature's immeasurable, all-pervading energy, which
ever and ever changing and moving, like a soul animates the inert
universe, electricity and magnetism are perhaps the most fascinating.
The effects of gravitation, of heat and light we observe daily, and soon
we get accustomed to them, and soon they lose for us the character of
the marvelous and wonderful; but electricity and magnetism, with their
singular relationship, with their seemingly dual character, unique among
the forces in nature, with their phenomena of attractions, repulsions
and rotations, strange manifestations of mysterious agents, stimulate
and excite the mind to thought and research. What is electricity, and
what is magnetism? These questions have been asked again and again. The
most able intellects have ceaselessly wrestled with the problem; still
the question has not as yet been fully answered. But while we cannot
even to-day state what these singular forces are, we have made good
headway towards the solution of the problem. We are now confident that
electric and magnetic phenomena are attributable to ether, and we are
perhaps justified in saying that the effects of static electricity are
effects of ether under strain, and those of dynamic electricity and
electro-magnetism effects of ether in motion. But this still leaves the
question, as to what electricity and magnetism are, unanswered.
First, we naturally inquire, What is electricity, and is there such a
thing as electricity? In interpreting electric phenomena, we may speak
of electricity or of an electric condition, state or effect. If we speak
of electric effects we must distinguish two such effects, opposite in
character and neutralizing each other, as observation shows that two
such opposite effects exist. This is unavoidable, for in a medium of the
properties of ether, we cannot possibly exert a strain, or produce a
displacement or motion of any kind, without causing in the surrounding
medium an equivalent and opposite effect. But if we speak of
electricity, meaning a _thing_, we must, I think, abandon the idea of
two electricities, as the existence of two such things is highly
improbable. For how can we imagine that there should be two things,
equivalent in amount, alike in their properties, but of opposite
character, both clinging to matter, both attracting and completely
neutralizing each other? Such an assumption, though suggested by many
phenomena, though most convenient for explaining them, has little to
commend it. If there _is_ such a thing as electricity, there can be only
_one_ such thing, and excess and want of that one thing, possibly; but
more probably its condition determines the positive and negative
character. The old theory of Franklin, though falling short in some
respects, is, from a certain point of view, after all, the most
plausible one. Still, in spite of this, the theory of the two
electricities is generally accepted, as it apparently explains electric
phenomena in a more satisfactory manner. But a theory which better
explains the facts is not necessarily true. Ingenious minds will invent
theories to suit observation, and almost every independent thinker has
his own views on the subject.
It is not with the object of advancing an opinion, but with the desire
of acquainting you better with some of the results, which I will
describe, to show you the reasoning I have followed, the departures I
have made--that I venture to express, in a few words, the views and
convictions which have led me to these results.
I adhere to the idea that there is a thing which we have been in the
habit of calling electricity. The question is, What is that thing? or,
What, of all things, the existence of which we know, have we the best
reason to call electricity? We know that it acts like an incompressible
fluid; that there must be a constant quantity of it in nature; that it
can be neither produced nor destroyed; and, what is more important, the
electro-magnetic theory of light and all facts observed teach us that
electric and ether phenomena are identical. The idea at once suggests
itself, therefore, that electricity might be called ether. In fact, this
view has in a certain sense been advanced by Dr. Lodge. His interesting
work has been read by everyone and many have been convinced by his
arguments. His great ability and the interesting nature of the subject,
keep the reader spellbound; but when the impressions fade, one realizes
that he has to deal only with ingenious explanations. I must confess,
that I cannot believe in two electricities, much less in a
doubly-constituted ether. The puzzling behavior of the ether as a solid
to waves of light and heat, and as a fluid to the motion of bodies
through it, is certainly explained in the most natural and satisfactory
manner by assuming it to be in motion, as Sir William Thomson has
suggested; but regardless of this, there is nothing which would enable
us to conclude with certainty that, while a fluid is not capable of
transmitting transverse vibrations of a few hundred or thousand per
second, it might not be capable of transmitting such vibrations when
they range into hundreds of million millions per second. Nor can anyone
prove that there are transverse ether waves emitted from an alternate
current machine, giving a small number of alternations per second; to
such slow disturbances, the ether, if at rest, may behave as a true
fluid.
Returning to the subject, and bearing in mind that the existence of two
electricities is, to say the least, highly improbable, we must remember,
that we have no evidence of electricity, nor can we hope to get it,
unless gross matter is present. Electricity, therefore, cannot be called
ether in the broad sense of the term; but nothing would seem to stand in
the way of calling electricity ether associated with matter, or bound
ether; or, in other words, that the so-called static charge of the
molecule is ether associated in some way with the molecule. Looking at
it in that light, we would be justified in saying, that electricity is
concerned in all molecular actions.
Now, precisely what the ether surrounding the molecules is, wherein it
differs from ether in general, can only be conjectured. It cannot differ
in density, ether being incompressible: it must, therefore, be under
some strain or in motion, and the latter is the most probable. To
understand its functions, it would be necessary to have an exact idea of
the physical construction of matter, of which, of course, we can only
form a mental picture.
But of all the views on nature, the one which assumes one matter and one
force, and a perfect uniformity throughout, is the most scientific and
most likely to be true. An infinitesimal world, with the molecules and
their atoms spinning and moving in orbits, in much the same manner as
celestial bodies, carrying with them and probably spinning with them
ether, or in other words, carrying with them static charges, seems to my
mind the most probable view, and one which, in a plausible manner,
accounts for most of the phenomena observed. The spinning of the
molecules and their ether sets up the ether tensions or electrostatic
strains; the equalization of ether tensions sets up ether motions or
electric currents, and the orbital movements produce the effects of
electro and permanent magnetism.
About fifteen years ago, Prof. Rowland demonstrated a most interesting
and important fact, namely, that a static charge carried around produces
the effects of an electric current. Leaving out of consideration the
precise nature of the mechanism, which produces the attraction and
repulsion of currents, and conceiving the electrostatically charged
molecules in motion, this experimental fact gives us a fair idea of
magnetism. We can conceive lines or tubes of force which physically
exist, being formed of rows of directed moving molecules; we can see
that these lines must be closed, that they must tend to shorten and
expand, etc. It likewise explains in a reasonable way, the most puzzling
phenomenon of all, permanent magnetism, and, in general, has all the
beauties of the Ampere theory without possessing the vital defect of the
same, namely, the assumption of molecular currents. Without enlarging
further upon the subject, I would say, that I look upon all
electrostatic, current and magnetic phenomena as being due to
electrostatic molecular forces.
The preceding remarks I have deemed necessary to a full understanding of
the subject as it presents itself to my mind.
Of all these phenomena the most important to study are the current
phenomena, on account of the already extensive and ever-growing use of
currents for industrial purposes. It is now a century since the first
practical source of current was produced, and, ever since, the phenomena
which accompany the flow of currents have been diligently studied, and
through the untiring efforts of scientific men the simple laws which
govern them have been discovered. But these laws are found to hold good
only when the currents are of a steady character. When the currents are
rapidly varying in strength, quite different phenomena, often
unexpected, present themselves, and quite different laws hold good,
which even now have not been determined as fully as is desirable, though
through the work, principally, of English scientists, enough knowledge
has been gained on the subject to enable us to treat simple cases which
now present themselves in daily practice.
The phenomena which are peculiar to the changing character of the
currents are greatly exalted when the rate of change is increased, hence
the study of these currents is considerably facilitated by the
employment of properly constructed apparatus. It was with this and other
objects in view that I constructed alternate current machines capable of
giving more than two million reversals of current per minute, and to
this circumstance it is principally due, that I am able to bring to your
attention some of the results thus far reached, which I hope will prove
to be a step in advance on account of their direct bearing upon one of
the most important problems, namely, the production of a practical and
efficient source of light.
The study of such rapidly alternating currents is very interesting.
Nearly every experiment discloses something new. Many results may, of
course, be predicted, but many more are unforeseen. The experimenter
makes many interesting observations. For instance, we take a piece of
iron and hold it against a magnet. Starting from low alternations and
running up higher and higher we feel the impulses succeed each other
faster and faster, get weaker and weaker, and finally disappear. We then
observe a continuous pull; the pull, of course, is not continuous; it
only appears so to us; our sense of touch is imperfect.
We may next establish an arc between the electrodes and observe, as the
alternations rise, that the note which accompanies alternating arcs gets
shriller and shriller, gradually weakens, and finally ceases. The air
vibrations, of course, continue, but they are too weak to be perceived;
our sense of hearing fails us.
We observe the small physiological effects, the rapid heating of the
iron cores and conductors, curious inductive effects, interesting
condenser phenomena, and still more interesting light phenomena with a
high tension induction coil. All these experiments and observations
would be of the greatest interest to the student, but their description
would lead me too far from the principal subject. Partly for this
reason, and partly on account of their vastly greater importance, I will
confine myself to the description of the light effects produced by these
currents.
In the experiments to this end a high tension induction coil or
equivalent apparatus for converting currents of comparatively low into
currents of high tension is used.
If you will be sufficiently interested in the results I shall describe
as to enter into an experimental study of this subject; if you will be
convinced of the truth of the arguments I shall advance--your aim will
be to produce high frequencies and high potentials; in other words,
powerful electrostatic effects. You will then encounter many
difficulties, which, if completely overcome, would allow us to produce
truly wonderful results.
First will be met the difficulty of obtaining the required frequencies
by means of mechanical apparatus, and, if they be obtained otherwise,
obstacles of a different nature will present themselves. Next it will be
found difficult to provide the requisite insulation without considerably
increasing the size of the apparatus, for the potentials required are
high, and, owing to the rapidity of the alternations, the insulation
presents peculiar difficulties. So, for instance, when a gas is present,
the discharge may work, by the molecular bombardment of the gas and
consequent heating, through as much as an inch of the best solid
insulating material, such as glass, hard rubber, porcelain, sealing wax,
etc.; in fact, through any known insulating substance. The chief
requisite in the insulation of the apparatus is, therefore, the
exclusion of any gaseous matter.
In general my experience tends to show that bodies which possess the
highest specific inductive capacity, such as glass, afford a rather
inferior insulation to others, which, while they are good insulators,
have a much smaller specific inductive capacity, such as oils, for
instance, the dielectric losses being no doubt greater in the former.
The difficulty of insulating, of course, only exists when the potentials
are excessively high, for with potentials such as a few thousand volts
there is no particular difficulty encountered in conveying currents from
a machine giving, say, 20,000 alternations per second, to quite a
distance. This number of alternations, however, is by far too small for
many purposes, though quite sufficient for some practical applications.
This difficulty of insulating is fortunately not a vital drawback; it
affects mostly the size of the apparatus, for, when excessively high
potentials would be used, the light-giving devices would be located not
far from the apparatus, and often they would be quite close to it. As
the air-bombardment of the insulated wire is dependent on condenser
action, the loss may be reduced to a trifle by using excessively thin
wires heavily insulated.
Another difficulty will be encountered in the capacity and
self-induction necessarily possessed by the coil. If the coil be large,
that is, if it contain a great length of wire, it will be generally
unsuited for excessively high frequencies; if it be small, it may be
well adapted for such frequencies, but the potential might then not be
as high as desired. A good insulator, and preferably one possessing a
small specific inductive capacity, would afford a two-fold advantage.
First, it would enable us to construct a very small coil capable of
withstanding enormous differences of potential; and secondly, such a
small coil, by reason of its smaller capacity and self-induction, would
be capable of a quicker and more vigorous vibration. The problem then of
constructing a coil or induction apparatus of any kind possessing the
requisite qualities I regard as one of no small importance, and it has
occupied me for a considerable time.
The investigator who desires to repeat the experiments which I will
describe, with an alternate current machine, capable of supplying
currents of the desired frequency, and an induction coil, will do well
to take the primary coil out and mount the secondary in such a manner as
to be able to look through the tube upon which the secondary is wound.
He will then be able to observe the streams which pass from the primary
to the insulating tube, and from their intensity he will know how far he
can strain the coil. Without this precaution he is sure to injure the
insulation. This arrangement permits, however, an easy exchange of the
primaries, which is desirable in these experiments.
The selection of the type of machine best suited for the purpose must be
left to the judgment of the experimenter. There are here illustrated
three distinct types of machines, which, besides others, I have used in
my experiments.
Fig. 97 represents the machine used in my experiments before this
Institute. The field magnet consists of a ring of wrought iron with 384
pole projections. The armature comprises a steel disc to which is
fastened a thin, carefully welded rim of wrought iron. Upon the rim are
wound several layers of fine, well annealed iron wire, which, when
wound, is passed through shellac. The armature wires are wound around
brass pins, wrapped with silk thread. The diameter of the armature wire
in this type of machine should not be more than 1/6 of the thickness of
the pole projections, else the local action will be considerable.
[Illustration: FIG. 97.]
Fig. 98 represents a larger machine of a different type. The field
magnet of this machine consists of two like parts which either enclose
an exciting coil, or else are independently wound. Each part has 480
pole projections, the projections of one facing those of the other. The
armature consists of a wheel of hard bronze, carrying the conductors
which revolve between the projections of the field magnet. To wind the
armature conductors, I have found it most convenient to proceed in the
following manner. I construct a ring of hard bronze of the required
size. This ring and the rim of the wheel are provided with the proper
number of pins, and both fastened upon a plate. The armature conductors
being wound, the pins are cut off and the ends of the conductors
fastened by two rings which screw to the bronze ring and the rim of the
wheel, respectively. The whole may then be taken off and forms a solid
structure. The conductors in such a type of machine should consist of
sheet copper, the thickness of which, of course, depends on the
thickness of the pole projections; or else twisted thin wires should be
employed.
Fig. 99 is a smaller machine, in many respects similar to the former,
only here the armature conductors and the exciting coil are kept
stationary, while only a block of wrought iron is revolved.
[Illustration: FIG. 98.]
It would be uselessly lengthening this description were I to dwell more
on the details of construction of these machines. Besides, they have
been described somewhat more elaborately in _The Electrical Engineer_,
of March 18, 1891. I deem it well, however, to call the attention of the
investigator to two things, the importance of which, though self
evident, he is nevertheless apt to underestimate; namely, to the local
action in the conductors which must be carefully avoided, and to the
clearance, which must be small. I may add, that since it is desirable to
use very high peripheral speeds, the armature should be of very large
diameter in order to avoid impracticable belt speeds. Of the several
types of these machines which have been constructed by me, I have found
that the type illustrated in Fig. 97 caused me the least trouble in
construction, as well as in maintenance, and on the whole, it has been a
good experimental machine.
In operating an induction coil with very rapidly alternating currents,
among the first luminous phenomena noticed are naturally those presented
by the high-tension discharge. As the number of alternations per second
is increased, or as--the number being high--the current through the
primary is varied, the discharge gradually changes in appearance. It
would be difficult to describe the minor changes which occur, and the
conditions which bring them about, but one may note five distinct forms
of the discharge.
[Illustration: FIG. 99.]
First, one may observe a weak, sensitive discharge in the form of a
thin, feeble-colored thread. (Fig. 100a.) It always occurs when, the
number of alternations per second being high, the current through the
primary is very small. In spite of the excessively small current, the
rate of change is great, and the difference of potential at the
terminals of the secondary is therefore considerable, so that the arc is
established at great distances; but the quantity of "electricity" set in
motion is insignificant, barely sufficient to maintain a thin,
threadlike arc. It is excessively sensitive and may be made so to such a
degree that the mere act of breathing near the coil will affect it, and
unless it is perfectly well protected from currents of air, it wriggles
around constantly. Nevertheless, it is in this form excessively
persistent, and when the terminals are approached to, say, one-third of
the striking distance, it can be blown out only with difficulty. This
exceptional persistency, when short, is largely due to the arc being
excessively thin; presenting, therefore, a very small surface to the
blast. Its great sensitiveness, when very long, is probably due to the
motion of the particles of dust suspended in the air.
[Illustration: FIG. 100a.]
[Illustration: FIG. 100b.]
When the current through the primary is increased, the discharge gets
broader and stronger, and the effect of the capacity of the coil becomes
visible until, finally, under proper conditions, a white flaming arc,
Fig. 100 B, often as thick as one's finger, and striking across the
whole coil, is produced. It develops remarkable heat, and may be further
characterized by the absence of the high note which accompanies the
less powerful discharges. To take a shock from the coil under these
conditions would not be advisable, although under different conditions,
the potential being much higher, a shock from the coil may be taken with
impunity. To produce this kind of discharge the number of alternations
per second must not be too great for the coil used; and, generally
speaking, certain relations between capacity, self-induction and
frequency must be observed.
The importance of these elements in an alternate current circuit is now
well-known, and under ordinary conditions, the general rules are
applicable. But in an induction coil exceptional conditions prevail.
First, the self-induction is of little importance before the arc is
established, when it asserts itself, but perhaps never as prominently as
in ordinary alternate current circuits, because the capacity is
distributed all along the coil, and by reason of the fact that the coil
usually discharges through very great resistances; hence the currents
are exceptionally small. Secondly, the capacity goes on increasing
continually as the potential rises, in consequence of absorption which
takes place to a considerable extent. Owing to this there exists no
critical relationship between these quantities, and ordinary rules would
not seem to be applicable. As the potential is increased either in
consequence of the increased frequency or of the increased current
through the primary, the amount of the energy stored becomes greater and
greater, and the capacity gains more and more in importance. Up to a
certain point the capacity is beneficial, but after that it begins to be
an enormous drawback. It follows from this that each coil gives the best
result with a given frequency and primary current. A very large coil,
when operated with currents of very high frequency, may not give as much
as 1/8 inch spark. By adding capacity to the terminals, the condition
may be improved, but what the coil really wants is a lower frequency.
When the flaming discharge occurs, the conditions are evidently such
that the greatest current is made to flow through the circuit. These
conditions may be attained by varying the frequency within wide limits,
but the highest frequency at which the flaming arc can still be
produced, determines, for a given primary current, the maximum striking
distance of the coil. In the flaming discharge the _eclat_ effect of the
capacity is not perceptible; the rate at which the energy is being
stored then just equals the rate at which it can be disposed of through
the circuit. This kind of discharge is the severest test for a coil; the
break, when it occurs, is of the nature of that in an overcharged Leyden
jar. To give a rough approximation I would state that, with an ordinary
coil of, say 10,000 ohms resistance, the most powerful arc would be
produced with about 12,000 alternations per second.
When the frequency is increased beyond that rate, the potential, of
course, rises, but the striking distance may, nevertheless, diminish,
paradoxical as it may seem. As the potential rises the coil attains more
and more the properties of a static machine until, finally, one may
observe the beautiful phenomenon of the streaming discharge, Fig. 101,
which may be produced across the whole length of the coil. At that stage
streams begin to issue freely from all points and projections. These
streams will also be seen to pass in abundance in the space between the
primary and the insulating tube. When the potential is excessively high
they will always appear, even if the frequency be low, and even if the
primary be surrounded by as much as an inch of wax, hard rubber, glass,
or any other insulating substance. This limits greatly the output of the
coil, but I will later show how I have been able to overcome to a
considerable extent this disadvantage in the ordinary coil.
Besides the potential, the intensity of the streams depends on the
frequency; but if the coil be very large they show themselves, no matter
how low the frequencies used. For instance, in a very large coil of a
resistance of 67,000 ohms, constructed by me some time ago, they appear
with as low as 100 alternations per second and less, the insulation of
the secondary being 3/4 inch of ebonite. When very intense they produce
a noise similar to that produced by the charging of a Holtz machine, but
much more powerful, and they emit a strong smell of ozone. The lower the
frequency, the more apt they are to suddenly injure the coil. With
excessively high frequencies they may pass freely without producing any
other effect than to heat the insulation slowly and uniformly.
[Illustration: FIG. 101.]
[Illustration: FIG. 102.]
The existence of these streams shows the importance of constructing an
expensive coil so as to permit of one's seeing through the tube
surrounding the primary, and the latter should be easily exchangeable;
or else the space between the primary and secondary should be completely
filled up with insulating material so as to exclude all air. The
non-observance of this simple rule in the construction of commercial
coils is responsible for the destruction of many an expensive coil.
At the stage when the streaming discharge occurs, or with somewhat
higher frequencies, one may, by approaching the terminals quite nearly,
and regulating properly the effect of capacity, produce a veritable
spray of small silver-white sparks, or a bunch of excessively thin
silvery threads (Fig. 102) amidst a powerful brush--each spark or thread
possibly corresponding to one alternation. This, when produced under
proper conditions, is probably the most beautiful discharge, and when an
air blast is directed against it, it presents a singular appearance. The
spray of sparks, when received through the body, causes some
inconvenience, whereas, when the discharge simply streams, nothing at
all is likely to be felt if large conducting objects are held in the
hands to protect them from receiving small burns.
If the frequency is still more increased, then the coil refuses to give
any spark unless at comparatively small distances, and the fifth typical
form of discharge may be observed (Fig. 103). The tendency to stream out
and dissipate is then so great that when the brush is produced at one
terminal no sparking occurs, even if, as I have repeatedly tried, the
hand, or any conducting object, is held within the stream; and, what is
more singular, the luminous stream is not at all easily deflected by the
approach of a conducting body.
[Illustration: FIG. 103.]
[Illustration: FIG. 104.]
At this stage the streams seemingly pass with the greatest freedom
through considerable thicknesses of insulators, and it is particularly
interesting to study their behavior. For this purpose it is convenient
to connect to the terminals of the coil two metallic spheres which may
be placed at any desired distance, Fig. 104. Spheres are preferable to
plates, as the discharge can be better observed. By inserting dielectric
bodies between the spheres, beautiful discharge phenomena may be
observed. If the spheres be quite close and a spark be playing between
them, by interposing a thin plate of ebonite between the spheres the
spark instantly ceases and the discharge spreads into an intensely
luminous circle several inches in diameter, provided the spheres are
sufficiently large. The passage of the streams heats, and, after a
while, softens, the rubber so much that two plates may be made to stick
together in this manner. If the spheres are so far apart that no spark
occurs, even if they are far beyond the striking distance, by inserting
a thick plate of glass the discharge is instantly induced to pass from
the spheres to the glass in the form of luminous streams. It appears
almost as though these streams pass _through_ the dielectric. In reality
this is not the case, as the streams are due to the molecules of the air
which are violently agitated in the space between the oppositely charged
surfaces of the spheres. When no dielectric other than air is present,
the bombardment goes on, but is too weak to be visible; by inserting a
dielectric the inductive effect is much increased, and besides, the
projected air molecules find an obstacle and the bombardment becomes so
intense that the streams become luminous. If by any mechanical means we
could effect such a violent agitation of the molecules we could produce
the same phenomenon. A jet of air escaping through a small hole under
enormous pressure and striking against an insulating substance, such as
glass, may be luminous in the dark, and it might be possible to produce
a phosphorescence of the glass or other insulators in this manner.
The greater the specific inductive capacity of the interposed
dielectric, the more powerful the effect produced. Owing to this, the
streams show themselves with excessively high potentials even if the
glass be as much as one and one-half to two inches thick. But besides
the heating due to bombardment, some heating goes on undoubtedly in the
dielectric, being apparently greater in glass than in ebonite. I
attribute this to the greater specific inductive capacity of the glass,
in consequence of which, with the same potential difference, a greater
amount of energy is taken up in it than in rubber. It is like connecting
to a battery a copper and a brass wire of the same dimensions. The
copper wire, though a more perfect conductor, would heat more by reason
of its taking more current. Thus what is otherwise considered a virtue
of the glass is here a defect. Glass usually gives way much quicker than
ebonite; when it is heated to a certain degree, the discharge suddenly
breaks through at one point, assuming then the ordinary form of an arc.
The heating effect produced by molecular bombardment of the dielectric
would, of course, diminish as the pressure of the air is increased, and
at enormous pressure it would be negligible, unless the frequency would
increase correspondingly.
It will be often observed in these experiments that when the spheres are
beyond the striking distance, the approach of a glass plate, for
instance, may induce the spark to jump between the spheres. This occurs
when the capacity of the spheres is somewhat below the critical value
which gives the greatest difference of potential at the terminals of the
coil. By approaching a dielectric, the specific inductive capacity of
the space between the spheres is increased, producing the same effect as
if the capacity of the spheres were increased. The potential at the
terminals may then rise so high that the air space is cracked. The
experiment is best performed with dense glass or mica.
Another interesting observation is that a plate of insulating material,
when the discharge is passing through it, is strongly attracted by
either of the spheres, that is by the nearer one, this being obviously
due to the smaller mechanical effect of the bombardment on that side,
and perhaps also to the greater electrification.
From the behavior of the dielectrics in these experiments, we may
conclude that the best insulator for these rapidly alternating currents
would be the one possessing the smallest specific inductive capacity and
at the same time one capable of withstanding the greatest differences of
potential; and thus two diametrically opposite ways of securing the
required insulation are indicated, namely, to use either a perfect
vacuum or a gas under great pressure; but the former would be
preferable. Unfortunately neither of these two ways is easily carried
out in practice.
It is especially interesting to note the behavior of an excessively high
vacuum in these experiments. If a test tube, provided with external
electrodes and exhausted to the highest possible degree, be connected to
the terminals of the coil, Fig. 105, the electrodes of the tube are
instantly brought to a high temperature and the glass at each end of the
tube is rendered intensely phosphorescent, but the middle appears
comparatively dark, and for a while remains cool.
When the frequency is so high that the discharge shown in Fig. 103 is
observed, considerable dissipation no doubt occurs in the coil.
Nevertheless the coil may be worked for a long time, as the heating is
gradual.
In spite of the fact that the difference of potential may be enormous,
little is felt when the discharge is passed through the body, provided
the hands are armed. This is to some extent due to the higher frequency,
but principally to the fact that less energy is available externally,
when the difference of potential reaches an enormous value, owing to the
circumstance that, with the rise of potential, the energy absorbed in
the coil increases as the square of the potential. Up to a certain point
the energy available externally increases with the rise of potential,
then it begins to fall off rapidly. Thus, with the ordinary high tension
induction coil, the curious paradox exists, that, while with a given
current through the primary the shock might be fatal, with many times
that current it might be perfectly harmless, even if the frequency be
the same. With high frequencies and excessively high potentials when the
terminals are not connected to bodies of some size, practically all the
energy supplied to the primary is taken up by the coil. There is no
breaking through, no local injury, but all the material, insulating and
conducting, is uniformly heated.
[Illustration: FIG. 105.]
[Illustration: FIG. 106.]
To avoid misunderstanding in regard to the physiological effect of
alternating currents of very high frequency, I think it necessary to
state that, while it is an undeniable fact that they are incomparably
less dangerous than currents of low frequencies, it should not be
thought that they are altogether harmless. What has just been said
refers only to currents from an ordinary high tension induction coil,
which currents are necessarily very small; if received directly from a
machine or from a secondary of low resistance, they produce more or less
powerful effects, and may cause serious injury, especially when used in
conjunction with condensers.
The streaming discharge of a high tension induction coil differs in many
respects from that of a powerful static machine. In color it has neither
the violet of the positive, nor the brightness of the negative, static
discharge, but lies somewhere between, being, of course, alternatively
positive and negative. But since the streaming is more powerful when the
point or terminal is electrified positively, than when electrified
negatively, it follows that the point of the brush is more like the
positive, and the root more like the negative, static discharge. In the
dark, when the brush is very powerful, the root may appear almost white.
The wind produced by the escaping streams, though it may be very
strong--often indeed to such a degree that it may be felt quite a
distance from the coil--is, nevertheless, considering the quantity of
the discharge, smaller than that produced by the positive brush of a
static machine, and it affects the flame much less powerfully. From the
nature of the phenomenon we can conclude that the higher the frequency,
the smaller must, of course, be the wind produced by the streams, and
with sufficiently high frequencies no wind at all would be produced at
the ordinary atmospheric pressures. With frequencies obtainable by means
of a machine, the mechanical effect is sufficiently great to revolve,
with considerable speed, large pin-wheels, which in the dark present a
beautiful appearance owing to the abundance of the streams (Fig. 106).
[Illustration: FIG. 107.]
[Illustration: FIG. 108.]
In general, most of the experiments usually performed with a static
machine can be performed with an induction coil when operated with very
rapidly alternating currents. The effects produced, however, are much
more striking, being of incomparably greater power. When a small length
of ordinary cotton covered wire, Fig. 107, is attached to one terminal
of the coil, the streams issuing from all points of the wire may be so
intense as to produce a considerable light effect. When the potentials
and frequencies are very high, a wire insulated with gutta percha or
rubber and attached to one of the terminals, appears to be covered with
a luminous film. A very thin bare wire when attached to a terminal emits
powerful streams and vibrates continually to and fro or spins in a
circle, producing a singular effect (Fig. 108). Some of these
experiments have been described by me in _The Electrical World_, of
February 21, 1891.
Another peculiarity of the rapidly alternating discharge of the
induction coil is its radically different behavior with respect to
points and rounded surfaces.
If a thick wire, provided with a ball at one end and with a point at the
other, be attached to the positive terminal of a static machine,
practically all the charge will be lost through the point, on account of
the enormously greater tension, dependent on the radius of curvature.
But if such a wire is attached to one of the terminals of the induction
coil, it will be observed that with very high frequencies streams issue
from the ball almost as copiously as from the point (Fig. 109).
It is hardly conceivable that we could produce such a condition to an
equal degree in a static machine, for the simple reason, that the
tension increases as the square of the density, which in turn is
proportional to the radius of curvature; hence, with a steady potential
an enormous charge would be required to make streams issue from a
polished ball while it is connected with a point. But with an induction
coil the discharge of which alternates with great rapidity it is
different. Here we have to deal with two distinct tendencies. First,
there is the tendency to escape which exists in a condition of rest, and
which depends on the radius of curvature; second, there is the tendency
to dissipate into the surrounding air by condenser action, which depends
on the surface. When one of these tendencies is a maximum, the other is
at a minimum. At the point the luminous stream is principally due to the
air molecules coming bodily in contact with the point; they are
attracted and repelled, charged and discharged, and, their atomic
charges being thus disturbed, vibrate and emit light waves. At the ball,
on the contrary, there is no doubt that the effect is to a great extent
produced inductively, the air molecules not _necessarily_ coming in
contact with the ball, though they undoubtedly do so. To convince
ourselves of this we only need to exalt the condenser action, for
instance, by enveloping the ball, at some distance, by a better
conductor than the surrounding medium, the conductor being, of course,
insulated; or else by surrounding it with a better dielectric and
approaching an insulated conductor; in both cases the streams will break
forth more copiously. Also, the larger the ball with a given frequency,
or the higher the frequency, the more will the ball have the advantage
over the point. But, since a certain intensity of action is required to
render the streams visible, it is obvious that in the experiment
described the ball should not be taken too large.
In consequence of this two-fold tendency, it is possible to produce by
means of points, effects identical to those produced by capacity. Thus,
for instance, by attaching to one terminal of the coil a small length of
soiled wire, presenting many points and offering great facility to
escape, the potential of the coil may be raised to the same value as by
attaching to the terminal a polished ball of a surface many times
greater than that of the wire.
[Illustration: FIG. 109.]
[Illustration: FIG. 110.]
An interesting experiment, showing the effect of the points, may be
performed in the following manner: Attach to one of the terminals of the
coil a cotton covered wire about two feet in length, and adjust the
conditions so that streams issue from the wire. In this experiment the
primary coil should be preferably placed so that it extends only about
half way into the secondary coil. Now touch the free terminal of the
secondary with a conducting object held in the hand, or else connect it
to an insulated body of some size. In this manner the potential on the
wire may be enormously raised. The effect of this will be either to
increase, or to diminish, the streams. If they increase, the wire is too
short; if they diminish, it is too long. By adjusting the length of the
wire, a point is found where the touching of the other terminal does not
at all affect the streams. In this case the rise of potential is exactly
counteracted by the drop through the coil. It will be observed that
small lengths of wire produce considerable difference in the magnitude
and luminosity of the streams. The primary coil is placed sidewise for
two reasons: First, to increase the potential at the wire; and, second,
to increase the drop through the coil. The sensitiveness is thus
augmented.