a Leyden jar. One, of course, anticipates that since the frequency is
high the capacity of the jar should be small. He therefore takes a very
small jar, about the size of a small wine glass, but he finds that even
with this jar the coil is practically short-circuited. He then reduces
the capacity until he comes to about the capacity of two spheres, say,
ten centimetres in diameter and two to four centimetres apart. The
discharge then assumes the form of a serrated band exactly like a
succession of sparks viewed in a rapidly revolving mirror; the
serrations, of course, corresponding to the condenser discharges. In
this case one may observe a queer phenomenon. The discharge starts at
the nearest points, works gradually up, breaks somewhere near the top of
the spheres, begins again at the bottom, and so on. This goes on so fast
that several serrated bands are seen at once. One may be puzzled for a
few minutes, but the explanation is simple enough. The discharge begins
at the nearest points, the air is heated and carries the arc upward
until it breaks, when it is re-established at the nearest points, etc.
Since the current passes easily through a condenser of even small
capacity, it will be found quite natural that connecting only one
terminal to a body of the same size, no matter how well insulated,
impairs considerably the striking distance of the arc.
Experiments with Geissler tubes are of special interest. An exhausted
tube, devoid of electrodes of any kind, will light up at some distance
from the coil. If a tube from a vacuum pump is near the coil the whole
of the pump is brilliantly lighted. An incandescent lamp approached to
the coil lights up and gets perceptibly hot. If a lamp have the
terminals connected to one of the binding posts of the coil and the hand
is approached to the bulb, a very curious and rather unpleasant
discharge from the glass to the hand takes place, and the filament may
become incandescent. The discharge resembles to some extent the stream
issuing from the plates of a powerful Toepler machine, but is of
incomparably greater quantity. The lamp in this case acts as a
condenser, the rarefied gas being one coating, the operator's hand the
other. By taking the globe of a lamp in the hand, and by bringing the
metallic terminals near to or in contact with a conductor connected to
the coil, the carbon is brought to bright incandescence and the glass is
rapidly heated. With a 100-volt 10 C. P. lamp one may without great
discomfort stand as much current as will bring the lamp to a
considerable brilliancy; but it can be held in the hand only for a few
minutes, as the glass is heated in an incredibly short time. When a tube
is lighted by bringing it near to the coil it may be made to go out by
interposing a metal plate on the hand between the coil and tube; but if
the metal plate be fastened to a glass rod or otherwise insulated, the
tube may remain lighted if the plate be interposed, or may even
increase in luminosity. The effect depends on the position of the plate
and tube relatively to the coil, and may be always easily foretold by
_assuming_ that conduction takes place from one terminal of the coil to
the other. According to the position of the plate, it may either divert
from or direct the current to the tube.
In another line of work the writer has in frequent experiments
maintained incandescent lamps of 50 or 100 volts burning at any desired
candle power with both the terminals of each lamp connected to a stout
copper wire of no more than a few feet in length. These experiments seem
interesting enough, but they are not more so than the queer experiment
of Faraday, which has been revived and made much of by recent
investigators, and in which a discharge is made to jump between two
points of a bent copper wire. An experiment may be cited here which may
seem equally interesting. If a Geissler tube, the terminals of which are
joined by a copper wire, be approached to the coil, certainly no one
would be prepared to see the tube light up. Curiously enough, it does
light up, and, what is more, the wire does not seem to make much
difference. Now one is apt to think in the first moment that the
impedance of the wire might have something to do with the phenomenon.
But this is of course immediately rejected, as for this an enormous
frequency would be required. This result, however, seems puzzling only
at first; for upon reflection it is quite clear that the wire can make
but little difference. It may be explained in more than one way, but it
agrees perhaps best with observation to assume that conduction takes
place from the terminals of the coil through the space. On this
assumption, if the tube with the wire be held in any position, the wire
can divert little more than the current which passes through the space
occupied by the wire and the metallic terminals of the tube; through the
adjacent space the current passes practically undisturbed. For this
reason, if the tube be held in any position at right angles to the line
joining the binding posts of the coil, the wire makes hardly any
difference, but in a position more or less parallel with that line it
impairs to a certain extent the brilliancy of the tube and its facility
to light up. Numerous other phenomena may be explained on the same
assumption. For instance, if the ends of the tube be provided with
washers of sufficient size and held in the line joining the terminals of
the coil, it will not light up, and then nearly the whole of the
current, which would otherwise pass uniformly through the space between
the washers, is diverted through the wire. But if the tube be inclined
sufficiently to that line, it will light up in spite of the washers.
Also, if a metal plate be fastened upon a glass rod and held at right
angles to the line joining the binding posts, and nearer to one of them,
a tube held more or less parallel with the line will light up instantly
when one of the terminals touches the plate, and will go out when
separated from the plate. The greater the surface of the plate, up to a
certain limit, the easier the tube will light up. When a tube is placed
at right angles to the straight line joining the binding posts, and then
rotated, its luminosity steadily increases until it is parallel with
that line. The writer must state, however, that he does not favor the
idea of a leakage or current through the space any more than as a
suitable explanation, for he is convinced that all these experiments
could not be performed with a static machine yielding a constant
difference of potential, and that condenser action is largely concerned
in these phenomena.
It is well to take certain precautions when operating a Ruhmkorff coil
with very rapidly alternating currents. The primary current should not
be turned on too long, else the core may get so hot as to melt the
gutta-percha or paraffin, or otherwise injure the insulation, and this
may occur in a surprisingly short time, considering the current's
strength. The primary current being turned on, the fine wire terminals
may be joined without great risk, the impedance being so great that it
is difficult to force enough current through the fine wire so as to
injure it, and in fact the coil may be on the whole much safer when the
terminals of the fine wire are connected than when they are insulated;
but special care should be taken when the terminals are connected to the
coatings of a Leyden jar, for with anywhere near the critical capacity,
which just counteracts the self-induction at the existing frequency, the
coil might meet the fate of St. Polycarpus. If an expensive vacuum pump
is lighted up by being near to the coil or touched with a wire connected
to one of the terminals, the current should be left on no more than a
few moments, else the glass will be cracked by the heating of the
rarefied gas in one of the narrow passages--in the writer's own
experience _quod erat demonstrandum_.[5]
[5] It is thought necessary to remark that, although the induction
coil may give quite a good result when operated with such
rapidly alternating currents, yet its construction, quite
irrespective of the iron core, makes it very unfit for such
high frequencies, and to obtain the best results the
construction should be greatly modified.
There are a good many other points of interest which may be observed in
connection with such a machine. Experiments with the telephone, a
conductor in a strong field or with a condenser or arc, seem to afford
certain proof that sounds far above the usual accepted limit of hearing
would be perceived. A telephone will emit notes of twelve to thirteen
thousand vibrations per second; then the inability of the core to follow
such rapid alternations begins to tell. If, however, the magnet and core
be replaced by a condenser and the terminals connected to the
high-tension secondary of a transformer, higher notes may still be
heard. If the current be sent around a finely laminated core and a small
piece of thin sheet iron be held gently against the core, a sound may be
still heard with thirteen to fourteen thousand alternations per second,
provided the current is sufficiently strong. A small coil, however,
tightly packed between the poles of a powerful magnet, will emit a sound
with the above number of alternations, and arcs may be audible with a
still higher frequency. The limit of audition is variously estimated. In
Sir William Thomson's writings it is stated somewhere that ten thousand
per second, or nearly so, is the limit. Other, but less reliable,
sources give it as high as twenty-four thousand per second. The above
experiments have convinced the writer that notes of an incomparably
higher number of vibrations per second would be perceived provided they
could be produced with sufficient power. There is no reason why it
should not be so. The condensations and rarefactions of the air would
necessarily set the diaphragm in a corresponding vibration and some
sensation would be produced, whatever--within certain limits--the
velocity of transmission to their nerve centres, though it is probable
that for want of exercise the ear would not be able to distinguish any
such high note. With the eye it is different; if the sense of vision is
based upon some resonance effect, as many believe, no amount of increase
in the intensity of the ethereal vibration could extend our range of
vision on either side of the visible spectrum.
The limit of audition of an arc depends on its size. The greater the
surface by a given heating effect in the arc, the higher the limit of
audition. The highest notes are emitted by the high-tension discharges
of an induction coil in which the arc is, so to speak, all surface. If
_R_ be the resistance of an arc, and _C_ the current, and the linear
dimensions be _n_ times increased, then the resistance is _R_/_n_, and
with the same current density the current would be _n_^2_C_; hence the
heating effect is _n_^3 times greater, while the surface is only _n_^2
times as great. For this reason very large arcs would not emit any
rhythmical sound even with a very low frequency. It must be observed,
however, that the sound emitted depends to some extent also on the
composition of the carbon. If the carbon contain highly refractory
material, this, when heated, tends to maintain the temperature of the
arc uniform and the sound is lessened; for this reason it would seem
that an alternating arc requires such carbons.
With currents of such high frequencies it is possible to obtain
noiseless arcs, but the regulation of the lamp is rendered extremely
difficult on account of the excessively small attractions or repulsions
between conductors conveying these currents.
An interesting feature of the arc produced by these rapidly alternating
currents is its persistency. There are two causes for it, one of which
is always present, the other sometimes only. One is due to the character
of the current and the other to a property of the machine. The first
cause is the more important one, and is due directly to the rapidity of
the alternations. When an arc is formed by a periodically undulating
current, there is a corresponding undulation in the temperature of the
gaseous column, and, therefore, a corresponding undulation in the
resistance of the arc. But the resistance of the arc varies enormously
with the temperature of the gaseous column, being practically infinite
when the gas between the electrodes is cold. The persistence of the arc,
therefore, depends on the inability of the column to cool. It is for
this reason impossible to maintain an arc with the current alternating
only a few times a second. On the other hand, with a practically
continuous current, the arc is easily maintained, the column being
constantly kept at a high temperature and low resistance. The higher the
frequency the smaller the time interval during which the arc may cool
and increase considerably in resistance. With a frequency of 10,000 per
second or more in an arc of equal size excessively small variations of
temperature are superimposed upon a steady temperature, like ripples on
the surface of a deep sea. The heating effect is practically continuous
and the arc behaves like one produced by a continuous current, with the
exception, however, that it may not be quite as easily started, and that
the electrodes are equally consumed; though the writer has observed
some irregularities in this respect.
The second cause alluded to, which possibly may not be present, is due
to the tendency of a machine of such high frequency to maintain a
practically constant current. When the arc is lengthened, the
electromotive force rises in proportion and the arc appears to be more
persistent.
Such a machine is eminently adapted to maintain a constant current, but
it is very unfit for a constant potential. As a matter of fact, in
certain types of such machines a nearly constant current is an almost
unavoidable result. As the number of poles or polar projections is
greatly increased, the clearance becomes of great importance. One has
really to do with a great number of very small machines. Then there is
the impedance in the armature, enormously augmented by the high
frequency. Then, again, the magnetic leakage is facilitated. If there
are three or four hundred alternate poles, the leakage is so great that
it is virtually the same as connecting, in a two-pole machine, the poles
by a piece of iron. This disadvantage, it is true, may be obviated more
or less by using a field throughout of the same polarity, but then one
encounters difficulties of a different nature. All these things tend to
maintain a constant current in the armature circuit.
In this connection it is interesting to notice that even to-day
engineers are astonished at the performance of a constant current
machine, just as, some years ago, they used to consider it an
extraordinary performance if a machine was capable of maintaining a
constant potential difference between the terminals. Yet one result is
just as easily secured as the other. It must only be remembered that in
an inductive apparatus of any kind, if constant potential is required,
the inductive relation between the primary or exciting and secondary or
armature circuit must be the closest possible; whereas, in an apparatus
for constant current just the opposite is required. Furthermore, the
opposition to the current's flow in the induced circuit must be as small
as possible in the former and as great as possible in the latter case.
But opposition to a current's flow may be caused in more than one way.
It may be caused by ohmic resistance or self-induction. One may make the
induced circuit of a dynamo machine or transformer of such high
resistance that when operating devices of considerably smaller
resistance within very wide limits a nearly constant current is
maintained. But such high resistance involves a great loss in power,
hence it is not practicable. Not so self-induction. Self-induction does
not necessarily mean loss of power. The moral is, use self-induction
instead of resistance. There is, however, a circumstance which favors
the adoption of this plan, and this is, that a very high self-induction
may be obtained cheaply by surrounding a comparatively small length of
wire more or less completely with iron, and, furthermore, the effect may
be exalted at will by causing a rapid undulation of the current. To sum
up, the requirements for constant current are: Weak magnetic connection
between the induced and inducing circuits, greatest possible
self-induction with the least resistance, greatest practicable rate of
change of the current. Constant potential, on the other hand, requires:
Closest magnetic connection between the circuits, steady induced
current, and, if possible, no reaction. If the latter conditions could
be fully satisfied in a constant potential machine, its output would
surpass many times that of a machine primarily designed to give constant
current. Unfortunately, the type of machine in which these conditions
may be satisfied is of little practical value, owing to the small
electromotive force obtainable and the difficulties in taking off the
current.
With their keen inventor's instinct, the now successful arc-light men
have early recognized the desiderata of a constant current machine.
Their arc light machines have weak fields, large armatures, with a great
length of copper wire and few commutator segments to produce great
variations in the current's strength and to bring self-induction into
play. Such machines may maintain within considerable limits of variation
in the resistance of the circuit a practically constant current. Their
output is of course correspondingly diminished, and, perhaps with the
object in view not to cut down the output too much, a simple device
compensating exceptional variations is employed. The undulation of the
current is almost essential to the commercial success of an arc-light
system. It introduces in the circuit a steadying element taking the
place of a large ohmic resistance, without involving a great loss in
power, and, what is more important, it allows the use of simple clutch
lamps, which with a current of a certain number of impulses per second,
best suitable for each particular lamp, will, if properly attended to,
regulate even better than the finest clock-work lamps. This discovery
has been made by the writer--several years too late.
It has been asserted by competent English electricians that in a
constant-current machine or transformer the regulation is effected by
varying the phase of the secondary current. That this view is erroneous
may be easily proved by using, instead of lamps, devices each possessing
self-induction and capacity or self-induction and resistance--that is,
retarding and accelerating components--in such proportions as to not
affect materially the phase of the secondary current. Any number of such
devices may be inserted or cut out, still it will be found that the
regulation occurs, a constant current being maintained, while the
electromotive force is varied with the number of the devices. The change
of phase of the secondary current is simply a result following from the
changes in resistance, and, though secondary reaction is always of more
or less importance, yet the real cause of the regulation lies in the
existence of the conditions above enumerated. It should be stated,
however, that in the case of a machine the above remarks are to be
restricted to the cases in which the machine is independently excited.
If the excitation be effected by commutating the armature current, then
the fixed position of the brushes makes any shifting of the neutral line
of the utmost importance, and it may not be thought immodest of the
writer to mention that, as far as records go, he seems to have been the
first who has successfully regulated machines by providing a bridge
connection between a point of the external circuit and the commutator by
means of a third brush. The armature and field being properly
proportioned and the brushes placed in their determined positions, a
constant current or constant potential resulted from the shifting of the
diameter of commutation by the varying loads.
In connection with machines of such high frequencies, the condenser
affords an especially interesting study. It is easy to raise the
electromotive force of such a machine to four or five times the value by
simply connecting the condenser to the circuit, and the writer has
continually used the condenser for the the purposes of regulation, as
suggested by Blakesley in his book on alternate currents, in which he
has treated the most frequently occurring condenser problems with
exquisite simplicity and clearness. The high frequency allows the use of
small capacities and renders investigation easy. But, although in most
of the experiments the result may be foretold, some phenomena observed
seem at first curious. One experiment performed three or four months ago
with such a machine and a condenser may serve as an illustration. A
machine was used giving about 20,000 alternations per second. Two bare
wires about twenty feet long and two millimetres in diameter, in close
proximity to each other, were connected to the terminals of the machine
at the one end, and to a condenser at the other. A small transformer
without an iron core, of course, was used to bring the reading within
range of a Cardew voltmeter by connecting the voltmeter to the
secondary. On the terminals of the condenser the electromotive force was
about 120 volts, and from there inch by inch it gradually fell until at
the terminals of the machine it was about 65 volts. It was virtually as
though the condenser were a generator, and the line and armature circuit
simply a resistance connected to it. The writer looked for a case of
resonance, but he was unable to augment the effect by varying the
capacity very carefully and gradually or by changing the speed of the
machine. A case of pure resonance he was unable to obtain. When a
condenser was connected to the terminals of the machine--the
self-induction of the armature being first determined in the maximum and
minimum position and the mean value taken--the capacity which gave the
highest electromotive force corresponded most nearly to that which just
counteracted the self-induction with the existing frequency. If the
capacity was increased or diminished, the electromotive force fell as
expected.
With frequencies as high as the above mentioned, the condenser effects
are of enormous importance. The condenser becomes a highly efficient
apparatus capable of transferring considerable energy.
* * * * *
In an appendix to this book will be found a description of the Tesla
oscillator, which its inventor believes will among other great
advantages give him the necessary high frequency conditions, while
relieving him of the inconveniences that attach to generators of the
type described at the beginning of this chapter.
CHAPTER XXX.
ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.[6]
[6] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
May 6, 1891.
About a year and a half ago while engaged in the study of alternate
currents of short period, it occurred to me that such currents could be
obtained by rotating charged surfaces in close proximity to conductors.
Accordingly I devised various forms of experimental apparatus of which
two are illustrated in the accompanying engravings.
[Illustration: FIG. 208.]
In the apparatus shown in Fig. 208, A is a ring of dry shellacked hard
wood provided on its inside with two sets of tin-foil coatings, _a_ and
_b_, all the _a_ coatings and all the _b_ coatings being connected
together, respectively, but independent from each other. These two sets
of coatings are connected to two terminals, T. For the sake of
clearness only a few coatings are shown. Inside of the ring A, and in
close proximity to it there is arranged to rotate a cylinder B, likewise
of dry, shellacked hard wood, and provided with two similar sets of
coatings, _a^1_ and _b^1_, all the coatings _a^1_ being connected to one
ring and all the others, _b^1_, to another marked + and -. These two
sets, _a^1_ and _b^1_ are charged to a high potential by a Holtz or
Wimshurst machine, and may be connected to a jar of some capacity. The
inside of ring A is coated with mica in order to increase the induction
and also to allow higher potentials to be used.
[Illustration: FIG. 209.]
When the cylinder B with the charged coatings is rotated, a circuit
connected to the terminals T is traversed by alternating currents.
Another form of apparatus is illustrated in Fig. 209. In this apparatus
the two sets of tin-foil coatings are glued on a plate of ebonite, and a
similar plate which is rotated, and the coatings of which are charged as
in Fig. 208, is provided.
The output of such an apparatus is very small, but some of the effects
peculiar to alternating currents of short periods may be observed. The
effects, however, cannot be compared with those obtainable with an
induction coil which is operated by an alternate current machine of high
frequency, some of which were described by me a short while ago.
CHAPTER XXXI.
"MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.[7]
[7] Article by Mr. Tesla in _The Electrical Engineer_ of Dec. 23d,
1891.
I trust that the present brief communication will not be interpreted as
an effort on my part to put myself on record as a "patent medicine" man,
for a serious worker cannot despise anything more than the misuse and
abuse of electricity which we have frequent occasion to witness. My
remarks are elicited by the lively interest which prominent medical
practitioners evince at every real advance in electrical investigation.
The progress in recent years has been so great that every electrician
and electrical engineer is confident that electricity will become the
means of accomplishing many things that have been heretofore, with our
existing knowledge, deemed impossible. No wonder then that progressive
physicians also should expect to find in it a powerful tool and help in
new curative processes. Since I had the honor to bring before the
American Institute of Electrical Engineers some results in utilizing
alternating currents of high tension, I have received many letters from
noted physicians inquiring as to the physical effects of such currents
of high frequency. It may be remembered that I then demonstrated that a
body perfectly well insulated in air can be heated by simply connecting
it with a source of rapidly alternating high potential. The heating in
this case is due in all probability to the bombardment of the body by
air, or possibly by some other medium, which is molecular or atomic in
construction, and the presence of which has so far escaped our
analysis--for according to my ideas, the true ether radiation with such
frequencies as even a few millions per second must be very small. This
body may be a good conductor or it may be a very poor conductor of
electricity with little change in the result. The human body is, in such
a case, a fine conductor, and if a person insulated in a room, or no
matter where, is brought into contact with such a source of rapidly
alternating high potential, the skin is heated by bombardment. It is a
mere question of the dimensions and character of the apparatus to
produce any degree of heating desired.
It has occurred to me whether, with such apparatus properly prepared, it
would not be possible for a skilled physician to find in it a means for
the effective treatment of various types of disease. The heating will,
of course, be superficial, that is, on the skin, and would result,
whether the person operated on were in bed or walking around a room,
whether dressed in thick clothes or whether reduced to nakedness. In
fact, to put it broadly, it is conceivable that a person entirely nude
at the North Pole might keep himself comfortably warm in this manner.
Without vouching for all the results, which must, of course, be
determined by experience and observation, I can at least warrant the
fact that heating would occur by the use of this method of subjecting
the human body to bombardment by alternating currents of high potential
and frequency such I have long worked with. It is only reasonable to
expect that some of the novel effects will be wholly different from
those obtainable with the old familiar therapeutic methods generally
used. Whether they would all be beneficial or not remains to be proved.
CHAPTER XXXII.
ELECTRIC DISCHARGE IN VACUUM TUBES.[8]
[8] Article by Mr. Tesla in _The Electrical Engineer_. N. Y.,
July 1, 1891.
In _The Electrical Engineer_ of June 10 I have noted the description of
some experiments of Prof. J. J. Thomson, on the "Electric Discharge in
Vacuum Tubes," and in your issue of June 24 Prof. Elihu Thomson
describes an experiment of the same kind. The fundamental idea in these
experiments is to set up an electromotive force in a vacuum
tube---preferably devoid of any electrodes--by means of electro-magnetic
induction, and to excite the tube in this manner.
As I view the subject I should, think that to any experimenter who had
carefully studied the problem confronting us and who attempted to find a
solution of it, this idea must present itself as naturally as, for
instance, the idea of replacing the tinfoil coatings of a Leyden jar by
rarefied gas and exciting luminosity in the condenser thus obtained by
repeatedly charging and discharging it. The idea being obvious, whatever
merit there is in this line of investigation must depend upon the
completeness of the study of the subject and the correctness of the
observations. The following lines are not penned with any desire on my
part to put myself on record as one who has performed similar
experiments, but with a desire to assist other experimenters by pointing
out certain peculiarities of the phenomena observed, which, to all
appearances, have not been noted by Prof. J. J. Thomson, who, however,
seems to have gone about systematically in his investigations, and who
has been the first to make his results known. These peculiarities noted
by me would seem to be at variance with the views of Prof. J. J.
Thomson, and present the phenomena in a different light.
My investigations in this line occupied me principally during the winter
and spring of the past year. During this time many different experiments
were performed, and in my exchanges of ideas on this subject with Mr.
Alfred S. Brown, of the Western Union Telegraph Company, various
different dispositions were suggested which were carried out by me in
practice. Fig. 210 may serve as an example of one of the many forms of
apparatus used. This consisted of a large glass tube sealed at one end
and projecting into an ordinary incandescent lamp bulb. The primary,
usually consisting of a few turns of thick, well-insulated copper sheet
was inserted within the tube, the inside space of the bulb furnishing
the secondary. This form of apparatus was arrived at after some
experimenting, and was used principally with the view of enabling me to
place a polished reflecting surface on the inside of the tube, and for
this purpose the last turn of the primary was covered with a thin silver
sheet. In all forms of apparatus used there was no special difficulty in
exciting a luminous circle or cylinder in proximity to the primary.
[Illustration: FIG. 210.]
As to the number of turns, I cannot quite understand why Prof. J. J.
Thomson should think that a few turns were "quite sufficient," but lest
I should impute to him an opinion he may not have, I will add that I
have gained this impression from the reading of the published abstracts
of his lecture. Clearly, the number of turns which gives the best result
in any case, is dependent on the dimensions of the apparatus, and, were
it not for various considerations, one turn would always give the best
result.
I have found that it is preferable to use in these experiments an
alternate current machine giving a moderate number of alternations per
second to excite the induction coil for charging the Leyden jar which
discharges through the primary--shown diagrammatically in Fig. 211,--as
in such case, before the disruptive discharge takes place, the tube or
bulb is slightly excited and the formation of the luminous circle is
decidedly facilitated. But I have also used a Wimshurst machine in some
experiments.
[Illustration: FIG. 211.]
Prof. J. J. Thomson's view of the phenomena under consideration seems to
be that they are wholly due to electro-magnetic action. I was, at one
time, of the same opinion, but upon carefully investigating the subject
I was led to the conviction that they are more of an electrostatic
nature. It must be remembered that in these experiments we have to deal
with primary currents of an enormous frequency or rate of change and of
high potential, and that the secondary conductor consists of a rarefied
gas, and that under such conditions electrostatic effects must play an
important part.
[Illustration: FIG. 212.]
In support of my view I will describe a few experiments made by me. To
excite luminosity in the tube it is not absolutely necessary that the
conductor should be closed. For instance, if an ordinary exhausted tube
(preferably of large diameter) be surrounded by a spiral of thick copper
wire serving as the primary, a feebly luminous spiral may be induced in
the tube, roughly shown in Fig. 212. In one of these experiments a
curious phenomenon was observed; namely, two intensely luminous circles,
each of them close to a turn of the primary spiral, were formed inside
of the tube, and I attributed this phenomenon to the existence of nodes
on the primary. The circles were connected by a faint luminous spiral
parallel to the primary and in close proximity to it. To produce this
effect I have found it necessary to strain the jar to the utmost. The
turns of the spiral tend to close and form circles, but this, of course,
would be expected, and does not necessarily indicate an electro-magnetic
effect; Whereas the fact that a glow can be produced along the primary
in the form of an open spiral argues for an electrostatic effect.
[Illustration: FIG. 213.]
In using Dr. Lodge's recoil circuit, the electrostatic action is
likewise apparent. The arrangement is illustrated in Fig. 213. In his
experiment two hollow exhausted tubes H H were slipped over the wires of
the recoil circuit and upon discharging the jar in the usual manner
luminosity was excited in the tubes.
Another experiment performed is illustrated in Fig. 214. In this case an
ordinary lamp-bulb was surrounded by one or two turns of thick copper
wire P and the luminous circle L excited in the bulb by discharging the
jar through the primary. The lamp-bulb was provided with a tinfoil
coating on the side opposite to the primary and each time the tinfoil
coating was connected to the ground or to a large object the luminosity
of the circle was considerably increased. This was evidently due to
electrostatic action.
In other experiments I have noted that when the primary touches the
glass the luminous circle is easier produced and is more sharply
defined; but I have not noted that, generally speaking, the circles
induced were very sharply defined, as Prof. J. J. Thomson has observed;
on the contrary, in my experiments they were broad and often the whole
of the bulb or tube was illuminated; and in one case I have observed an
intensely purplish glow, to which Prof. J. J. Thomson refers. But the
circles were always in close proximity to the primary and were
considerably easier produced when the latter was very close to the
glass, much more so than would be expected assuming the action to be
electromagnetic and considering the distance; and these facts speak for
an electrostatic effect.
[Illustration: FIG. 214.]
[Illustration: FIG. 215.]
Furthermore I have observed that there is a molecular bombardment in the
plane of the luminous circle at right angles to the glass--supposing the
circle to be in the plane of the primary--this bombardment being
evident from the rapid heating of the glass near the primary. Were the
bombardment not at right angles to the glass the heating could not be so
rapid. If there is a circumferential movement of the molecules
constituting the luminous circle, I have thought that it might be
rendered manifest by placing within the tube or bulb, radially to the
circle, a thin plate of mica coated with some phosphorescent material
and another such plate tangentially to the circle. If the molecules
would move circumferentially, the former plate would be rendered more
intensely phosphorescent. For want of time I have, however, not been
able to perform the experiment.
Another observation made by me was that when the specific inductive
capacity of the medium between the primary and secondary is increased,
the inductive effect is augmented. This is roughly illustrated in Fig.
215. In this case luminosity was excited in an exhausted tube or bulb B
and a glass tube T slipped between the primary and the bulb, when the
effect pointed out was noted. Were the action wholly electromagnetic no
change could possibly have been observed.
I have likewise noted that when a bulb is surrounded by a wire closed
upon itself and in the plane of the primary, the formation of the
luminous circle within the bulb is not prevented. But if instead of the
wire a broad strip of tinfoil is glued upon the bulb, the formation of
the luminous band was prevented, because then the action was distributed
over a greater surface. The effect of the closed tinfoil was no doubt of
an electrostatic nature, for it presented a much greater resistance than
the closed wire and produced therefore a much smaller electromagnetic
effect.
Some of the experiments of Prof. J. J. Thomson also would seem to show
some electrostatic action. For instance, in the experiment with the bulb
enclosed in a bell jar, I should think that when the latter is exhausted
so far that the gas enclosed reaches the maximum conductivity, the
formation of the circle in the bulb and jar is prevented because of the
space surrounding the primary being highly conducting; when the jar is
further exhausted, the conductivity of the space around the primary
diminishes and the circles appear necessarily first in the bell jar, as
the rarefied gas is nearer to the primary. But were the inductive effect
very powerful, they would probably appear in the bulb also. If, however,
the bell jar were exhausted to the highest degree they would very likely
show themselves in the bulb only, that is, supposing the vacuous space
to be non-conducting. On the assumption that in these phenomena
electrostatic actions are concerned we find it easily explicable why the
introduction of mercury or the heating of the bulb prevents the
formation of the luminous band or shortens the after-glow; and also why
in some cases a platinum wire may prevent the excitation of the tube.
Nevertheless some of the experiments of Prof. J. J. Thomson would seem
to indicate an electromagnetic effect. I may add that in one of my
experiments in which a vacuum was produced by the Torricellian method, I
was unable to produce the luminous band, but this may have been due to
the weak exciting current employed.
My principal argument is the following: I have experimentally proved
that if the same discharge which is barely sufficient to excite a
luminous band in the bulb when passed through the primary circuit be so
directed as to exalt the electrostatic inductive effect--namely, by
converting upwards--an exhausted tube, devoid of electrodes, may be
excited at a distance of several feet.
SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.[9]
BY PROF. J. J. THOMSON, M.A., F.R.S.
[9] Abstract of a paper read before Physical Society of London.
[Illustration: FIG. 216.]
[Illustration: FIG. 217.]
[Illustration: FIG. 218.]
[Illustration: FIG. 219.]
The phenomena of vacuum discharges were, Prof. Thomson said,
greatly simplified when their path was wholly gaseous, the
complication of the dark space surrounding the negative electrode,
and the stratifications so commonly observed in ordinary vacuum
tubes, being absent. To produce discharges in tubes devoid of
electrodes was, however, not easy to accomplish, for the only
available means of producing an electromotive force in the
discharge circuit was by electro-magnetic induction. Ordinary
methods of producing variable induction were valueless, and
recourse was had to the oscillatory discharge of a Leyden jar,
which combines the two essentials of a current whose maximum value
is enormous, and whose rapidity of alternation is immensely great.
The discharge circuits, which may take the shape of bulbs, or of
tubes bent in the form of coils, were placed in close proximity to
glass tubes filled with mercury, which formed the path of the
oscillatory discharge. The parts thus corresponded to the windings
of an induction coil, the vacuum tubes being the secondary, and the
tubes filled with mercury the primary. In such an apparatus the
Leyden jar need not be large, and neither primary nor secondary
need have many turns, for this would increase the self-induction of
the former, and lengthen the discharge path in the latter.
Increasing the self-induction of the primary reduces the E. M. F.
induced in the secondary, whilst lengthening the secondary does not
increase the E. M. F. per unit length. The two or three turns, as
shown in Fig. 216, in each, were found to be quite sufficient, and,
on discharging the Leyden jar between two highly polished knobs in
the primary circuit, a plain uniform band of light was seen to pass
round the secondary. An exhausted bulb, Fig. 217, containing traces
of oxygen was placed within a primary spiral of three turns, and,
on passing the jar discharge, a circle of light was seen within the
bulb in close proximity to the primary circuit, accompanied by a
purplish glow, which lasted for a second or more. On heating the
bulb, the duration of the glow was greatly diminished, and it could
be instantly extinguished by the presence of an electro-magnet.
Another exhausted bulb, Fig. 218, surrounded by a primary spiral,
was contained in a bell-jar, and when the pressure of air in the
jar was about that of the atmosphere, the secondary discharge
occurred in the bulb, as is ordinarily the case. On exhausting the
jar, however, the luminous discharge grew fainter, and a point was
reached at which no secondary discharge was visible. Further
exhaustion of the jar caused the secondary discharge to appear
outside of the bulb. The fact of obtaining no luminous discharge,
either in the bulb or jar, the author could only explain on two
suppositions, viz.: that under the conditions then existing the
specific inductive capacity of the gas was very great, or that a
discharge could pass without being luminous. The author had also
observed that the conductivity of a vacuum tube without electrodes
increased as the pressure diminished, until a certain point was
reached, and afterwards diminished again, thus showing that the
high resistance of a nearly perfect vacuum is in no way due to the
presence of the electrodes. One peculiarity of the discharges was
their local nature, the rings of light being much more sharply
defined than was to be expected. They were also found to be most
easily produced when the chain of molecules in the discharge were
all of the same kind. For example, a discharge could be easily sent
through a tube many feet long, but the introduction of a small
pellet of mercury in the tube stopped the discharge, although the
conductivity of the mercury was much greater than that of the
vacuum. In some cases he had noticed that a very fine wire placed
within a tube, on the side remote from the primary circuit, would
prevent a luminous discharge in that tube.
Fig. 219 shows an exhausted secondary coil of one loop containing
bulbs; the discharge passed along the inner side of the bulbs, the
primary coils being placed within the secondary.
[9]In _The Electrical Engineer_ of August 12, I find some remarks of
Prof. J. J. Thomson, which appeared originally in the London
_Electrician_ and which have a bearing upon some experiments described
by me in your issue of July 1.
[9] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
August 26, 1891.
I did not, as Prof. J. J. Thomson seems to believe, misunderstand his
position in regard to the cause of the phenomena considered, but I
thought that in his experiments, as well as in my own, electrostatic
effects were of great importance. It did not appear, from the meagre
description of his experiments, that all possible precautions had been
taken to exclude these effects. I did not doubt that luminosity could be
excited in a closed tube when electrostatic action is completely
excluded. In fact, at the outset, I myself looked for a purely
electrodynamic effect and believed that I had obtained it. But many
experiments performed at that time proved to me that the electrostatic
effects were generally of far greater importance, and admitted of a more
satisfactory explanation of most of the phenomena observed.
In using the term _electrostatic_ I had reference rather to the nature
of the action than to a stationary condition, which is the usual
acceptance of the term. To express myself more clearly, I will suppose
that near a closed exhausted tube be placed a small sphere charged to a
very high potential. The sphere would act inductively upon the tube, and
by distributing electricity over the same would undoubtedly produce
luminosity (if the potential be sufficiently high), until a permanent
condition would be reached. Assuming the tube to be perfectly well
insulated, there would be only one instantaneous flash during the act of
distribution. This would be due to the electrostatic action simply.
But now, suppose the charged sphere to be moved at short intervals with
great speed along the exhausted tube. The tube would now be permanently
excited, as the moving sphere would cause a constant redistribution of
electricity and collisions of the molecules of the rarefied gas. We
would still have to deal with an electrostatic effect, and in addition
an electrodynamic effect would be observed. But if it were found that,
for instance, the effect produced depended more on the specific
inductive capacity than on the magnetic permeability of the
medium--which would certainly be the case for speeds incomparably lower
than that of light--then I believe I would be justified in saying that
the effect produced was more of an electrostatic nature. I do not mean
to say, however, that any similar condition prevails in the case of the
discharge of a Leyden jar through the primary, but I think that such an
action would be desirable.
It is in the spirit of the above example that I used the terms "more of
an electrostatic nature," and have investigated the influence of bodies
of high specific inductive capacity, and observed, for instance, the
importance of the quality of glass of which the tube is made. I also
endeavored to ascertain the influence of a medium of high permeability
by using oxygen. It appeared from rough estimation that an oxygen tube
when excited under similar conditions--that is, as far as could be
determined--gives more light; but this, of course, may be due to many
causes.
Without doubting in the least that, with the care and precautions taken
by Prof. J. J. Thomson, the luminosity excited was due solely to
electrodynamic action, I would say that in many experiments I have
observed curious instances of the ineffectiveness of the screening, and
I have also found that the electrification through the air is often of
very great importance, and may, in some cases, determine the excitation
of the tube.
In his original communication to the _Electrician_, Prof. J. J. Thomson
refers to the fact that the luminosity in a tube near a wire through
which a Leyden jar was discharged was noted by Hittorf. I think that the
feeble luminous effect referred to has been noted by many
experimenters, but in my experiments the effects were much more powerful
than those usually noted.
The following is the communication[10] referred to:--
[10] Note by Prof. J. J. Thomson in the London _Electrician_,
July 24, 1891.
"Mr. Tesla seems to ascribe the effects he observed to
electrostatic action, and I have no doubt, from the description he
gives of his method of conducting his experiments, that in them
electrostatic action plays a very important part. He seems,
however, to have misunderstood my position with respect to the
cause of these discharges, which is not, as he implies, that
luminosity in tubes without electrodes cannot be produced by
electrostatic action, but that it can also be produced when this
action is excluded. As a matter of fact, it is very much easier to
get the luminosity when these electrostatic effects are operative
than when they are not. As an illustration of this I may mention
that the first experiment I tried with the discharge of a Leyden
jar produced luminosity in the tube, but it was not until after six
weeks' continuous experimenting that I was able to get a discharge
in the exhausted tube which I was satisfied was due to what is
ordinarily called electrodynamic action. It is advisable to have a
clear idea of what we mean by electrostatic action. If, previous to
the discharge of the jar, the primary coil is raised to a high
potential, it will induce over the glass of the tube a distribution
of electricity. When the potential of the primary suddenly falls,
this electrification will redistribute itself, and may pass through
the rarefied gas and produce luminosity in doing so. Whilst the
discharge of the jar is going on, it is difficult, and, from a
theoretical point of view, undesirable, to separate the effect into
parts, one of which is called electrostatic, the other
electromagnetic; what we can prove is that in this case the
discharge is not such as would be produced by electromotive forces
derived from a potential function. In my experiments the primary
coil was connected to earth, and, as a further precaution, the
primary was separated from the discharge tube by a screen of
blotting paper, moistened with dilute sulphuric acid, and connected
to earth. Wet blotting paper is a sufficiently good conductor to
screen off a stationary electrostatic effect, though it is not a
good enough one to stop waves of alternating electromotive
intensity. When showing the experiments to the Physical Society I
could not, of course, keep the tubes covered up, but, unless my
memory deceives me, I stated the precautions which had been taken
against the electrostatic effect. To correct misapprehension I may
state that I did not read a formal paper to the Society, my object
being to exhibit a few of the most typical experiments. The account
of the experiments in the _Electrician_ was from a reporter's note,
and was not written, or even read, by me. I have now almost
finished writing out, and hope very shortly to publish, an account
of these and a large number of allied experiments, including some
analogous to those mentioned by Mr. Tesla on the effect of
conductors placed near the discharge tube, which I find, in some
cases, to produce a diminution, in others an increase, in the
brightness of the discharge, as well as some on the effect of the
presence of substances of large specific inductive capacity. These
seem to me to admit of a satisfactory explanation, for which,
however, I must refer to my paper."
PART III.
MISCELLANEOUS INVENTIONS AND WRITINGS.
CHAPTER XXXIII.
METHOD OF OBTAINING DRIECT FROM ALTERNATING CURRENTS.
This method consists in obtaining direct from alternating currents, or
in directing the waves of an alternating current so as to produce direct
or substantially direct currents by developing or producing in the
branches of a circuit including a source of alternating currents, either
permanently or periodically, and by electric, electro-magnetic, or
magnetic agencies, manifestations of energy, or what may be termed
active resistances of opposite electrical character, whereby the
currents or current waves of opposite sign will be diverted through
different circuits, those of one sign passing over one branch and those
of opposite sign over the other.
We may consider herein only the case of a circuit divided into two
paths, inasmuch as any further subdivision involves merely an extension
of the general principle. Selecting, then, any circuit through which is
flowing an alternating current, Mr. Tesla divides such circuit at any
desired point into two branches or paths. In one of these paths he
inserts some device to create an electromotive force counter to the
waves or impulses of current of one sign and a similar device in the
other branch which opposes the waves of opposite sign. Assume, for
example, that these devices are batteries, primary or secondary, or
continuous current dynamo machines. The waves or impulses of opposite
direction composing the main current have a natural tendency to divide
between the two branches; but by reason of the opposite electrical
character or effect of the two branches, one will offer an easy passage
to a current of a certain direction, while the other will offer a
relatively high resistance to the passage of the same current. The
result of this disposition is, that the waves of current of one sign
will, partly or wholly, pass over one of the paths or branches, while
those of the opposite sign pass over the other. There may thus be
obtained from an alternating current two or more direct currents without
the employment of any commutator such as it has been heretofore
regarded as necessary to use. The current in either branch may be
used in the same way and for the same purposes as any other direct
current--that is, it may be made to charge secondary batteries, energize
electro-magnets, or for any other analogous purpose.
Fig. 220 represents a plan of directing the alternating currents by
means of devices purely electrical in character. Figs. 221, 222, 223,
224, 225, and 226 are diagrams illustrative of other ways of carrying
out the invention.
[Illustration: FIG. 220.]
In Fig. 220, A designates a generator of alternating currents, and B B
the main or line circuit therefrom. At any given point in this circuit
at or near which it is desired to obtain direct currents, the circuit B
is divided into two paths or branches C D. In each of these branches is
placed an electrical generator, which for the present we will assume
produces direct or continuous currents. The direction of the current
thus produced is opposite in one branch to that of the current in the
other branch, or, considering the two branches as forming a closed
circuit, the generators E F are connected up in series therein, one
generator in each part or half of the circuit. The electromotive force
of the current sources E and F may be equal to or higher or lower than
the electromotive forces in the branches C D, or between the points X
and Y of the circuit B B. If equal, it is evident that current waves of
one sign will be opposed in one branch and assisted in the other to such
an extent that all the waves of one sign will pass over one branch and
those of opposite sign over the other. If, on the other hand, the
electromotive force of the sources E F be lower than that between X and
Y, the currents in both branches will be alternating, but the waves of
one sign will preponderate. One of the generators or sources of current
E or F may be dispensed with; but it is preferable to employ both, if
they offer an appreciable resistance, as the two branches will be
thereby better balanced. The translating or other devices to be acted
upon by the current are designated by the letters G, and they are
inserted in the branches C D in any desired manner; but in order to
better preserve an even balance between the branches due regard should,
of course, be had to the number and character of the devices.
[Illustration: FIG. 221.]
Figs. 221, 222, 223, and 224 illustrate what may termed
"electro-magnetic" devices for accomplishing a similar result--that is
to say, instead of producing directly by a generator an electromotive
force in each branch of the circuit, Mr. Tesla establishes a field or
fields of force and leads the branches through the same in such manner
that an active opposition of opposite effect or direction will be
developed therein by the passage, or tendency to pass, of the
alternations of current. In Fig. 221, for example, A is the generator of
alternating currents, B B the line circuit, and C D the branches over
which the alternating currents are directed. In each branch is included
the secondary of a transformer or induction coil, which, since they
correspond in their functions to the batteries of the previous figure,
are designated by the letters E F. The primaries H H' of the induction
coils or transformers are connected either in parallel or series with a
source of direct or continuous currents I, and the number of
convolutions is so calculated for the strength of the current from I
that the cores J J' will be saturated. The connections are such that the
conditions in the two transformers are of opposite character--that is to
say, the arrangement is such that a current wave or impulse
corresponding in direction with that of the direct current in one
primary, as H, is of opposite direction to that in the other primary H'.
It thus results that while one secondary offers a resistance or
opposition to the passage through it of a wave of one sign, the other
secondary similarly opposes a wave of opposite sign. In consequence, the
waves of one sign will, to a greater or less extent, pass by way of one
branch, while those of opposite sign in like manner pass over the other
branch.
In lieu of saturating the primaries by a source of continuous current,
we may include the primaries in the branches C D, respectively, and
periodically short-circuit by any suitable mechanical devices--such as
an ordinary revolving commutator--their secondaries. It will be
understood, of course, that the rotation and action of the commutator
must be in synchronism or in proper accord with the periods of the
alternations in order to secure the desired results. Such a disposition
is represented diagrammatically in Fig. 222. Corresponding to the
previous figures, A is the generator of alternating currents, B B the
line, and C D the two branches for the direct currents. In branch C are
included two primary coils E E', and in branch D are two similar
primaries F F' The corresponding secondaries for these coils and which
are on the same subdivided cores J or J', are in circuits the terminals
of which connect to opposite segments K K', and L L', respectively, of a
commutator. Brushes _b b_ bear upon the commutator and alternately
short-circuit the plates K and K', and L and L', through a connection
_c_. It is obvious that either the magnets and commutator, or the
brushes, may revolve.
[Illustration: FIG. 222.]
The operation will be understood from a consideration of the effects of
closing or short-circuiting the secondaries. For example, if at the
instant when a given wave of current passes, one set of secondaries be
short-circuited, nearly all the current flows through the corresponding
primaries; but the secondaries of the other branch being open-circuited,
the self-induction in the primaries is highest, and hence little or no
current will pass through that branch. If, as the current alternates,
the secondaries of the two branches are alternately short-circuited, the
result will be that the currents of one sign pass over one branch and
those of the opposite sign over the other. The disadvantages of this
arrangement, which would seem to result from the employment of sliding
contacts, are in reality very slight, inasmuch as the electromotive
force of the secondaries may be made exceedingly low, so that sparking
at the brushes is avoided.
[Illustration: FIG. 223.]
Fig. 223 is a diagram, partly in section, of another plan of carrying
out the invention. The circuit B in this case is divided, as before, and
each branch includes the coils of both the fields and revolving
armatures of two induction devices. The armatures O P are preferably
mounted on the same shaft, and are adjusted relatively to one another in
such manner that when the self-induction in one branch, as C, is
maximum, in the other branch D it is minimum. The armatures are rotated
in synchronism with the alternations from the source A. The winding or
position of the armature coils is such that a current in a given
direction passed through both armatures would establish in one, poles
similar to those in the adjacent poles of the field, and in the other,
poles unlike the adjacent field poles, as indicated by _n n s s_ in the
diagram. If the like poles are presented, as shown in circuit D, the
condition is that of a closed secondary upon a primary, or the position
of least inductive resistance; hence a given alternation of current will
pass mainly through D. A half revolution of the armatures produces an
opposite effect and the succeeding current impulse passes through C.
Using this figure as an illustration, it is evident that the fields N M
may be permanent magnets or independently excited and the armatures O P
driven, as in the present case, so as to produce alternate currents,
which will set up alternately impulses of opposite direction in the two
branches D C, which in such case would include the armature circuits and
translating devices only.
In Fig. 224 a plan alternative with that shown in Fig. 222 is
illustrated. In the previous case illustrated, each branch C and D
contained one or more primary coils, the secondaries of which were
periodically short circuited in synchronism with the alternations of
current from the main source A, and for this purpose a commutator was
employed. The latter may, however, be dispensed with and an armature
with a closed coil substituted.
[Illustration: FIG. 224.]
Referring to Fig. 224 in one of the branches, as C, are two coils M',
wound on laminated cores, and in the other branches D are similar coils
N'. A subdivided or laminated armature O', carrying a closed coil R', is
rotatably supported between the coils M' N', as shown. In the position
shown--that is, with the coil R' parallel with the convolutions of the
primaries N' M'--practically the whole current will pass through branch
D, because the self-induction in coils M' M' is maximum. If, therefore,
the armature and coil be rotated at a proper speed relatively to the
periods or alternations of the source A, the same results are obtained
as in the case of Fig. 222.
Fig. 225 is an instance of what may be called, in distinction to the
others, a "magnetic" means of securing the result. V and W are two
strong permanent magnets provided with armatures V' W', respectively.
The armatures are made of thin laminae of soft iron or steel, and the
amount of magnetic metal which they contain is so calculated that they
will be fully or nearly saturated by the magnets. Around the armatures
are coils E F, contained, respectively, in the circuits C and D. The
connections and electrical conditions in this case are similar to those
in Fig. 221, except that the current source of I, Fig. 221, is dispensed
with and the saturation of the core of coils E F obtained from the
permanent magnets.
[Illustration: FIG. 225.]
The previous illustrations have all shown the two branches or paths
containing the translating or induction devices as in derivation one to
the other; but this is not always necessary. For example, in Fig. 226, A
is an alternating-current generator; B B, the line wires or circuit. At
any given point in the circuit let us form two paths, as D D', and at
another point two paths, as C C'. Either pair or group of paths is
similar to the previous dispositions with the electrical source or
induction device in one branch only, while the two groups taken together
form the obvious equivalent of the cases in which an induction device or
generator is included in both branches. In one of the paths, as D, are
included the devices to be operated by the current. In the other branch,
as D', is an induction device that opposes the current impulses of one
direction and directs them through the branch D. So, also, in branch C
are translating devices G, and in branch C' an induction device or its
equivalent that diverts through C impulses of opposite direction to
those diverted by the device in branch D'. The diagram shows a special
form of induction device for this purpose. J J' are the cores, formed
with pole-pieces, upon which are wound the coils M N. Between these
pole-pieces are mounted at right angles to one another the magnetic
armatures O P, preferably mounted on the same shaft and designed to be
rotated in synchronism with the alternations of current. When one of the
armatures is in line with the poles or in the position occupied by
armature P, the magnetic circuit of the induction device is practically
closed; hence there will be the greatest opposition to the passage of a
current through coils N N. The alternation will therefore pass by way of
branch D. At the same time, the magnetic circuit of the other induction
device being broken by the position of the armature O, there will be
less opposition to the current in coils M, which will shunt the current
from branch C. A reversal of the current being attended by a shifting of
the armatures, the opposite effect is produced.
[Illustration: FIG. 226.]
Other modifications of these methods are possible, but need not be
pointed out. In all these plans, it will be observed, there is developed
in one or all of these branches of a circuit from a source of
alternating currents, an active (as distinguished from a dead)
resistance or opposition to the currents of one sign, for the purpose of
diverting the currents of that sign through the other or another path,
but permitting the currents of opposite sign to pass without substantial
opposition.
Whether the division of the currents or waves of current of opposite
sign be effected with absolute precision or not is immaterial, since it
will be sufficient if the waves are only partially diverted or directed,
for in such case the preponderating influence in each branch of the
circuit of the waves of one sign secures the same practical results in
many if not all respects as though the current were direct and
continuous.
An alternating and a direct current have been combined so that the waves
of one direction or sign were partially or wholly overcome by the direct
current; but by this plan only one set of alternations are utilized,
whereas by the system just described the entire current is rendered
available. By obvious applications of this discovery Mr. Tesla is
enabled to produce a self-exciting alternating dynamo, or to operate
direct current meters on alternating-current circuits or to run various
devices--such as arc lamps--by direct currents in the same circuit with
incandescent lamps or other devices operated by alternating currents.
It will be observed that if an intermittent counter or opposing force be
developed in the branches of the circuit and of higher electromotive
force than that of the generator, an alternating current will result in
each branch, with the waves of one sign preponderating, while a
constantly or uniformly acting opposition in the branches of higher
electromotive force than the generator would produce a pulsating
current, which conditions would be, under some circumstances, the
equivalent of those described.
CHAPTER XXXIV.
CONDENSERS WITH PLATES IN OIL.
[Illustration: FIG. 227.]
[Illustration: FIG. 228.]
In experimenting with currents of high frequency and high potential, Mr.
Tesla has found that insulating materials such as glass, mica, and in
general those bodies which possess the highest specific inductive
capacity, are inferior as insulators in such devices when currents of
the kind described are employed compared with those possessing high
insulating power, together with a smaller specific inductive capacity;
and he has also found that it is very desirable to exclude all gaseous
matter from the apparatus, or any access of the same to the electrified
surfaces, in order to prevent heating by molecular bombardment and the
loss or injury consequent thereon. He has therefore devised a method to
accomplish these results and produce highly efficient and reliable
condensers, by using oil as the dielectric[11]. The plan admits of a
particular construction of condenser, in which the distance between the
plates is adjustable, and of which he takes advantage.
[11] Mr. Tesla's experiments, as the careful reader of his three
lectures will perceive, have revealed a very important fact which
is taken advantage of in this invention. Namely, he has shown that
in a condenser a considerable amount of energy may be wasted, and
the condenser may break down merely because gaseous matter is
present between the surfaces. A number of experiments are described
in the lectures, which bring out this fact forcibly and serve as a
guide in the operation of high tension apparatus. But besides
bearing upon this point, these experiments also throw a light upon
investigations of a purely scientific nature and explain now the
lack of harmony among the observations of various investigators.
Mr. Tesla shows that in a fluid such as oil the losses are very
small as compared with those incurred in a gas.
In the accompanying illustrations, Fig. 227 is a section of a condenser
constructed in accordance with this principle and having stationary
plates; and Fig. 228 is a similar view of a condenser with adjustable
plates.
Any suitable box or receptacle A may be used to contain the plates or
armatures. These latter are designated by B and C and are connected,
respectively, to terminals D and E, which pass out through the sides of
the case. The plates ordinarily are separated by strips of porous
insulating material F, which are used merely for the purpose of
maintaining them in position. The space within the can is filled with
oil G. Such a condenser will prove highly efficient and will not become
heated or permanently injured.
In many cases it is desirable to vary or adjust the capacity of a
condenser, and this is provided for by securing the plates to adjustable
supports--as, for example, to rods H--passing through stuffing boxes K
in the sides of case A and furnished with nuts L, the ends of the rods
being threaded for engagement with the nuts.
It is well known that oils possess insulating properties, and it has
been a common practice to interpose a body of oil between two conductors
for purposes of insulation; but Mr. Tesla believes he has discovered
peculiar properties in oils which render them very valuable in this
particular form of device.
CHAPTER XXXV.
ELECTROLYTIC REGISTERING METER.
An ingenious form of electrolytic meter attributable to Mr. Tesla is one
in which a conductor is immersed in a solution, so arranged that metal
may be deposited from the solution or taken away in such a manner that
the electrical resistance of the conductor is varied in a definite
proportion to the strength of the current the energy of which is to be
computed, whereby this variation in resistance serves as a measure of
the energy and also may actuate registering mechanism, whenever the
resistance rises above or falls below certain limits.
In carrying out this idea Mr. Tesla employs an electrolytic cell,
through which extend two conductors parallel and in close proximity to
each other. These conductors he connects in series through a resistance,
but in such manner that there is an equal difference of potential
between them throughout their entire extent. The free ends or terminals
of the conductors are connected either in series in the circuit
supplying the current to the lamps or other devices, or in parallel to a
resistance in the circuit and in series with the current consuming
devices. Under such circumstances a current passing through the
conductors establishes a difference of potential between them which is
proportional to the strength of the current, in consequence of which
there is a leakage of current from one conductor to the other across the
solution. The strength of this leakage current is proportional to the
difference of potential, and, therefore, in proportion to the strength
of the current passing through the conductors. Moreover, as there is a
constant difference of potential between the two conductors throughout
the entire extent that is exposed to the solution, the current density
through such solution is the same at all corresponding points, and hence
the deposit is uniform along the whole of one of the conductors, while
the metal is taken away uniformly from the other. The resistance of one
conductor is by this means diminished, while that of the other is
increased, both in proportion to the strength of the current passing
through the conductors. From such variation in the resistance of either
or both of the conductors forming the positive and negative electrodes
of the cell, the current energy expended may be readily computed. Figs.
229 and 230 illustrate two forms of such a meter.
[Illustration: FIG. 229.]
In Fig. 229 G designates a direct-current generator. L L are the
conductors of the circuit extending therefrom. A is a tube of glass, the
ends of which are sealed, as by means of insulating plugs or caps B B. C
C' are two conductors extending through the tube A, their ends passing
out through the plugs B to terminals thereon. These conductors may be
corrugated or formed in other proper ways to offer the desired
electrical resistance. R is a resistance connected in series with the
two conductors C C', which by their free terminals are connected up in
circuit with one of the conductors L.
The method of using this device and computing by means thereof the
energy of the current will be readily understood. First, the resistances
of the two conductors C C', respectively, are accurately measured and
noted. Then a known current is passed through the instrument for a given
time, and by a second measurement the increase and diminution of the
resistances of the two conductors are respectively taken. From these
data the constant is obtained--that is to say, for example, the
increase of resistance of one conductor or the diminution of the
resistance of the other per lamp hour. These two measurements evidently
serve as a check, since the gain of one conductor should equal the loss
of the other. A further check is afforded by measuring both wires in
series with the resistance, in which case the resistance of the whole
should remain constant.
[Illustration: FIG. 230.]
In Fig. 230 the conductors C C' are connected in parallel, the current
device at X passing in one branch first through a resistance R' and then
through conductor C, while on the other branch it passes first through
conductor C', and then through resistance R''. The resistances R' R''
are equal, as also are the resistances of the conductors C C'. It is,
moreover, preferable that the respective resistances of the conductors C
C' should be a known and convenient fraction of the coils or resistances
R' R''. It will be observed that in the arrangement shown in Fig. 230
there is a constant potential difference between the two conductors C C'
throughout their entire length.
It will be seen that in both cases illustrated, the proportionality of
the increase or decrease of resistance to the current strength will
always be preserved, for what one conductor gains the other loses, and
the resistances of the conductors C C' being small as compared with the
resistances in series with them. It will be understood that after each
measurement or registration of a given variation of resistance in one or
both conductors, the direction of the current should be changed or the
instrument reversed, so that the deposit will be taken from the
conductor which has gained and added to that which has lost. This
principle is capable of many modifications. For instance, since there is
a section of the circuit--to wit, the conductor C or C'--that varies in
resistance in proportion to the current strength, such variation may be
utilized, as is done in many analogous cases, to effect the operation of
various automatic devices, such as registers. It is better, however, for
the sake of simplicity to compute the energy by measurements of
resistance.
The chief advantages of this arrangement are, first, that it is possible
to read off directly the amount of the energy expended by means of a
properly constructed ohm-meter and without resorting to weighing the
deposit; secondly it is not necessary to employ shunts, for the whole of
the current to be measured may be passed through the instrument; third,
the accuracy of the instrument and correctness of the indications are
but slightly affected by changes in temperature. It is also said that
such meters have the merit of superior economy and compactness, as well
as of cheapness in construction. Electrolytic meters seem to need every
auxiliary advantage to make them permanently popular and successful, no
matter how much ingenuity may be shown in their design.
CHAPTER XXXVI.
THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.
No electrical inventor of the present day dealing with the problems of
light and power considers that he has done himself or his opportunities
justice until he has attacked the subject of thermo-magnetism. As far
back as the beginning of the seventeenth century it was shown by Dr.
William Gilbert, the father of modern electricity, that a loadstone or
iron bar when heated to redness loses its magnetism; and since that time
the influence of heat on the magnetic metals has been investigated
frequently, though not with any material or practical result.
For a man of Mr. Tesla's inventive ability, the problems in this field
have naturally had no small fascination, and though he has but glanced
at them, it is to be hoped he may find time to pursue the study deeper
and further. For such as he, the investigation must undoubtedly bear
fruit. Meanwhile he has worked out one or two operative devices worthy
of note.[12] He obtains mechanical power by a reciprocating action
resulting from the joint operations of heat, magnetism, and a spring or
weight or other force--that is to say he subjects a body magnetized by
induction or otherwise to the action of heat until the magnetism is
sufficiently neutralized to allow a weight or spring to give motion to
the body and lessen the action of the heat, so that the magnetism may be
sufficiently restored to move the body in the opposite direction, and
again subject the same to the demagnetizing power of the heat.
[12] It will, of course, be inferred from the nature of these devices
that the vibration obtained in this manner is very slow owing to
the inability of the iron to follow rapid changes in temperature.
In an interview with Mr. Tesla on this subject, the compiler
learned of an experiment which will interest students. A simple
horseshoe magnet is taken and a piece of sheet iron bent in the
form of an L is brought in contact with one of the poles and
placed in such a position that it is kept in the attraction of
the opposite pole delicately suspended. A spirit lamp is placed
under the sheet iron piece and when the iron is heated to a
certain temperature it is easily set in vibration oscillating
as rapidly as 400 to 500 times a minute. The experiment is very
easily performed and is interesting principally on account of the
very rapid rate of vibration.
Use is made of either an electro-magnet or a permanent magnet, and the
heat is directed against a body that is magnetized by induction, rather
than directly against a permanent magnet, thereby avoiding the loss of
magnetism that might result in the permanent magnet by the action of
heat. Mr. Tesla also provides for lessening the volume of the heat or
for intercepting the same during that portion of the reciprocation in
which the cooling action takes place.
In the diagrams are shown some of the numerous arrangements that may be
made use of in carrying out this idea. In all of these figures the
magnet-poles are marked N S, the armature A, the Bunsen burner or other
source of heat H, the axis of motion M, and the spring or the equivalent
thereof--namely, a weight--is marked W.
[Illustration: FIG. 232.]
[Illustration: FIG. 231.]
[Illustration: FIG. 233.]
In Fig. 231 the permanent magnet N is connected with a frame, F,
supporting the axis M, from which the arm P hangs, and at the lower end
of which the armature A is supported. The stops 2 and 3 limit the extent
of motion, and the spring W tends to draw the armature A away from the
magnet N. It will now be understood that the magnetism of N is
sufficient to overcome the spring W and draw the armature A toward the
magnet N. The heat acting upon the armature A neutralizes its induced
magnetism sufficiently for the spring W to draw the armature A away from
the magnet N and also from the heat at H. The armature now cools, and
the attraction of the magnet N overcomes the spring W and draws the
armature A back again above the burner H, so that the same is again
heated and the operations are repeated. The reciprocating movements thus
obtained are employed as a source of mechanical power in any desired
manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would
be made use of, as indicated in Fig. 240.
[Illustration: FIG. 234.]
[Illustration: FIG. 236.]
[Illustration: FIG. 235.]
Fig. 232 represents the same parts as before described; but an
electro-magnet is illustrated in place of a permanent magnet. The
operations, however, are the same.
In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they
are differently arranged. The armature A, instead of swinging, is
stationary and held by arm P', and the core N S of the electro-magnet is
made to swing within the helix Q, the core being suspended by the arm P
from the pivot M. A shield, R, is connected with the magnet-core and
swings with it, so that after the heat has demagnetized the armature A
to such an extent that the spring W draws the core N S away from the
armature A, the shield R comes between the flame H and armature A,
thereby intercepting the action of the heat and allowing the armature to
cool, so that the magnetism, again preponderating, causes the movement
of the core N S toward the armature A and the removal of the shield R
from above the flame, so that the heat again acts to lessen or
neutralize the magnetism. A rotary or other movement may be obtained
from this reciprocation.
Fig. 234 corresponds in every respect with Fig. 233, except that a
permanent horseshoe-magnet, N S is represented as taking the place of
the electro-magnet in Fig. 233.
In Fig. 235 is shown a helix, Q, with an armature adapted to swing
toward or from the helix. In this case there may be a soft-iron core in
the helix, or the armature may assume the form of a solenoid core, there
being no permanent core within the helix.
[Illustration: FIG. 237.]
[Illustration: FIG. 238.]
[Illustration: FIG. 239.]
Fig. 236 is an end view, and Fig. 237 a plan view, illustrating the
method as applied to a swinging armature, A, and a stationary permanent
magnet, N S. In this instance Mr. Tesla applies the heat to an auxiliary
armature or keeper, T, which is adjacent to and preferably in direct
contact with the magnet. This armature T, in the form of a plate of
sheet-iron, extends across from one pole to the other and is of
sufficient section to practically form a keeper for the magnet, so that
when the armature T is cool nearly all the lines of force pass over the
same and very little free magnetism is exhibited. Then the armature A,
which swings freely on the pivots M in front of the poles N S, is very
little attracted and the spring W pulls the same way from the poles into
the position indicated in the diagram. The heat is directed upon the
iron plate T at some distance from the magnet, so as to allow the magnet
to keep comparatively cool. This heat is applied beneath the plate by
means of the burners H, and there is a connection from the armature A or
its pivot to the gas-cock 6, or other device for regulating the heat.
The heat acting upon the middle portion of the plate T, the magnetic
conductivity of the heated portion is diminished or destroyed, and a
great number of the lines of force are deflected over the armature A,
which is now powerfully attracted and drawn into line, or nearly so,
with the poles N S. In so doing the cock 6 is nearly closed and the
plate T cools, the lines of force are again deflected over the same, the
attraction exerted upon the armature A is diminished, and the spring W
pulls the same away from the magnet into the position shown by full
lines, and the operations are repeated. The arrangement shown in Fig.
236 has the advantages that the magnet and armature are kept cool and
the strength of the permanent magnet is better preserved, as the
magnetic circuit is constantly closed.
In the plan view, Fig. 238, is shown a permanent magnet and keeper
plate, T, similar to those in Figs. 236 and 237, with the burners H for
the gas beneath the same; but the armature is pivoted at one end to one
pole of the magnet and the other end swings toward and from the other
pole of the magnet. The spring W acts against a lever arm that projects
from the armature, and the supply of heat has to be partly cut off by a
connection to the swinging armature, so as to lessen the heat acting
upon the keeper plate when the armature A has been attracted.
[Illustration: FIG. 240.]
[Illustration: FIG. 241.]
Fig. 239 is similar to Fig. 238, except that the keeper T is not made
use of and the armature itself swings into and out of the range of the
intense action of the heat from the burner H. Fig. 240 is a diagram
similar to Fig. 231, except that in place of using a spring and stops,
the armature is shown as connected by a link, to the crank of a
fly-wheel, so that the fly-wheel will be revolved as rapidly as the
armature can be heated and cooled to the necessary extent. A spring may
be used in addition, as in Fig. 231. In Fig. 241 the armatures A A are
connected by a link, so that one will be heating while the other is
cooling, and the attraction exerted to move the cooled armature is
availed of to draw away the heated armature instead of using a spring.
Mr. Tesla has also devoted his attention to the development of a
pyromagnetic generator of electricity[13] based upon the following laws:
First, that electricity or electrical energy is developed in any
conducting body by subjecting such body to a varying magnetic influence;
and second, that the magnetic properties of iron or other magnetic
substance may be partially or entirely destroyed or caused to disappear
by raising it to a certain temperature, but restored and caused to
reappear by again lowering its temperature to a certain degree. These
laws may be applied in the production of electrical currents in many
ways, the principle of which is in all cases the same, viz., to subject
a conductor to a varying magnetic influence, producing such variations
by the application of heat, or, more strictly speaking, by the
application or action of a varying temperature upon the source of the
magnetism. This principle of operation may be illustrated by a simple
experiment: Place end to end, and preferably in actual contact, a
permanently magnetized steel bar and a strip or bar of soft iron. Around
the end of the iron bar or plate wind a coil of insulated wire. Then
apply to the iron between the coil and the steel bar a flame or other
source of heat which will be capable of raising that portion of the iron
to an orange red, or a temperature of about 600 deg. centigrade. When this
condition is reached, the iron somewhat suddenly loses its magnetic
properties, if it be very thin, and the same effect is produced as
though the iron had been moved away from the magnet or the heated
section had been removed. This change of position, however, is
accompanied by a shifting of the magnetic lines, or, in other words, by
a variation in the magnetic influence to which the coil is exposed, and
a current in the coil is the result. Then remove the flame or in any
other way reduce the temperature of the iron. The lowering of its
temperature is accompanied by a return of its magnetic properties, and
another change of magnetic conditions occurs, accompanied by a current
in an opposite direction in the coil. The same operation may be
repeated indefinitely, the effect upon the coil being similar to that
which would follow from moving the magnetized bar to and from the end of
the iron bar or plate.
[13] The chief point to be noted is that Mr. Tesla attacked this
problem in a way which was, from the standpoint of theory, and
that of an engineer, far better than that from which some
earlier trials in this direction started. The enlargement of
these ideas will be found in Mr. Tesla's work on the pyromagnetic
generator, treated in this chapter. The chief effort of the
inventor was to economize the heat, which was accomplished by
inclosing the iron in a source of heat well insulated, and by
cooling the iron by means of steam, utilizing the steam over
again. The construction also permits of more rapid magnetic
changes per unit of time, meaning larger output.
The device illustrated below is a means of obtaining this result, the
features of novelty in the invention being, first, the employment of an
artificial cooling device, and, second, inclosing the source of heat and
that portion of the magnetic circuit exposed to the heat and
artificially cooling the heated part.
These improvements are applicable generally to the generators
constructed on the plan above described--that is to say, we may use an
artificial cooling device in conjunction with a variable or varied or
uniform source of heat.
[Illustration: FIG. 242.]
[Illustration: FIG. 243.]
Fig. 242 is a central vertical longitudinal section of the complete
apparatus and Fig. 243 is a cross-section of the magnetic armature-core
of the generator.
Let A represent a magnetized core or permanent magnet the poles of which
are bridged by an armature-core composed of a casing or shell B
inclosing a number of hollow iron tubes C. Around this core are wound
the conductors E E', to form the coils in which the currents are
developed. In the circuits of these coils are current-consuming devices,
as F F'.
D is a furnace or closed fire-box, through which the central portion of
the core B extends. Above the fire is a boiler K, containing water. The
flue L from the fire-box may extend up through the boiler.
G is a water-supply pipe, and H is the steam-exhaust pipe, which
communicates with all the tubes C in the armature B, so that steam
escaping from the boiler will pass through the tubes.
In the steam-exhaust pipe H is a valve V, to which is connected the
lever I, by the movement of which the valve is opened or closed. In such
a case as this the heat of the fire may be utilized for other purposes
after as much of it as may be needed has been applied to heating the
core B. There are special advantages in the employment of a cooling
device, in that the metal of the core B is not so quickly oxidized.
Moreover, the difference between the temperature of the applied heat and
of the steam, air, or whatever gas or fluid be applied as the cooling
medium, may be increased or decreased at will, whereby the rapidity of
the magnetic changes or fluctuations may be regulated.
CHAPTER XXXVII.
ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.
In direct current dynamos of great electromotive force--such, for
instance, as those used for arc lighting--when one commutator bar or
plate comes out of contact with the collecting-brush a spark is apt to
appear on the commutator. This spark may be due to the break of the
complete circuit, or to a shunt of low resistance formed by the brush
between two or more commutator-bars. In the first case the spark is more
apparent, as there is at the moment when the circuit is broken a
discharge of the magnets through the field helices, producing a great
spark or flash which causes an unsteady current, rapid wear of the
commutator bars and brushes, and waste of power. The sparking may be
reduced by various devices, such as providing a path for the current at
the moment when the commutator segment or bar leaves the brush, by
short-circuiting the field-helices, by increasing the number of the
commutator-bars, or by other similar means; but all these devices are
expensive or not fully available, and seldom attain the object desired.
To prevent this sparking in a simple manner, Mr. Tesla some years ago
employed with the commutator-bars and intervening insulating material,
mica, asbestos paper or other insulating and incombustible material,
arranged to bear on the surface of the commutator, near to and behind
the brush.
In the drawings, Fig. 244 is a section of a commutator with an asbestos
insulating device; and Fig. 245 is a similar view, representing two
plates of mica upon the back of the brush.
In 244, C represents the commutator and intervening insulating material;
B B, the brushes. _d d_ are sheets of asbestos paper or other suitable
non-conducting material. _f f_ are springs, the pressure of which may be
adjusted by means of the screws _g g_.
In Fig. 245 a simple arrangement is shown with two plates of mica or
other material. It will be seen that whenever one commutator segment
passes out of contact with the brush, the formation of the arc will be
prevented by the intervening insulating material coming in contact with
the insulating material on the brush.
[Illustration: FIG. 244.]
[Illustration: FIG. 245.]
Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia,
or other suitable material, may be used, as the paper and cloth are
soft, and serve at the same time to wipe and polish the commutator; but
mica or any other suitable material can be employed, provided the
material be an insulator or a bad conductor of electricity.
A few years later Mr. Tesla turned his attention again to the same
subject, as, perhaps, was very natural in view of the fact that the
commutator had always been prominent in his thoughts, and that so much
of his work was even aimed at dispensing with it entirely as an
objectionable and unnecessary part of dynamos and motors. In these later
efforts to remedy commutator troubles, Mr. Tesla constructs a commutator
and the collectors therefor in two parts mutually adapted to one
another, and, so far as the essential features are concerned, alike in
mechanical structure. Selecting as an illustration a commutator of two
segments adapted for use with an armature the coils or coil of which
have but two free ends, connected respectively to the segments, the
bearing-surface is the face of a disc, and is formed of two metallic
quadrant segments and two insulating segments of the same dimensions,
and the face of the disc is smoothed off, so that the metal and
insulating segments are flush. The part which takes the place of the
usual brushes, or the "collector," is a disc of the same character as
the commutator and has a surface similarly formed with two insulating
and two metallic segments. These two parts are mounted with their faces
in contact and in such manner that the rotation of the armature causes
the commutator to turn upon the collector, whereby the currents induced
in the coils are taken off by the collector segments and thence
conveyed off by suitable conductors leading from the collector segments.
This is the general plan of the construction adopted. Aside from certain
adjuncts, the nature and functions of which are set forth later, this
means of commutation will be seen to possess many important advantages.
In the first place the short-circuiting and the breaking of the armature
coil connected to the commutator-segments occur at the same instant, and
from the nature of the construction this will be done with the greatest
precision; secondly, the duration of both the break and of the short
circuit will be reduced to a minimum. The first results in a reduction
which amounts practically to a suppression of the spark, since the break
and the short circuit produce opposite effects in the armature-coil. The
second has the effect of diminishing the destructive effect of a spark,
since this would be in a measure proportional to the duration of the
spark; while lessening the duration of the short circuit obviously
increases the efficiency of the machine.
[Illustration: FIG. 246.]
[Illustration: FIG. 247.]
The mechanical advantages will be better understood by referring to the
accompanying diagrams, in which Fig. 246 is a central longitudinal
section of the end of a shaft with the improved commutator carried
thereon. Fig. 247 is a view of the inner or bearing face of the
collector. Fig. 248 is an end view from the armature side of a modified
form of commutator. Figs. 249 and 250 are views of details of Fig. 248.
Fig. 251 is a longitudinal central section of another modification, and
Fig. 252 is a sectional view of the same. A is the end of the
armature-shaft of a dynamo-electric machine or motor. A' is a sleeve of
insulating material around the shaft, secured in place by a screw, _a'_.
[Illustration: FIG. 248.]
[Illustration: FIG. 249.]
[Illustration: FIG. 250.]
The commutator proper is in the form of a disc which is made up of four
segments D D' G G', similar to those shown in Fig. 248. Two of these
segments, as D D', are of metal and are in electrical connection with
the ends of the coils on the armature. The other two segments are of
insulating material. The segments are held in place by a band, B, of
insulating material. The disc is held in place by friction or by screws,
_g' g'_, Fig. 248, which secure the disc firmly to the sleeve A'.
The collector is made in the same form as the commutator. It is composed
of the two metallic segments E E' and the two insulating segments F F',
bound together by a band, C. The metallic segments E E' are of the same
or practically the same width or extent as the insulating segments or
spaces of the commutator. The collector is secured to a sleeve, B', by
screws _g g_, and the sleeve is arranged to turn freely on the shaft A.
The end of the sleeve B' is closed by a plate, _f_, upon which presses a
pivot-pointed screw, _h_, adjustable in a spring, H, which acts to
maintain the collector in close contact with the commutator and to
compensate for the play of the shaft. The collector is so fixed that it
cannot turn with the shaft. For example, the diagram shows a slotted
plate, K, which is designed to be attached to a stationary support, and
an arm extending from the collector and carrying a clamping screw, L, by
which the collector may be adjusted and set to the desired position.
Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit the
insulating segments of both commutator and collector loosely and to
provide some means--as, for example, light springs, _e e_, secured to
the bands A' B', respectively, and bearing against the segments--to
exert a light pressure upon them and keep them in close contact and to
compensate for wear. The metal segments of the commutator may be moved
forward by loosening the screw _a'_.
The line wires are fed from the metal segments of the collector, being
secured thereto in any convenient manner, the plan of connections being
shown as applied to a modified form of the commutator in Fig. 251. The
commutator and the collector in thus presenting two flat and smooth
bearing surfaces prevent most effectually by mechanical action the
occurrence of sparks.
The insulating segments are made of some hard material capable of being
polished and formed with sharp edges. Such materials as glass, marble,
or soapstone may be advantageously used. The metal segments are
preferably of copper or brass; but they may have a facing or edge of
durable material--such as platinum or the like--where the sparks are
liable to occur.
[Illustration: FIG. 251.]
[Illustration: FIG. 252.]
In Fig. 248 a somewhat modified form of the invention is shown, a form
designed to facilitate the construction and replacing of the parts. In
this modification the commutator and collector are made in substantially
the same manner as previously described, except that the bands B C are
omitted. The four segments of each part, however, are secured to their
respective sleeves by screws _g' g'_, and one edge of each segment is
cut away, so that small plates _a b_ may be slipped into the spaces thus
formed. Of these plates _a a_ are of metal, and are in contact with the
metal segments D D', respectively. The other two, _b b_, are of glass or
marble, and they are all better square, as shown in Figs. 249 and 250,
so that they may be turned to present new edges should any edge become
worn by use. Light springs _d_ bear upon these plates and press those in
the commutator toward those in the collector, and insulating strips _c
c_ are secured to the periphery of the discs to prevent the blocks from
being thrown out by centrifugal action. These plates are, of course,
useful at those edges of the segments only where sparks are liable to
occur, and, as they are easily replaced, they are of great advantage. It
is considered best to coat them with platinum or silver.
In Figs. 251 and 252 is shown a construction where, instead of solid
segments, a fluid is employed. In this case the commutator and collector
are made of two insulating discs, S T, and in lieu of the metal segments
a space is cut out of each part, as at R R', corresponding in shape and
size to a metal segment. The two parts are fitted smoothly and the
collector T held by the screw _h_ and spring H against the commutator S.
As in the other cases, the commutator revolves while the collector
remains stationary. The ends of the coils are connected to binding-posts
_s s_, which are in electrical connection with metal plates _t t_ within
the recesses in the two parts S T. These chambers or recesses are filled
with mercury, and in the collector part are tubes W W, with screws _w
w_, carrying springs X and pistons X', which compensate for the
expansion and contraction of the mercury under varying temperatures, but
which are sufficiently strong not to yield to the pressure of the fluid
due to centrifugal action, and which serve as binding-posts.
In all the above cases the commutators are adapted for a single coil,
and the device is particularly suited to such purposes. The number of
segments may be increased, however, or more than one commutator used
with a single armature. Although the bearing-surfaces are shown as
planes at right angles to the shaft or axis, it is evident that in this
particular the construction may be very greatly modified.
CHAPTER XXXVIII.
AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.
An interesting method devised by Mr. Tesla for the regulation of direct
current dynamos, is that which has come to be known as the "third brush"
method. In machines of this type, devised by him as far back as 1885, he
makes use of two main brushes to which the ends of the field magnet
coils are connected, an auxiliary brush, and a branch or shunt
connection from an intermediate point of the field wire to the auxiliary
brush.[14]
[14] The compiler has learned partially from statements made on
several occasions in journals and partially by personal inquiry
of Mr. Tesla, that a great deal of work in this interesting line
is unpublished. In these inventions as will be seen, the brushes
are automatically shifted, but in the broad method barely
suggested here the regulation is effected without any change in
the position of the brushes. This auxiliary brush invention, it
will be remembered, was very much discussed a few years ago, and
it may be of interest that this work of Mr. Tesla, then unknown
in this field, is now brought to light.
The relative positions of the respective brushes are varied, either
automatically or by hand, so that the shunt becomes inoperative when the
auxiliary brush has a certain position upon the commutator; but when the
auxiliary brush is moved in its relation to the main brushes, or the
latter are moved in their relation to the auxiliary brush, the electric
condition is disturbed and more or less of the current through the
field-helices is diverted through the shunt or a current is passed over
the shunt to the field-helices. By varying the relative position upon
the commutator of the respective brushes automatically in proportion to
the varying electrical conditions of the working-circuit, the current
developed can be regulated in proportion to the demands in the
working-circuit.
Fig. 253 is a diagram illustrating the invention, showing one core of
the field-magnets with one helix wound in the same direction throughout.
Figs. 254 and 255 are diagrams showing one core of the field-magnets
with a portion of the helices wound in opposite directions. Figs. 256
and 257 are diagrams illustrating the electric devices that may be
employed for automatically adjusting the brushes, and Fig. 258 is a
diagram illustrating the positions of the brushes when the machine is
being energized at the start.
_a_ and _b_ are the positive and negative brushes of the main or
working-circuit, and _c_ the auxiliary brush. The working-circuit D
extends from the brushes _a_ and _b_, as usual, and contains electric
lamps or other devices, D', either in series or in multiple arc.
M M' represent the field-helices, the ends of which are connected to the
main brushes _a_ and _b_. The branch or shunt wire _c'_ extends from the
auxiliary brush _c_ to the circuit of the field-helices, and is
connected to the same at an intermediate point, _x_.
[Illustration: FIG. 253.]
H represents the commutator, with the plates of ordinary construction.
When the auxiliary brush _c_ occupies such a position upon the
commutator that the electro-motive force between the brushes _a_ and _c_
is to the electro-motive force between the brushes _c_ and _b_ as the
resistance of the circuit _a_ M _c' c_ A is to the resistance of the
circuit _b_ M' _c' c_ B, the potentials of the points _x_ and Y will be
equal, and no current will flow over the auxiliary brush; but when the
brush _c_ occupies a different position the potentials of the points _x_
and Y will be different, and a current will flow over the auxiliary
brush to and from the commutator, according to the relative position of
the brushes. If, for instance, the commutator-space between the brushes
_a_ and _c_, when the latter is at the neutral point, is diminished, a
current will flow from the point Y over the shunt _c_ to the brush _b_,
thus strengthening the current in the part M', and partly neutralizing
the current in part M; but if the space between the brushes _a_ and _c_
is increased, the current will flow over the auxiliary brush in an
opposite direction, and the current in M will be strengthened, and in
M', partly neutralized.
By combining with the brushes _a_, _b_, and _c_ any usual automatic
regulating mechanism, the current developed can be regulated in
proportion to the demands in the working circuit. The parts M and M' of
the field wire may be wound in the same direction. In this case they are
arranged as shown in Fig. 253; or the part M may be wound in the
opposite direction, as shown in Figs. 254 and 255.
[Illustration: FIG. 254.]