to reach any such frequencies as might--assuming some of the views
before expressed to be true--produce any of the results which I have
pointed out as mere possibilities. This may be so, but in the course of
these investigations, from the observation of many phenomena, I have
gained the conviction that these frequencies would be much lower than
one is apt to estimate at first. In a flame we set up light vibrations
by causing molecules, or atoms, to collide. But what is the ratio of the
frequency of the collisions and that of the vibrations set up? Certainly
it must be incomparably smaller than that of the strokes of the bell and
the sound vibrations, or that of the discharges and the oscillations of
the condenser. We may cause the molecules of the gas to collide by the
use of alternate electric impulses of high frequency, and so we may
imitate the process in a flame; and from experiments with frequencies
which we are now able to obtain, I think that the result is producible
with impulses which are transmissible through a conductor.
In connection with thoughts of a similar nature, it appeared to me of
great interest to demonstrate the rigidity of a vibrating gaseous
column. Although with such low frequencies as, say 10,000 per second,
which I was able to obtain without difficulty from a specially
constructed alternator, the task looked discouraging at first, I made a
series of experiments. The trials with air at ordinary pressure led to
no result, but with air moderately rarefied I obtain what I think to be
an unmistakable experimental evidence of the property sought for. As a
result of this kind might lead able investigators to conclusions of
importance, I will describe one of the experiments performed.
It is well known that when a tube is slightly exhausted, the discharge
may be passed through it in the form of a thin luminous thread. When
produced with currents of low frequency, obtained from a coil operated
as usual, this thread is inert. If a magnet be approached to it, the
part near the same is attracted or repelled, according to the direction
of the lines of force of the magnet. It occurred to me that if such a
thread would be produced with currents of very high frequency, it should
be more or less rigid, and as it was visible it could be easily studied.
Accordingly I prepared a tube about one inch in diameter and one metre
long, with outside coating at each end. The tube was exhausted to a
point at which, by a little working, the thread discharge could be
obtained. It must be remarked here that the general aspect of the tube,
and the degree of exhaustion, are quite other than when ordinary low
frequency currents are used. As it was found preferable to work with one
terminal, the tube prepared was suspended from the end of a wire
connected to the terminal, the tinfoil coating being connected to the
wire, and to the lower coating sometimes a small insulated plate was
attached. When the thread was formed, it extended through the upper part
of the tube and lost itself in the lower end. If it possessed rigidity
it resembled, not exactly an elastic cord stretched tight between two
supports, but a cord suspended from a height with a small weight
attached at the end. When the finger or a small magnet was approached to
the upper end of the luminous thread, it could be brought locally out of
position by electrostatic or magnetic action; and when the disturbing
object was very quickly removed, an analogous result was produced, as
though a suspended cord would be displaced and quickly released near the
point of suspension. In doing this the luminous thread was set in
vibration, and two very sharply marked nodes, and a third indistinct
one, were formed. The vibration, once set up, continued for fully eight
minutes, dying gradually out. The speed of the vibration often varied
perceptibly, and it could be observed that the electrostatic attraction
of the glass affected the vibrating thread; but it was clear that the
electrostatic action was not the cause of the vibration, for the thread
was most generally stationary, and could always be set in vibration by
passing the finger quickly near the upper part of the tube. With a
magnet the thread could be split in two and both parts vibrated. By
approaching the hand to the lower coating of the tube, or insulation
plate if attached, the vibration was quickened; also, as far as I could
see, by raising the potential or frequency. Thus, either increasing the
frequency or passing a stronger discharge of the same frequency
corresponded to a tightening of the cord. I did not obtain any
experimental evidence with condenser discharges. A luminous band excited
in the bulb by repeated discharges of a Leyden jar must possess
rigidity, and if deformed and suddenly released, should vibrate. But
probably the amount of vibrating matter is so small that in spite of the
extreme speed, the inertia cannot prominently assert itself. Besides,
the observation in such a case is rendered extremely difficult on
account of the fundamental vibration.
The demonstration of the fact--which still needs better experimental
confirmation--that a vibrating gaseous column possesses rigidity, might
greatly modify the views of thinkers. When with low frequencies and
insignificant potentials indications of that property may be noted, how
must a gaseous medium behave under the influence of enormous
electrostatic stresses which may be active in the interstellar space,
and which may alternate with inconceivable rapidity? The existence of
such an electrostatic, rhythmically throbbing force--of a vibrating
electrostatic field--would show a possible way how solids might have
formed from the ultra-gaseous uterus, and how transverse and all kinds
of vibrations may be transmitted through a gaseous medium filling all
space. Then, ether might be a true fluid, devoid of rigidity, and at
rest, it being merely necessary as a connecting link to enable
interaction. What determines the rigidity of a body? It must be the
speed and the amount of motive matter. In a gas the speed maybe
considerable, but the density is exceedingly small; in a liquid the
speed would be likely to be small, though the density may be
considerable; and in both cases the inertia resistance offered to
displacement is practically _nil_. But place a gaseous (or liquid)
column in an intense, rapidly alternating electrostatic field, set the
particles vibrating with enormous speeds, then the inertia resistance
asserts itself. A body might move with more or less freedom through the
vibrating mass, but as a whole it would be rigid.
There is a subject which I must mention in connection with these
experiments: it is that of high vacua. This is a subject, the study of
which is not only interesting, but useful, for it may lead to results of
great practical importance. In commercial apparatus, such as
incandescent lamps, operated from ordinary systems of distribution, a
much higher vacuum than is obtained at present would not secure a very
great advantage. In such a case the work is performed on the filament,
and the gas is little concerned; the improvement, therefore, would be
but trifling. But when we begin to use very high frequencies and
potentials, the action of the gas becomes all important, and the degree
of exhaustion materially modifies the results. As long as ordinary
coils, even very large ones, were used, the study of the subject was
limited, because just at a point when it became most interesting it had
to be interrupted on account of the "non-striking" vacuum being reached.
But at present we are able to obtain from a small disruptive discharge
coil potentials much higher than even the largest coil was capable of
giving, and, what is more, we can make the potential alternate with
great rapidity. Both of these results enable us now to pass a luminous
discharge through almost any vacua obtainable, and the field of our
investigations is greatly extended. Think we as we may, of all the
possible directions to develop a practical illuminant, the line of high
vacua seems to be the most promising at present. But to reach extreme
vacua the appliances must be much more improved, and ultimate perfection
will not be attained until we shall have discharged the mechanical and
perfected an _electrical_ vacuum pump. Molecules and atoms can be thrown
out of a bulb under the action of an enormous potential: _this_ will be
the principle of the vacuum pump of the future. For the present, we must
secure the best results we can with mechanical appliances. In this
respect, it might not be out of the way to say a few words about the
method of, and apparatus for, producing excessively high degrees of
exhaustion of which I have availed myself in the course of these
investigations. It is very probable that other experimenters have used
similar arrangements; but as it is possible that there may be an item of
interest in their description, a few remarks, which will render this
investigation more complete, might be permitted.
[Illustration: FIG. 159.]
The apparatus is illustrated in a drawing shown in Fig. 159. S
represents a Sprengel pump, which has been specially constructed to
better suit the work required. The stop-cock which is usually employed
has been omitted, and instead of it a hollow stopper _s_ has been fitted
in the neck of the reservoir R. This stopper has a small hole _h_,
through which the mercury descends; the size of the outlet _o_ being
properly determined with respect to the section of the fall tube _t_,
which is sealed to the reservoir instead of being connected to it in the
usual manner. This arrangement overcomes the imperfections and troubles
which often arise from the use of the stopcock on the reservoir and the
connections of the latter with the fall tube.
The pump is connected through a U-shaped tube _t_ to a very large
reservoir R_{1}. Especial care was taken in fitting the grinding
surfaces of the stoppers p and p_{1}, and both of these and the mercury
caps above them were made exceptionally long. After the U-shaped tube
was fitted and put in place, it was heated, so as to soften and take
off the strain resulting from imperfect fitting. The U-shaped tube was
provided with a stopcock C, and two ground connections g and g_{1},--one
for a small bulb _b_, usually containing caustic potash, and the other
for the receiver _r_, to be exhausted.
The reservoir R_{1}, was connected by means of a rubber tube to a
slightly larger reservoir R_{2}, each of the two reservoirs being
provided with a stopcock C_{1} and C_{2}, respectively. The reservoir
R_{2} could be raised and lowered by a wheel and rack, and the range of
its motion was so determined that when it was filled with mercury and
the stopcock C_{2} closed, so as to form a Torricellian vacuum in it
when raised, it could be lifted so high that the reservoir R_{1} would
stand a little above stopcock C_{1}; and when this stopcock was closed
and the reservoir R_{2} descended, so as to form a Torricellian vacuum
in reservoir R_{1}, it could be lowered so far as to completely empty
the latter, the mercury filling the reservoir R_{2} up to a little above
stopcock C_{2}.
The capacity of the pump and of the connections was taken as small as
possible relatively to the volume of reservoir, R_{1}, since, of course,
the degree of exhaustion depended upon the ratio of these quantities.
With this apparatus I combined the usual means indicated by former
experiments for the production of very high vacua. In most of the
experiments it was most convenient to use caustic potash. I may venture
to say, in regard to its use, that much time is saved and a more perfect
action of the pump insured by fusing and boiling the potash as soon as,
or even before, the pump settles down. If this course is not followed,
the sticks, as ordinarily employed, may give off moisture at a certain
very slow rate, and the pump may work for many hours without reaching a
very high vacuum. The potash was heated either by a spirit lamp or by
passing a discharge through it, or by passing a current through a wire
contained in it. The advantage in the latter case was that the heating
could be more rapidly repeated.
Generally the process of exhaustion was the following:--At the start,
the stop-cocks C and C_{1} being open, and all other connections closed,
the reservoir R_{2} was raised so far that the mercury filled the
reservoir R_{1} and a part of the narrow connecting U-shaped tube.
When the pump was set to work, the mercury would, of course, quickly
rise in the tube, and reservoir R_{2} was lowered, the experimenter
keeping the mercury at about the same level. The reservoir R_{2} was
balanced by a long spring which facilitated the operation, and the
friction of the parts was generally sufficient to keep it in almost any
position. When the Sprengel pump had done its work, the reservoir R_{2}
was further lowered and the mercury descended in R_{1} and filled R_{2},
whereupon stopcock C_{2} was closed. The air adhering to the walls of
R_{1} and that absorbed by the mercury was carried off, and to free the
mercury of all air the reservoir R_{2} was for a long time worked up and
down. During this process some air, which would gather below stopcock
C_{2}, was expelled from R_{2} by lowering it far enough and opening the
stopcock, closing the latter again before raising the reservoir. When
all the air had been expelled from the mercury, and no air would gather
in R_{2} when it was lowered, the caustic potash was resorted to. The
reservoir R_{2} was now again raised until the mercury in R_{1}, stood
above stopcock C_{1}. The caustic potash was fused and boiled, and
moisture partly carried off by the pump and partly re-absorbed; and this
process of heating and cooling was repeated many times, and each time,
upon the moisture being absorbed or carried off, the reservoir R_{2} was
for a long time raised and lowered. In this manner all the moisture was
carried off from the mercury, and both the reservoirs were in proper
condition to be used. The reservoir R_{2} was then again raised to the
top, and the pump was kept working for a long time. When the highest
vacuum obtainable with the pump had been reached, the potash bulb was
usually wrapped with cotton which was sprinkled with ether so as to keep
the potash at a very low temperature, then the reservoir R_{2} was
lowered, and upon reservoir R_{1} being emptied the receiver was quickly
sealed up.
When a new bulb was put on, the mercury was always raised above stopcock
C_{1}, which was closed, so as to always keep the mercury and both the
reservoirs in fine condition, and the mercury was never withdrawn from
R_{1} except when the pump had reached the highest degree of exhaustion.
It is necessary to observe this rule if it is desired to use the
apparatus to advantage.
By means of this arrangement I was able to proceed very quickly, and
when the apparatus was in perfect order it was possible to reach the
phosphorescent stage in a small bulb in less than fifteen minutes, which
is certainly very quick work for a small laboratory arrangement
requiring all in all about 100 pounds of mercury. With ordinary small
bulbs the ratio of the capacity of the pump, receiver, and connections,
and that of reservoir R was about 1 to 20, and the degrees of exhaustion
reached were necessarily very high, though I am unable to make a precise
and reliable statement how far the exhaustion was carried.
What impresses the investigator most in the course of these experiences
is the behavior of gases when subjected to great rapidly alternating
electrostatic stresses. But he must remain in doubt as to whether the
effects observed are due wholly to the molecules, or atoms, of the gas
which chemical analysis discloses to us, or whether there enters into
play another medium of a gaseous nature, comprising atoms, or molecules,
immersed in a fluid pervading the space. Such a medium surely must
exist, and I am convinced that, for instance, even if air were absent,
the surface and neighborhood of a body in space would be heated by
rapidly alternating the potential of the body; but no such heating of
the surface or neighborhood could occur if all free atoms were removed
and only a homogeneous, incompressible, and elastic fluid--such as ether
is supposed to be--would remain, for then there would be no impacts, no
collisions. In such a case, as far as the body itself is concerned, only
frictional losses in the inside could occur.
It is a striking fact that the discharge through a gas is established
with ever-increasing freedom as the frequency of the impulses is
augmented. It behaves in this respect quite contrarily to a metallic
conductor. In the latter the impedance enters prominently into play as
the frequency is increased, but the gas acts much as a series of
condensers would; the facility with which the discharge passes through,
seems to depend on the rate of change of potential. If it acts so, then
in a vacuum tube even of great length, and no matter how strong the
current, self-induction could not assert itself to any appreciable
degree. We have, then, as far as we can now see, in the gas a conductor
which is capable of transmitting electric impulses of any frequency
which we may be able to produce. Could the frequency be brought high
enough, then a queer system of electric distribution, which would be
likely to interest gas companies, might be realized: metal pipes
filled with gas--the metal being the insulator, the gas the
conductor--supplying phosphorescent bulbs, or perhaps devices as yet
uninvented. It is certainly possible to take a hollow core of copper,
rarefy the gas in the same, and by passing impulses of sufficiently high
frequency through a circuit around it, bring the gas inside to a high
degree of incandescence; but as to the nature of the forces there would
be considerable uncertainty, for it would be doubtful whether with such
impulses the copper core would act as a static screen. Such paradoxes
and apparent impossibilities we encounter at every step in this line of
work, and therein lies, to a great extent, the charm of the study.
I have here a short and wide tube which is exhausted to a high degree
and covered with a substantial coating of bronze, the coating barely
allowing the light to shine through. A metallic cap, with a hook for
suspending the tube, is fastened around the middle portion of the
latter, the clasp being in contact with the bronze coating. I now want
to light the gas inside by suspending the tube on a wire connected to
the coil. Any one who would try the experiment for the first time, not
having any previous experience, would probably take care to be quite
alone when making the trial, for fear that he might become the joke of
his assistants. Still, the bulb lights in spite of the metal coating,
and the light can be distinctly perceived through the latter. A long
tube covered with aluminum bronze lights when held in one hand--the
other touching the terminal of the coil--quite powerfully. It might be
objected that the coatings are not sufficiently conducting; still, even
if they were highly resistant, they ought to screen the gas. They
certainly screen it perfectly in a condition of rest, but far from
perfectly when the charge is surging in the coating. But the loss of
energy which occurs within the tube, notwithstanding the screen, is
occasioned principally by the presence of the gas. Were we to take a
large hollow metallic sphere and fill it with a perfect, incompressible,
fluid dielectric, there would be no loss inside of the sphere, and
consequently the inside might be considered as perfectly screened,
though the potential be very rapidly alternating. Even were the sphere
filled with oil, the loss would be incomparably smaller than when the
fluid is replaced by a gas, for in the latter case the force produces
displacements; that means impact and collisions in the inside.
No matter what the pressure of the gas may be, it becomes an important
factor in the heating of a conductor when the electric density is great
and the frequency very high. That in the heating of conductors by
lightning discharges, air is an element of great importance, is almost
as certain as an experimental fact. I may illustrate the action of the
air by the following experiment: I take a short tube which is exhausted
to a moderate degree and has a platinum wire running through the middle
from one end to the other. I pass a steady or low frequency current
through the wire, and it is heated uniformly in all parts. The heating
here is due to conduction, or frictional losses, and the gas around the
wire has--as far as we can see--no function to perform. But now let me
pass sudden discharges, or high frequency currents, through the wire.
Again the wire is heated, this time principally on the ends and least in
the middle portion; and if the frequency of the impulses, or the rate of
change, is high enough, the wire might as well be cut in the middle as
not, for practically all heating is due to the rarefied gas. Here the
gas might only act as a conductor of no impedance diverting the current
from the wire as the impedance of the latter is enormously increased,
and merely heating the ends of the wire by reason of their resistance to
the passage of the discharge. But it is not at all necessary that the
gas in the tube should be conducting; it might be at an extremely low
pressure, still the ends of the wire would be heated--as, however, is
ascertained by experience--only the two ends would in such case not be
electrically connected through the gaseous medium. Now what with these
frequencies and potentials occurs in an exhausted tube, occurs in the
lightning discharges at ordinary pressure. We only need remember one of
the facts arrived at in the course of these investigations, namely, that
to impulses of very high frequency the gas at ordinary pressure behaves
much in the same manner as though it were at moderately low pressure. I
think that in lightning discharges frequently wires or conducting
objects are volatilized merely because air is present, and that, were
the conductor immersed in an insulating liquid, it would be safe, for
then the energy would have to spend itself somewhere else. From the
behavior of gases under sudden impulses of high potential, I am led to
conclude that there can be no surer way of diverting a lightning
discharge than by affording it a passage through a volume of gas, if
such a thing can be done in a practical manner.
There are two more features upon which I think it necessary to dwell in
connection with these experiments--the "radiant state" and the
"non-striking vacuum."
Any one who has studied Crookes' work must have received the impression
that the "radiant state" is a property of the gas inseparably connected
with an extremely high degree of exhaustion. But it should be remembered
that the phenomena observed in an exhausted vessel are limited to the
character and capacity of the apparatus which is made use of. I think
that in a bulb a molecule, or atom, does not precisely move in a
straight line because it meets no obstacle, but because the velocity
imparted to it is sufficient to propel it in a sensibly straight line.
The mean free path is one thing, but the velocity--the energy associated
with the moving body--is another, and under ordinary circumstances I
believe that it is a mere question of potential or speed. A disruptive
discharge coil, when the potential is pushed very far, excites
phosphorescence and projects shadows, at comparatively low degrees of
exhaustion. In a lightning discharge, matter moves in straight lines at
ordinary pressure when the mean free path is exceedingly small, and
frequently images of wires or other metallic objects have been produced
by the particles thrown off in straight lines.
I have prepared a bulb to illustrate by an experiment the correctness of
these assertions. In a globe L, Fig. 160, I have mounted upon a lamp
filament _f_ a piece of lime _l_. The lamp filament is connected with a
wire which leads into the bulb, and the general construction of the
latter is as indicated in Fig. 148, before described. The bulb being
suspended from a wire connected to the terminal of the coil, and the
latter being set to work, the lime piece _l_ and the projecting parts of
the filament _f_ are bombarded. The degree of exhaustion is just such
that with the potential the coil is capable of giving, phosphorescence
of the glass is produced, but disappears as soon as the vacuum is
impaired. The lime containing moisture, and moisture being given off as
soon as heating occurs, the phosphorescence lasts only for a few
moments. When the lime has been sufficiently heated, enough moisture has
been given off to impair materially the vacuum of the bulb. As the
bombardment goes on, one point of the lime piece is more heated than
other points, and the result is that finally practically all the
discharge passes through that point which is intensely heated, and a
white stream of lime particles (Fig. 160) then breaks forth from that
point. This stream is composed of "radiant" matter, yet the degree of
exhaustion is low. But the particles move in straight lines because the
velocity imparted to them is great, and this is due to three causes--to
the great electric density, the high temperature of the small point, and
the fact that the particles of the lime are easily torn and thrown
off--far more easily than those of carbon. With frequencies such as we
are able to obtain, the particles are bodily thrown off and projected to
a considerable distance; but with sufficiently high frequencies no such
thing would occur; in such case only a stress would spread or a
vibration would be propagated through the bulb. It would be out of the
question to reach any such frequency on the assumption that the atoms
move with the speed of light; but I believe that such a thing is
impossible; for this an enormous potential would be required. With
potentials which we are able to obtain, even with a disruptive discharge
coil, the speed must be quite insignificant.
[Illustration: FIG. 160.]
As to the "non-striking vacuum," the point to be noted is, that it can
occur only with low frequency impulses, and it is necessitated by the
impossibility of carrying off enough energy with such impulses in high
vacuum, since the few atoms which are around the terminal upon coming in
contact with the same, are repelled and kept at a distance for a
comparatively long period of time, and not enough work can be performed
to render the effect perceptible to the eye. If the difference of
potential between the terminals is raised, the dielectric breaks down.
But with very high frequency impulses there is no necessity for such
breaking down, since any amount of work can be performed by continually
agitating the atoms in the exhausted vessel, provided the frequency is
high enough. It is easy to reach--even with frequencies obtained from an
alternator as here used--a stage at which the discharge does not pass
between two electrodes in a narrow tube, each of these being connected
to one of the terminals of the coil, but it is difficult to reach a
point at which a luminous discharge would not occur around each
electrode.
[Illustration: FIG. 161.]
[Illustration: FIG. 162.]
A thought which naturally presents itself in connection with high
frequency currents, is to make use of their powerful electrodynamic
inductive action to produce light effects in a sealed glass globe. The
leading-in wire is one of the defects of the present incandescent lamp,
and if no other improvement were made, that imperfection at least should
be done away with. Following this thought, I have carried on
experiments in various directions, of which some were indicated in my
former paper. I may here mention one or two more lines of experiment
which have been followed up.
Many bulbs were constructed as shown in Fig. 161 and Fig. 162.
In Fig. 161, a wide tube, T, was sealed to a smaller W
shaped tube U, of phosphorescent glass. In the tube T, was placed a coil
C, of aluminum wire, the ends of which were provided with small spheres,
t and t_{1}, of aluminum, and reached into the U tube. The tube T
was slipped into a socket containing a primary coil, through which
usually the discharges of Leyden jars were directed, and the rarefied
gas in the small U tube was excited to strong luminosity by the
high-tension current induced in the coil C. When Leyden jar discharges
were used to induce currents in the coil C, it was found necessary to
pack the tube T tightly with insulating powder, as a discharge would
occur frequently between the turns of the coil, especially when the
primary was thick and the air gap, through which the jars discharged,
large, and no little trouble was experienced in this way.
In Fig. 162 is illustrated another form of the bulb constructed. In this
case a tube T is sealed to a globe L. The tube contains a coil C, the
ends of which pass through two small glass tubes t and t_{1}, which
are sealed to the tube T. Two refractory buttons m and m_{1}, are
mounted on lamp filaments which are fastened to the ends of the wires
passing through the glass tubes t and t_{1}. Generally in bulbs made
on this plan the globe L communicated with the tube T. For this purpose
the ends of the small tubes t and t_{1} were heated just a trifle in
the burner, merely to hold the wires, but not to interfere with the
communication. The tube T, with the small tubes, wires through the same,
and the refractory buttons m and m_{1}, were first prepared, and
then sealed to globe L, whereupon the coil C was slipped in and the
connections made to its ends. The tube was then packed with insulating
powder, jamming the latter as tight as possible up to very nearly the
end; then it was closed and only a small hole left through which the
remainder of the powder was introduced, and finally the end of the tube
was closed. Usually in bulbs constructed as shown in Fig. 162 an
aluminum tube _a_ was fastened to the upper end _s_ of each of the tubes
t and t_{1} in order to protect that end against the heat. The
buttons m and m_{1} could be brought to any degree of incandescence
by passing the discharges of Leyden jars around the coil C. In such
bulbs with two buttons a very curious effect is produced by the
formation of the shadows of each of the two buttons.
Another line of experiment, which has been assiduously followed, was to
induce by electro-dynamic induction a current or luminous discharge in
an exhausted tube or bulb. This matter has received such able treatment
at the hands of Prof. J. J. Thomson, that I could add but little to what
he has made known, even had I made it the special subject of this
lecture. Still, since experiments in this line have gradually led me to
the present views and results, a few words must be devoted here to this
subject.
It has occurred, no doubt, to many that as a vacuum tube is made longer,
the electromotive force per unit length of the tube, necessary to pass a
luminous discharge through the latter, becomes continually smaller;
therefore, if the exhausted tube be made long enough, even with low
frequencies a luminous discharge could be induced in such a tube closed
upon itself. Such a tube might be placed around a hall or on a ceiling,
and at once a simple appliance capable of giving considerable light
would be obtained. But this would be an appliance hard to manufacture
and extremely unmanageable. It would not do to make the tube up of small
lengths, because there would be with ordinary frequencies considerable
loss in the coatings, and besides, if coatings were used, it would be
better to supply the current directly to the tube by connecting the
coatings to a transformer. But even if all objections of such nature
were removed, with low frequencies the light conversion itself would be
inefficient, as I have before stated. In using extremely high
frequencies the length of the secondary--in other words, the size of the
vessel--can be reduced as much as desired, and the efficiency of the
light conversion is increased, provided that means are invented for
efficiently obtaining such high frequencies. Thus one is led, from
theoretical and practical considerations, to the use of high
frequencies, and this means high electromotive forces and small currents
in the primary. When one works with condenser charges--and they are the
only means up to the present known for reaching these extreme
frequencies--one gets to electromotive forces of several thousands of
volts per turn of the primary. We cannot multiply the electro-dynamic
inductive effect by taking more turns in the primary, for we arrive at
the conclusion that the best way is to work with one single turn--though
we must sometimes depart from this rule--and we must get along with
whatever inductive effect we can obtain with one turn. But before one
has long experimented with the extreme frequencies required to set up in
a small bulb an electromotive force of several thousands of volts, one
realizes the great importance of electrostatic effects, and these
effects grow relatively to the electro-dynamic in significance as the
frequency is increased.
Now, if anything is desirable in this case, it is to increase the
frequency, and this would make it still worse for the electrodynamic
effects. On the other hand, it is easy to exalt the electrostatic action
as far as one likes by taking more turns on the secondary, or combining
self-induction and capacity to raise the potential. It should also be
remembered that, in reducing the current to the smallest value and
increasing the potential, the electric impulses of high frequency can be
more easily transmitted through a conductor.
These and similar thoughts determined me to devote more attention to the
electrostatic phenomena, and to endeavor to produce potentials as high
as possible, and alternating as fast as they could be made to alternate.
I then found that I could excite vacuum tubes at considerable distance
from a conductor connected to a properly constructed coil, and that I
could, by converting the oscillatory current of a conductor to a higher
potential, establish electrostatic alternating fields which acted
through the whole extent of the room, lighting up a tube no matter where
it was held in space. I thought I recognized that I had made a step in
advance, and I have persevered in this line; but I wish to say that I
share with all lovers of science and progress the one and only
desire--to reach a result of utility to men in any direction to which
thought or experiment may lead me. I think that this departure is the
right one, for I cannot see, from the observation of the phenomena which
manifest themselves as the frequency is increased, what there would
remain to act between two circuits conveying, for instance, impulses of
several hundred millions per second, except electrostatic forces. Even
with such trifling frequencies the energy would be practically all
potential, and my conviction has grown strong that, to whatever kind of
motion light may be due, it is produced by tremendous electrostatic
stresses vibrating with extreme rapidity.
[Illustration: FIG. 163.]
[Illustration: FIG. 164.]
Of all these phenomena observed with currents, or electric impulses, of
high frequency, the most fascinating for an audience are certainly those
which are noted in an electrostatic field acting through considerable
distance; and the best an unskilled lecturer can do is to begin and
finish with the exhibition of these singular effects. I take a tube in
my hand and move it about, and it is lighted wherever I may hold it;
throughout space the invisible forces act. But I may take another tube
and it might not light, the vacuum being very high. I excite it by means
of a disruptive discharge coil, and now it will light in the
electrostatic field. I may put it away for a few weeks or months, still
it retains the faculty of being excited. What change have I produced in
the tube in the act of exciting it? If a motion imparted to atoms, it is
difficult to perceive how it can persist so long without being arrested
by frictional losses; and if a strain exerted in the dielectric, such as
a simple electrification would produce, it is easy to see how it may
persist indefinitely, but very difficult to understand why such a
condition should aid the excitation when we have to deal with potentials
which are rapidly alternating.
Since I have exhibited these phenomena for the first time, I have
obtained some other interesting effects. For instance, I have produced
the incandescence of a button, filament, or wire enclosed in a tube. To
get to this result it was necessary to economize the energy which is
obtained from the field, and direct most of it on the small body to be
rendered incandescent. At the beginning the task appeared difficult, but
the experiences gathered permitted me to reach the result easily. In
Fig. 163 and Fig. 164, two such tubes are illustrated, which are
prepared for the occasion. In Fig. 163 a short tube T_{1}, sealed to
another long tube T, is provided with a stem _s_, with a platinum wire
sealed in the latter. A very thin lamp filament _l_, is fastened to this
wire and connection to the outside is made through a thin copper wire
_w_. The tube is provided with outside and inside coatings, C and C_{1},
respectively, and is filled as far as the coatings reach with
conducting, and the space above with insulating, powder. These coatings
are merely used to enable me to perform two experiments with the
tube--namely, to produce the effect desired either by direct connection
of the body of the experimenter or of another body to the wire _w_, or
by acting inductively through the glass. The stem _s_ is provided with
an aluminum tube _a_, for purposes before explained, and only a small
part of the filament reaches out of this tube. By holding the tube T_{1}
anywhere in the electrostatic field, the filament is rendered
incandescent.
A more interesting piece of apparatus is illustrated in Fig. 164. The
construction is the same as before, only instead of the lamp filament a
small platinum wire _p_, sealed in a stem _s_, and bent above it in a
circle, is connected to the copper wire _w_, which is joined to an
inside coating C. A small stem s_{1} is provided with a needle, on the
point of which is arranged, to rotate very freely, a very light fan of
mica _v_. To prevent the fan from falling out, a thin stem of glass _g_,
is bent properly and fastened to the aluminum tube. When the glass tube
is held anywhere in the electrostatic field the platinum wire becomes
incandescent, and the mica vanes are rotated very fast.
Intense phosphorescence may be excited in a bulb by merely connecting it
to a plate within the field, and the plate need not be any larger than
an ordinary lamp shade. The phosphorescence excited with these currents
is incomparably more powerful than with ordinary apparatus. A small
phosphorescent bulb, when attached to a wire connected to a coil, emits
sufficient light to allow reading ordinary print at a distance of five
to six paces. It was of interest to see how some of the phosphorescent
bulbs of Professor Crookes would behave with these currents, and he has
had the kindness to lend me a few for the occasion. The effects produced
are magnificent, especially by the sulphide of calcium and sulphide of
zinc. With the disruptive discharge coil they glow intensely merely by
holding them in the hand and connecting the body to the terminal of the
coil.
To whatever results investigations of this kind may lead, the chief
interest lies, for the present, in the possibilities they offer for the
production of an efficient illuminating device. In no branch of electric
industry is an advance more desired than in the manufacture of light.
Every thinker, when considering the barbarous methods employed, the
deplorable losses incurred in our best systems of light production, must
have asked himself, What is likely to be the light of the future? Is it
to be an incandescent solid, as in the present lamp, or an incandescent
gas, or a phosphorescent body, or something like a burner, but
incomparably more efficient?
There is little chance to perfect a gas burner; not, perhaps, because
human ingenuity has been bent upon that problem for centuries without a
radical departure having been made--though the argument is not devoid of
force--but because in a burner the highest vibrations can never be
reached, except by passing through all the low ones. For how is a flame
to proceed unless by a fall of lifted weights? Such process cannot be
maintained without renewal, and renewal is repeated passing from low to
high vibrations. One way only seems to be open to improve a burner, and
that is by trying to reach higher degrees of incandescence. Higher
incandescence is equivalent to a quicker vibration: that means more
light from the same material, and that again, means more economy. In
this direction some improvements have been made, but the progress is
hampered by many limitations. Discarding, then, the burner, there
remains the three ways first mentioned, which are essentially
electrical.
Suppose the light of the immediate future to be a solid, rendered
incandescent by electricity. Would it not seem that it is better to
employ a small button than a frail filament? From many considerations it
certainly must be concluded that a button is capable of a higher
economy, assuming, of course, the difficulties connected with the
operation of such a lamp to be effectively overcome. But to light such
a lamp we require a high potential; and to get this economically, we
must use high frequencies.
Such considerations apply even more to the production of light by the
incandescence of a gas, or by phosphorescence. In all cases we require
high frequencies and high potentials. These thoughts occurred to me a
long time ago.
Incidentally we gain, by the use of high frequencies, many advantages,
such as higher economy in the light production, the possibility of
working with one lead, the possibility of doing away with the leading-in
wire, etc.
The question is, how far can we go with frequencies? Ordinary conductors
rapidly lose the facility of transmitting electric impulses when the
frequency is greatly increased. Assume the means for the production of
impulses of very great frequency brought to the utmost perfection, every
one will naturally ask how to transmit them when the necessity arises.
In transmitting such impulses through conductors we must remember that
we have to deal with _pressure_ and _flow_, in the ordinary
interpretation of these terms. Let the pressure increase to an enormous
value, and let the flow correspondingly diminish, then such
impulses--variations merely of pressure, as it were--can no doubt be
transmitted through a wire even if their frequency be many hundreds of
millions per second. It would, of course, be out of question to transmit
such impulses through a wire immersed in a gaseous medium, even if the
wire were provided with a thick and excellent insulation, for most of
the energy would be lost in molecular bombardment and consequent
heating. The end of the wire connected to the source would be heated,
and the remote end would receive but a trifling part of the energy
supplied. The prime necessity, then, if such electric impulses are to be
used, is to find means to reduce as much as possible the dissipation.
The first thought is, to employ the thinnest possible wire surrounded by
the thickest practicable insulation. The next thought is to employ
electrostatic screens. The insulation of the wire may be covered with a
thin conducting coating and the latter connected to the ground. But this
would not do, as then all the energy would pass through the conducting
coating to the ground and nothing would get to the end of the wire. If a
ground connection is made it can only be made through a conductor
offering an enormous impedance, or through a condenser of extremely
small capacity. This, however, does not do away with other difficulties.
If the wave length of the impulses is much smaller than the length of
the wire, then corresponding short waves will be set up in the
conducting coating, and it will be more or less the same as though the
coating were directly connected to earth. It is therefore necessary to
cut up the coating in sections much shorter than the wave length. Such
an arrangement does not still afford a perfect screen, but it is ten
thousand times better than none. I think it preferable to cut up the
conducting coating in small sections, even if the current waves be much
longer than the coating.
If a wire were provided with a perfect electrostatic screen, it would be
the same as though all objects were removed from it at infinite
distance. The capacity would then be reduced to the capacity of the wire
itself, which would be very small. It would then be possible to send
over the wire current vibrations of very high frequencies at enormous
distances, without affecting greatly the character of the vibrations. A
perfect screen is of course out of the question, but I believe that with
a screen such as I have just described telephony could be rendered
practicable across the Atlantic. According to my ideas, the gutta-percha
covered wire should be provided with a third conducting coating
subdivided in sections. On the top of this should be again placed a
layer of gutta-percha and other insulation, and on the top of the whole
the armor. But such cables will not be constructed, for ere long
intelligence--transmitted without wires--will throb through the earth
like a pulse through a living organism. The wonder is that, with the
present state of knowledge and the experiences gained, no attempt is
being made to disturb the electrostatic or magnetic condition of the
earth, and transmit, if nothing else, intelligence.
It has been my chief aim in presenting these results to point out
phenomena or features of novelty, and to advance ideas which I am
hopeful will serve as starting points of new departures. It has been my
chief desire this evening to entertain you with some novel experiments.
Your applause, so frequently and generously accorded, has told me that I
have succeeded.
In conclusion, let me thank you most heartily for your kindness and
attention, and assure you that the honor I have had in addressing such
a distinguished audience, the pleasure I have had in presenting these
results to a gathering of so many able men--and among them also some of
those in whose work for many years past I have found enlightenment and
constant pleasure--I shall never forget.
CHAPTER XXVIII.
ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA.[3]
[3] A lecture delivered before the Franklin Institute, Philadelphia,
February, 1893, and before the National Electric Light
Association, St. Louis, March, 1893.
INTRODUCTORY.--SOME THOUGHTS ON THE EYE.
When we look at the world around us, on Nature, we are impressed with
its beauty and grandeur. Each thing we perceive, though it may be
vanishingly small, is in itself a world, that is, like the whole of the
universe, matter and force governed by law,--a world, the contemplation
of which fills us with feelings of wonder and irresistibly urges us to
ceaseless thought and inquiry. But in all this vast world, of all
objects our senses reveal to us, the most marvellous, the most appealing
to our imagination, appears no doubt a highly developed organism, a
thinking being. If there is anything fitted to make us admire Nature's
handiwork, it is certainly this inconceivable structure, which performs
its innumerable motions of obedience to external influence. To
understand its workings, to get a deeper insight into this Nature's
masterpiece, has ever been for thinkers a fascinating aim, and after
many centuries of arduous research men have arrived at a fair
understanding of the functions of its organs and senses. Again, in all
the perfect harmony of its parts, of the parts which constitute the
material or tangible of our being, of all its organs and senses, the eye
is the most wonderful. It is the most precious, the most indispensable
of our perceptive or directive organs, it is the great gateway through
which all knowledge enters the mind. Of all our organs, it is the one,
which is in the most intimate relation with that which we call
intellect. So intimate is this relation, that it is often said, the very
soul shows itself in the eye.
It can be taken as a fact, which the theory of the action of the eye
implies, that for each external impression, that is, for each image
produced upon the retina, the ends of the visual nerves, concerned in
the conveyance of the impression to the mind, must be under a peculiar
stress or in a vibratory state. It now does not seem improbable that,
when by the power of thought an image is evoked, a distinct reflex
action, no matter how weak, is exerted upon certain ends of the visual
nerves, and therefore upon the retina. Will it ever be within human
power to analyze the condition of the retina when disturbed by thought
or reflex action, by the help of some optical or other means of such
sensitiveness, that a clear idea of its state might be gained at any
time? If this were possible, then the problem of reading one's thoughts
with precision, like the characters of an open book, might be much
easier to solve than many problems belonging to the domain of positive
physical science, in the solution of which many, if not the majority, of
scientific men implicitly believe. Helmholtz, has shown that the fundi
of the eye are themselves, luminous, and he was able to _see_, in total
darkness, the movement of his arm by the light of his own eyes. This is
one of the most remarkable experiments recorded in the history of
science, and probably only a few men could satisfactorily repeat it, for
it is very likely, that the luminosity of the eyes is associated with
uncommon activity of the brain and great imaginative power. It is
fluorescence of brain action, as it were.
Another fact having a bearing on this subject which has probably been
noted by many, since it is stated in popular expressions, but which I
cannot recollect to have found chronicled as a positive result of
observation is, that at times, when a sudden idea or image presents
itself to the intellect, there is a distinct and sometimes painful
sensation of luminosity produced in the eye, observable even in broad
daylight.
The saying then, that the soul shows itself in the eye, is deeply
founded, and we feel that it expresses a great truth. It has a profound
meaning even for one who, like a poet or artist, only following his
inborn instinct or love for Nature, finds delight in aimless thoughts
and in the mere contemplation of natural phenomena, but a still more
profound meaning for one who, in the spirit of positive scientific
investigation, seeks to ascertain the causes of the effects. It is
principally the natural philosopher, the physicist, for whom the eye is
the subject of the most intense admiration.
Two facts about the eye must forcibly impress the mind of the physicist,
notwithstanding he may think or say that it is an imperfect optical
instrument, forgetting, that the very conception of that which is
perfect or seems so to him, has been gained through this same
instrument. First, the eye is, as far as our positive knowledge goes,
the only organ which is _directly_ affected by that subtile medium,
which as science teaches us, must fill all space; secondly, it is the
most sensitive of our organs, incomparably more sensitive to external
impressions than any other.
The organ of hearing implies the impact of ponderable bodies, the organ
of smell the transference of detached material particles, and the organs
of taste, and of touch or force, the direct contact, or at least some
interference of ponderable matter, and this is true even in those
instances of animal organisms, in which some of these organs are
developed to a degree of truly marvelous perfection. This being so, it
seems wonderful that the organ of sight solely should be capable of
being stirred by that, which all our other organs are powerless to
detect, yet which plays an essential part in all natural phenomena,
which transmits all energy and sustains all motion and, that most
intricate of all, life, but which has properties such that even a
scientifically trained mind cannot help drawing a distinction between it
and all that is called matter. Considering merely this, and the fact
that the eye, by its marvelous power, widens our otherwise very narrow
range of perception far beyond the limits of the small world which is
our own, to embrace myriads of other worlds, suns and stars in the
infinite depths of the universe, would make it justifiable to assert,
that it is an organ of a higher order. Its performances are beyond
comprehension. Nature as far as we know never produced anything more
wonderful. We can get barely a faint idea of its prodigious power by
analyzing what it does and by comparing. When ether waves impinge upon
the human body, they produce the sensations of warmth or cold, pleasure
or pain, or perhaps other sensations of which we are not aware, and any
degree or intensity of these sensations, which degrees are infinite in
number, hence an infinite number of distinct sensations. But our sense
of touch, or our sense of force, cannot reveal to us these differences
in degree or intensity, unless they are very great. Now we can readily
conceive how an organism, such as the human, in the eternal process of
evolution, or more philosophically speaking, adaptation to Nature, being
constrained to the use of only the sense of touch or force, for
instance, might develop this sense to such a degree of sensitiveness or
perfection, that it would be capable of distinguishing the minutest
differences in the temperature of a body even at some distance, to a
hundredth, or thousandth, or millionth part of a degree. Yet, even this
apparently impossible performance would not begin to compare with that
of the eye, which is capable of distinguishing and conveying to the mind
in a single instant innumerable peculiarities of the body, be it in
form, or color, or other respects. This power of the eye rests upon two
things, namely, the rectilinear propagation of the disturbance by which
it is effected, and upon its sensitiveness. To say that the eye is
sensitive is not saying anything. Compared with it, all other organs are
monstrously crude. The organ of smell which guides a dog on the trail of
a deer, the organ of touch or force which guides an insect in its
wanderings, the organ of hearing, which is affected by the slightest
disturbances of the air, are sensitive organs, to be sure, but what are
they compared with the human eye! No doubt it responds to the faintest
echoes or reverberations of the medium; no doubt, it brings us tidings
from other worlds, infinitely remote, but in a language we cannot as yet
always understand. And why not? Because we live in a medium filled with
air and other gases, vapors and a dense mass of solid particles flying
about. These play an important part in many phenomena; they fritter away
the energy of the vibrations before they can reach the eye; they too,
are the carriers of germs of destruction, they get into our lungs and
other organs, clog up the channels and imperceptibly, yet inevitably,
arrest the stream of life. Could we but do away with all ponderable
matter in the line of sight of the telescope, it would reveal to us
undreamt of marvels. Even the unaided eye, I think, would be capable of
distinguishing in the pure medium, small objects at distances measured
probably by hundreds or perhaps thousands of miles.
But there is something else about the eye which impresses us still more
than these wonderful features which we observed, viewing it from the
standpoint of a physicist, merely as an optical instrument,--something
which appeals to us more than its marvelous faculty of being directly
affected by the vibrations of the medium, without interference of gross
matter, and more than its inconceivable sensitiveness and discerning
power. It is its significance in the processes of life. No matter what
one's views on nature and life may be, he must stand amazed when, for
the first time in his thoughts, he realizes the importance of the eye in
the physical processes and mental performances of the human organism.
And how could it be otherwise, when he realizes, that the eye is the
means through which the human race has acquired the entire knowledge it
possesses, that it controls all our motions, more still, all our
actions.
There is no way of acquiring knowledge except through the eye. What is
the foundation of all philosophical systems of ancient and modern times,
in fact, of all the philosophy of man? _I am, I think; I think,
therefore I am._ But how could I think and how would I know that I
exist, if I had not the eye? For knowledge involves consciousness;
consciousness involves ideas, conceptions; conceptions involve pictures
or images, and images the sense of vision, and therefore the organ of
sight. But how about blind men, will be asked? Yes, a blind man may
depict in magnificent poems, forms and scenes from real life, from a
world he physically does not see. A blind man may touch the keys of an
instrument with unerring precision, may model the fastest boat, may
discover and invent, calculate and construct, may do still greater
wonders--but all the blind men who have done such things have descended
from those who had seeing eyes. Nature may reach the same result in many
ways. Like a wave in the physical world, in the infinite ocean of the
medium which pervades all, so in the world of organisms, in life, an
impulse started proceeds onward, at times, may be, with the speed of
light, at times, again, so slowly that for ages and ages it seems to
stay, passing through processes of a complexity inconceivable to men,
but in all its forms, in all its stages, its energy ever and ever
integrally present. A single ray of light from a distant star falling
upon the eye of a tyrant in bygone times, may have altered the course of
his life, may have changed the destiny of nations, may have transformed
the surface of the globe, so intricate, so inconceivably complex are the
processes in Nature. In no way can we get such an overwhelming idea of
the grandeur of Nature, as when we consider, that in accordance with the
law of the conservation of energy, throughout the infinite, the forces
are in a perfect balance, and hence the energy of a single thought may
determine the motion of a Universe. It is not necessary that every
individual, not even that every generation or many generations, should
have the physical instrument of sight, in order to be able to form
images and to think, that is, form ideas or conceptions; but sometime or
other, during the process of evolution, the eye certainly must have
existed, else thought, as we understand it, would be impossible; else
conceptions, like spirit, intellect, mind, call it as you may, could not
exist. It is conceivable, that in some other world, in some other
beings, the eye is replaced by a different organ, equally or more
perfect, but these beings cannot be men.
Now what prompts us all to voluntary motions and actions of any kind?
Again the eye. If I am conscious of the motion, I must have an idea or
conception, that is, an image, therefore the eye. If I am not precisely
conscious of the motion, it is, because the images are vague or
indistinct, being blurred by the superimposition of many. But when I
perform the motion, does the impulse which prompts me to the action come
from within or from without? The greatest physicists have not disdained
to endeavor to answer this and similar questions and have at times
abandoned themselves to the delights of pure and unrestrained thought.
Such questions are generally considered not to belong to the realm of
positive physical science, but will before long be annexed to its
domain. Helmholtz has probably thought more on life than any modern
scientist. Lord Kelvin expressed his belief that life's process is
electrical and that there is a force inherent to the organism and
determining its motions. Just as much as I am convinced of any physical
truth I am convinced that the motive impulse must come from the outside.
For, consider the lowest organism we know--and there are probably many
lower ones--an aggregation of a few cells only. If it is capable of
voluntary motion it can perform an infinite number of motions, all
definite and precise. But now a mechanism consisting of a finite number
of parts and few at that, cannot perform an infinite number of definite
motions, hence the impulses which govern its movements must come from
the environment. So, the atom, the ulterior element of the Universe's
structure, is tossed about in space, eternally, a play to external
influences, like a boat in a troubled sea. Were it to stop its motion
_it would die_. Matter at rest, if such a thing could exist, would be
matter dead. Death of matter! Never has a sentence of deeper
philosophical meaning been uttered. This is the way in which Prof.
Dewar forcibly expresses it in the description of his admirable
experiments, in which liquid oxygen is handled as one handles water, and
air at ordinary pressure is made to condense and even to solidify by the
intense cold. Experiments, which serve to illustrate, in his language,
the last feeble manifestations of life, the last quiverings of matter
about to die. But human eyes shall not witness such death. There is no
death of matter, for throughout the infinite universe, all has to move,
to vibrate, that is, to live.
I have made the preceding statements at the peril of treading upon
metaphysical ground, in my desire to introduce the subject of this
lecture in a manner not altogether uninteresting, I may hope, to an
audience such as I have the honor to address. But now, then, returning
to the subject, this divine organ of sight, this indispensable
instrument for thought and all intellectual enjoyment, which lays open
to us the marvels of this universe, through which we have acquired what
knowledge we possess, and which prompts us to, and controls, all our
physical and mental activity. By what is it affected? By light! What is
light?
We have witnessed the great strides which have been made in all
departments of science in recent years. So great have been the advances
that we cannot refrain from asking ourselves, Is this all true, or is it
but a dream? Centuries ago men have lived, have thought, discovered,
invented, and have believed that they were soaring, while they were
merely proceeding at a snail's pace. So we too may be mistaken. But
taking the truth of the observed events as one of the implied facts of
science, we must rejoice in the immense progress already made and still
more in the anticipation of what must come, judging from the
possibilities opened up by modern research. There is, however, an
advance which we have been witnessing, which must be particularly
gratifying to every lover of progress. It is not a discovery, or an
invention, or an achievement in any particular direction. It is an
advance in all directions of scientific thought and experiment. I mean
the generalization of the natural forces and phenomena, the looming up
of a certain broad idea on the scientific horizon. It is this idea which
has, however, long ago taken possession of the most advanced minds, to
which I desire to call your attention, and which I intend to illustrate
in a general way, in these experiments, as the first step in answering
the question "What is light?" and to realize the modern meaning of this
word.
It is beyond the scope of my lecture to dwell upon the subject of light
in general, my object being merely to bring presently to your notice a
certain class of light effects and a number of phenomena observed in
pursuing the study of these effects. But to be consistent in my remarks
it is necessary to state that, according to that idea, now accepted by
the majority of scientific men as a positive result of theoretical and
experimental investigation, the various forms or manifestations of
energy which were generally designated as "electric" or more precisely
"electromagnetic" are energy manifestations of the same nature as those
of radiant heat and light. Therefore the phenomena of light and heat and
others besides these, may be called electrical phenomena. Thus
electrical science has become the mother science of all and its study
has become all important. The day when we shall know exactly what
"electricity" is, will chronicle an event probably greater, more
important than any other recorded in the history of the human race. The
time will come when the comfort, the very existence, perhaps, of man
will depend upon that wonderful agent. For our existence and comfort we
require heat, light and mechanical power. How do we now get all these?
We get them from fuel, we get them by consuming material. What will man
do when the forests disappear, when the coal fields are exhausted? Only
one thing, according to our present knowledge will remain; that is, to
transmit power at great distances. Men will go to the waterfalls, to the
tides, which are the stores of an infinitesimal part of Nature's
immeasurable energy. There will they harness the energy and transmit the
same to their settlements, to warm their homes by, to give them light,
and to keep their obedient slaves, the machines, toiling. But how will
they transmit this energy if not by electricity? Judge then, if the
comfort, nay, the very existence, of man will not depend on electricity.
I am aware that this view is not that of a practical engineer, but
neither is it that of an illusionist, for it is certain, that power
transmission, which at present is merely a stimulus to enterprise, will
some day be a dire necessity.
It is more important for the student, who takes up the study of light
phenomena, to make himself thoroughly acquainted with certain modern
views, than to peruse entire books on the subject of light itself, as
disconnected from these views. Were I therefore to make these
demonstrations before students seeking information--and for the sake of
the few of those who may be present, give me leave to so assume--it
would be my principal endeavor to impress these views upon their minds
in this series of experiments.
It might be sufficient for this purpose to perform a simple and
well-known experiment. I might take a familiar appliance, a Leyden jar,
charge it from a frictional machine, and then discharge it. In
explaining to you its permanent state when charged, and its transitory
condition when discharging, calling your attention to the forces which
enter into play and to the various phenomena they produce, and pointing
out the relation of the forces and phenomena, I might fully succeed in
illustrating that modern idea. No doubt, to the thinker, this simple
experiment would appeal as much as the most magnificent display. But
this is to be an experimental demonstration, and one which should
possess, besides instructive, also entertaining features and as such, a
simple experiment, such as the one cited, would not go very far towards
the attainment of the lecturer's aim. I must therefore choose another
way of illustrating, more spectacular certainly, but perhaps also more
instructive. Instead of the frictional machine and Leyden jar, I shall
avail myself in these experiments, of an induction coil of peculiar
properties, which was described in detail by me in a lecture before the
London Institution of Electrical Engineers, in Feb., 1892. This
induction coil is capable of yielding currents of enormous potential
differences, alternating with extreme rapidity. With this apparatus I
shall endeavor to show you three distinct classes of effects, or
phenomena, and it is my desire that each experiment, while serving for
the purposes of illustration, should at the same time teach us some
novel truth, or show us some novel aspect of this fascinating science.
But before doing this, it seems proper and useful to dwell upon the
apparatus employed, and method of obtaining the high potentials and
high-frequency currents which are made use of in these experiments.
[Illustration: FIG. 165.]
ON THE APPARATUS AND METHOD OF CONVERSION.
These high-frequency currents are obtained in a peculiar manner. The
method employed was advanced by me about two years ago in an
experimental lecture before the American Institute of Electrical
Engineers. A number of ways, as practiced in the laboratory, of
obtaining these currents either from continuous or low frequency
alternating currents, is diagramatically indicated in Fig. 165, which
will be later described in detail. The general plan is to charge
condensers, from a direct or alternate-current source, preferably of
high-tension, and to discharge them disruptively while observing
well-known conditions necessary to maintain the oscillations of the
current. In view of the general interest taken in high-frequency
currents and effects producible by them, it seems to me advisable to
dwell at some length upon this method of conversion. In order to give
you a clear idea of the action, I will suppose that a continuous-current
generator is employed, which is often very convenient. It is desirable
that the generator should possess such high tension as to be able to
break through a small air space. If this is not the case, then auxiliary
means have to be resorted to, some of which will be indicated
subsequently. When the condensers are charged to a certain potential,
the air, or insulating space, gives way and a disruptive discharge
occurs. There is then a sudden rush of current and generally a large
portion of accumulated electrical energy spends itself. The condensers
are thereupon quickly charged and the same process is repeated in more
or less rapid succession. To produce such sudden rushes of current it is
necessary to observe certain conditions. If the rate at which the
condensers are discharged is the same as that at which they are charged,
then, clearly, in the assumed case the condensers do not come into play.
If the rate of discharge be smaller than the rate of charging, then,
again, the condensers cannot play an important part. But if, on the
contrary, the rate of discharging is greater than that of charging, then
a succession of rushes of current is obtained. It is evident that, if
the rate at which the energy is dissipated by the discharge is very much
greater than the rate of supply to the condensers, the sudden rushes
will be comparatively few, with long-time intervals between. This always
occurs when a condenser of considerable capacity is charged by means of
a comparatively small machine. If the rates of supply and dissipation
are not widely different, then the rushes of current will be in quicker
succession, and this the more, the more nearly equal both the rates are,
until limitations incident to each case and depending upon a number of
causes are reached. Thus we are able to obtain from a continuous-current
generator as rapid a succession of discharges as we like. Of course, the
higher the tension of the generator, the smaller need be the capacity of
the condensers, and for this reason, principally, it is of advantage to
employ a generator of very high tension. Besides, such a generator
permits the attaining of greater rates of vibration.
The rushes of current may be of the same direction under the conditions
before assumed, but most generally there is an oscillation superimposed
upon the fundamental vibration of the current. When the conditions are
so determined that there are no oscillations, the current impulses are
unidirectional and thus a means is provided of transforming a continuous
current of high tension, into a direct current of lower tension, which I
think may find employment in the arts.
This method of conversion is exceedingly interesting and I was much
impressed by its beauty when I first conceived it. It is ideal in
certain respects. It involves the employment of no mechanical devices of
any kind, and it allows of obtaining currents of any desired frequency
from an ordinary circuit, direct or alternating. The frequency of the
fundamental discharges depending on the relative rates of supply and
dissipation can be readily varied within wide limits, by simple
adjustments of these quantities, and the frequency of the superimposed
vibration by the determination of the capacity, self-induction and
resistance of the circuit. The potential of the currents, again, may be
raised as high as any insulation is capable of withstanding safely by
combining capacity and self-induction or by induction in a secondary,
which need have but comparatively few turns.
As the conditions are often such that the intermittence or oscillation
does not readily establish itself, especially when a direct current
source is employed, it is of advantage to associate an interrupter with
the arc, as I have, some time ago, indicated the use of an air-blast or
magnet, or other such device readily at hand. The magnet is employed
with special advantage in the conversion of direct currents, as it is
then very effective. If the primary source is an alternate current
generator, it is desirable, as I have stated on another occasion, that
the frequency should be low, and that the current forming the arc be
large, in order to render the magnet more effective.
A form of such discharger with a magnet which has been found convenient,
and adopted after some trials, in the conversion of direct currents
particularly, is illustrated in Fig. 166. N S are the pole pieces of a
very strong magnet which is excited by a coil C. The pole pieces are
slotted for adjustment and can be fastened in any position by screws s
s_{1}. The discharge rods d d_{1}, thinned down on the ends in order
to allow a closer approach of the magnetic pole pieces, pass through the
columns of brass b b_{1} and are fastened in position by screws s_{2}
s_{2}. Springs r r_{1} and collars c c_{1} are slipped on the
rods, the latter serving to set the points of the rods at a certain
suitable distance by means of screws s_{3} s_{3}, and the former to
draw the points apart. When it is desired to start the arc, one of the
large rubber handles h h_{1} is tapped quickly with the hand, whereby
the points of the rods are brought in contact but are instantly
separated by the springs r r_{1}. Such an arrangement has been found
to be often necessary, namely in cases when the E. M. F. was not large
enough to cause the discharge to break through the gap, and also when it
was desirable to avoid short circuiting of the generator by the metallic
contact of the rods. The rapidity of the interruptions of the current
with a magnet depends on the intensity of the magnetic field and on the
potential difference at the end of the arc. The interruptions are
generally in such quick succession as to produce a musical sound. Years
ago it was observed that when a powerful induction coil is discharged
between the poles of a strong magnet, the discharge produces a loud
noise, not unlike a small pistol shot. It was vaguely stated that the
spark was intensified by the presence of the magnetic field. It is now
clear that the discharge current, flowing for some time, was interrupted
a great number of times by the magnet, thus producing the sound. The
phenomenon is especially marked when the field circuit of a large magnet
or dynamo is broken in a powerful magnetic field.
[Illustration: FIG. 166.]
When the current through the gap is comparatively large, it is of
advantage to slip on the points of the discharge rods pieces of very
hard carbon and let the arc play between the carbon pieces. This
preserves the rods, and besides has the advantage of keeping the air
space hotter, as the heat is not conducted away as quickly through the
carbons, and the result is that a smaller E. M. F. in the arc gap is
required to maintain a succession of discharges.
[Illustration: FIG. 167.]
Another form of discharger, which may be employed with advantage in
some cases, is illustrated in Fig. 167. In this form the discharge rods
d d_{1} pass through perforations in a wooden box B, which is thickly
coated with mica on the inside, as indicated by the heavy lines. The
perforations are provided with mica tubes m m_{1} of some thickness,
which are preferably not in contact with the rods d d_{1}. The box has
a cover C which is a little larger and descends on the outside of the
box. The spark gap is warmed by a small lamp _l_ contained in the box. A
plate _p_ above the lamp allows the draught to pass only through the
chimney _e_ of the lamp, the air entering through holes _o o_ in or near
the bottom of the box and following the path indicated by the arrows.
When the discharger is in operation, the door of the box is closed so
that the light of the arc is not visible outside. It is desirable to
exclude the light as perfectly as possible, as it interferes with some
experiments. This form of discharger is simple and very effective when
properly manipulated. The air being warmed to a certain temperature, has
its insulating power impaired; it becomes dielectrically weak, as it
were, and the consequence is that the arc can be established at much
greater distance. The arc should, of course, be sufficiently insulating
to allow the discharge to pass through the gap _disruptively_. The arc
formed under such conditions, when long, may be made extremely
sensitive, and the weak draught through the lamp chimney _c_ is quite
sufficient to produce rapid interruptions. The adjustment is made by
regulating the temperature and velocity of the draught. Instead of using
the lamp, it answers the purpose to provide for a draught of warm air in
other ways. A very simple way which has been practiced is to enclose the
arc in a long vertical tube, with plates on the top and bottom for
regulating the temperature and velocity of the air current. Some
provision had to be made for deadening the sound.
The air may be rendered dielectrically weak also by rarefaction.
Dischargers of this kind have likewise been used by me in connection
with a magnet. A large tube is for this purpose provided with heavy
electrodes of carbon or metal, between which the discharge is made to
pass, the tube being placed in a powerful magnetic field. The exhaustion
of the tube is carried to a point at which the discharge breaks through
easily, but the pressure should be more than 75 millimetres, at which
the ordinary thread discharge occurs. In another form of discharger,
combining the features before mentioned, the discharge was made to pass
between two adjustable magnetic pole pieces, the space between them
being kept at an elevated temperature.
It should be remarked here that when such, or interrupting devices of
any kind, are used and the currents are passed through the primary of a
disruptive discharge coil, it is not, as a rule, of advantage to produce
a number of interruptions of the current per second greater than the
natural frequency of vibration of the dynamo supply circuit, which is
ordinarily small. It should also be pointed out here, that while the
devices mentioned in connection with the disruptive discharge are
advantageous under certain conditions, they may be sometimes a source of
trouble, as they produce intermittences and other irregularities in the
vibration which it would be very desirable to overcome.
There is, I regret to say, in this beautiful method of conversion a
defect, which fortunately is not vital, and which I have been gradually
overcoming. I will best call attention to this defect and indicate a
fruitful line of work, by comparing the electrical process with its
mechanical analogue. The process may be illustrated in this manner.
Imagine a tank with a wide opening at the bottom, which is kept closed
by spring pressure, but so that it snaps off _suddenly_ when the liquid
in the tank has reached a certain height. Let the fluid be supplied to
the tank by means of a pipe feeding at a certain rate. When the critical
height of the liquid is reached, the spring gives way and the bottom of
the tank drops out. Instantly the liquid falls through the wide opening,
and the spring, reasserting itself, closes the bottom again. The tank is
now filled, and after a certain time interval the same process is
repeated. It is clear, that if the pipe feeds the fluid quicker than the
bottom outlet is capable of letting it pass through, the bottom will
remain off and the tank will still overflow. If the rates of supply are
exactly equal, then the bottom lid will remain partially open and no
vibration of the same and of the liquid column will generally occur,
though it might, if started by some means. But if the inlet pipe does
not feed the fluid fast enough for the outlet, then there will be always
vibration. Again, in such case, each time the bottom flaps up or down,
the spring and the liquid column, if the pliability of the spring and
the inertia of the moving parts are properly chosen, will perform
independent vibrations. In this analogue the fluid may be likened to
electricity or electrical energy, the tank to the condenser, the spring
to the dielectric, and the pipe to the conductor through which
electricity is supplied to the condenser. To make this analogy quite
complete it is necessary to make the assumption, that the bottom, each
time it gives way, is knocked violently against a non-elastic stop, this
impact involving some loss of energy; and that, besides, some
dissipation of energy results due to frictional losses. In the preceding
analogue the liquid is supposed to be under a steady pressure. If the
presence of the fluid be assumed to vary rhythmically, this may be taken
as corresponding to the case of an alternating current. The process is
then not quite as simple to consider, but the action is the same in
principle.
It is desirable, in order to maintain the vibration economically, to
reduce the impact and frictional losses as much as possible. As regards
the latter, which in the electrical analogue correspond to the losses
due to the resistance of the circuits, it is impossible to obviate them
entirely, but they can be reduced to a minimum by a proper selection of
the dimensions of the circuits and by the employment of thin conductors
in the form of strands. But the loss of energy caused by the first
breaking through of the dielectric--which in the above example
corresponds to the violent knock of the bottom against the inelastic
stop--would be more important to overcome. At the moment of the breaking
through, the air space has a very high resistance, which is probably
reduced to a very small value when the current has reached some
strength, and the space is brought to a high temperature. It would
materially diminish the loss of energy if the space were always kept at
an extremely high temperature, but then there would be no disruptive
break. By warming the space moderately by means of a lamp or otherwise,
the economy as far as the arc is concerned is sensibly increased. But
the magnet or other interrupting device does not diminish the loss in
the arc. Likewise, a jet of air only facilitates the carrying off of the
energy. Air, or a gas in general, behaves curiously in this respect.
When two bodies charged to a very high potential, discharge disruptively
through an air space, any amount of energy may be carried off by the
air. This energy is evidently dissipated by bodily carriers, in impact
and collisional losses of the molecules. The exchange of the molecules
in the space occurs with inconceivable rapidity. A powerful discharge
taking place between two electrodes, they may remain entirely cool, and
yet the loss in the air may represent any amount of energy. It is
perfectly practicable, with very great potential differences in the gap,
to dissipate several horse-power in the arc of the discharge without
even noticing a small increase in the temperature of the electrodes. All
the frictional losses occur then practically in the air. If the exchange
of the air molecules is prevented, as by enclosing the air hermetically,
the gas inside of the vessel is brought quickly to a high temperature,
even with a very small discharge. It is difficult to estimate how much
of the energy is lost in sound waves, audible or not, in a powerful
discharge. When the currents through the gap are large, the electrodes
may become rapidly heated, but this is not a reliable measure of the
energy wasted in the arc, as the loss through the gap itself may be
comparatively small. The air or a gas in general is, at ordinary
pressure at least, clearly not the best medium through which a
disruptive discharge should occur. Air or other gas under great pressure
is of course a much more suitable medium for the discharge gap. I have
carried on long-continued experiments in this direction, unfortunately
less practicable on account of the difficulties and expense in getting
air under great pressure. But even if the medium in the discharge space
is solid or liquid, still the same losses take place, though they are
generally smaller, for just as soon as the arc is established, the solid
or liquid is volatilized. Indeed, there is no body known which would not
be disintegrated by the arc, and it is an open question among scientific
men, whether an arc discharge could occur at all in the air itself
without the particles of the electrodes being torn off. When the current
through the gap is very small and the arc very long, I believe that a
relatively considerable amount of heat is taken up in the disintegration
of the electrodes, which partially on this account may remain quite
cold.
The ideal medium for a discharge gap should only _crack_, and the ideal
electrode should be of some material which cannot be disintegrated. With
small currents through the gap it is best to employ aluminum, but not
when the currents are large. The disruptive break in the air, or more or
less in any ordinary medium, is not of the nature of a crack, but it is
rather comparable to the piercing of innumerable bullets through a mass
offering great frictional resistances to the motion of the bullets, this
involving considerable loss of energy. A medium which would merely crack
when strained electrostatically--and this possibly might be the case
with a perfect vacuum, that is, pure ether--would involve a very small
loss in the gap, so small as to be entirely negligible, at least
theoretically, because a crack may be produced by an infinitely small
displacement. In exhausting an oblong bulb provided with two aluminum
terminals, with the greatest care, I have succeeded in producing such a
vacuum that the secondary discharge of a disruptive discharge coil would
break disruptively through the bulb in the form of fine spark streams.
The curious point was that the discharge would completely ignore the
terminals and start far behind the two aluminum plates which served as
electrodes. This extraordinary high vacuum could only be maintained for
a very short while. To return to the ideal medium, think, for the sake
of illustration, of a piece of glass or similar body clamped in a vice,
and the latter tightened more and more. At a certain point a minute
increase of the pressure will cause the glass to crack. The loss of
energy involved in splitting the glass may be practically nothing, for
though the force is great, the displacement need be but extremely small.
Now imagine that the glass would possess the property of closing again
perfectly the crack upon a minute diminution of the pressure. This is
the way the dielectric in the discharge space should behave. But
inasmuch as there would be always some loss in the gap, the medium,
which should be continuous, should exchange through the gap at a rapid
rate. In the preceding example, the glass being perfectly closed, it
would mean that the dielectric in the discharge space possesses a great
insulating power; the glass being cracked, it would signify that the
medium in the space is a good conductor. The dielectric should vary
enormously in resistance by minute variations of the E. M. F. across the
discharge space. This condition is attained, but in an extremely
imperfect manner, by warming the air space to a certain critical
temperature, dependent on the E. M. F. across the gap, or by otherwise
impairing the insulating power of the air. But as a matter of fact the
air does never break down _disruptively_, if this term be rigorously
interpreted, for before the sudden rush of the current occurs, there is
always a weak current preceding it, which rises first gradually and then
with comparative suddenness. That is the reason why the rate of change
is very much greater when glass, for instance, is broken through, than
when the break takes place through an air space of equivalent dielectric
strength. As a medium for the discharge space, a solid, or even a
liquid, would be preferable therefor. It is somewhat difficult to
conceive of a solid body which would possess the property of closing
instantly after it has been cracked. But a liquid, especially under
great pressure, behaves practically like a solid, while it possesses the
property of closing the crack. Hence it was thought that a liquid
insulator might be more suitable as a dielectric than air. Following out
this idea, a number of different forms of dischargers in which a variety
of such insulators, sometimes under great pressure, were employed, have
been experimented upon. It is thought sufficient to dwell in a few words
upon one of the forms experimented upon. One of these dischargers is
illustrated in Figs. 168_a_ and 168_b_.
[Illustration: FIG. 168a.]
[Illustration: FIG. 168b.]
A hollow metal pulley P (Fig. 168_a_), was fastened upon an arbor _a_,
which by suitable means was rotated at a considerable speed. On the
inside of the pulley, but disconnected from the same, was supported a
thin disc _h_ (which is shown thick for the sake of clearness), of hard
rubber in which there were embedded two metal segments _s s_ with
metallic extensions _e e_ into which were screwed conducting terminals
_t t_ covered with thick tubes of hard rubber _t t_. The rubber disc _h_
with its metallic segments _s s_, was finished in a lathe, and its
entire surface highly polished so as to offer the smallest possible
frictional resistance to the motion through a fluid. In the hollow of
the pulley an insulating liquid such as a thin oil was poured so as to
reach very nearly to the opening left in the flange _f_, which was
screwed tightly on the front side of the pulley. The terminals _t t_,
were connected to the opposite coatings of a battery of condensers so
that the discharge occurred through the liquid. When the pulley was
rotated, the liquid was forced against the rim of the pulley and
considerable fluid pressure resulted. In this simple way the discharge
gap was filled with a medium which behaved practically like a solid,
which possessed the quality of closing instantly upon the occurrence of
the break, and which moreover was circulating through the gap at a rapid
rate. Very powerful effects were produced by discharges of this kind
with liquid interrupters, of which a number of different forms were
made. It was found that, as expected, a longer spark for a given length
of wire was obtainable in this way than by using air as an interrupting
device. Generally the speed, and therefore also the fluid pressure, was
limited by reason of the fluid friction, in the form of discharger
described, but the practically obtainable speed was more than sufficient
to produce a number of breaks suitable for the circuits ordinarily used.
In such instances the metal pulley P was provided with a few projections
inwardly, and a definite number of breaks was then produced which could
be computed from the speed of rotation of the pulley. Experiments were
also carried on with liquids of different insulating power with the view
of reducing the loss in the arc. When an insulating liquid is moderately
warmed, the loss in the arc is diminished.
A point of some importance was noted in experiments with various
discharges of this kind. It was found, for instance, that whereas the
conditions maintained in these forms were favorable for the production
of a great spark length, the current so obtained was not best suited to
the production of light effects. Experience undoubtedly has shown, that
for such purposes a harmonic rise and fall of the potential is
preferable. Be it that a solid is rendered incandescent, or
phosphorescent, or be it that energy is transmitted by condenser coating
through the glass, it is quite certain that a harmonically rising and
falling potential produces less destructive action, and that the vacuum
is more permanently maintained. This would be easily explained if it
were ascertained that the process going on in an exhausted vessel is of
an electrolytic nature.
In the diagrammatical sketch, Fig. 165, which has been already referred
to, the cases which are most likely to be met with in practice are
illustrated. One has at his disposal either direct or alternating
currents from a supply station. It is convenient for an experimenter in
an isolated laboratory to employ a machine G, such as illustrated,
capable of giving both kinds of currents. In such case it is also
preferable to use a machine with multiple circuits, as in many
experiments it is useful and convenient to have at one's disposal
currents of different phases. In the sketch, D represents the direct and
A the alternating circuit. In each of these, three branch circuits are
shown, all of which are provided with double line switches _s s s s s
s_. Consider first the direct current conversion; I_a_ represents the
simplest case. If the E. M. F. of the generator is sufficient to break
through a small air space, at least when the latter is warmed or
otherwise rendered poorly insulating, there is no difficulty in
maintaining a vibration with fair economy by judicious adjustment of the
capacity, self-induction and resistance of the circuit L containing the
devices _l l m_. The magnet N, S, can be in this case advantageously
combined with the air space. The discharger _d d_ with the magnet may be
placed either way, as indicated by the full or by the dotted lines. The
circuit I_a_ with the connections and devices is supposed to possess
dimensions such as are suitable for the maintenance of a vibration. But
usually the E. M. F. on the circuit or branch I_a_ will be something
like a 100 volts or so, and in this case it is not sufficient to break
through the gap. Many different means may be used to remedy this by
raising the E. M. F. across the gap. The simplest is probably to insert
a large self-induction coil in series with the circuit L. When the arc
is established, as by the discharger illustrated in Fig. 166, the magnet
blows the arc out the instant it is formed. Now the extra current of the
break, being of high E. M. F., breaks through the gap, and a path of low
resistance for the dynamo current being again provided, there is a
sudden rush of current from the dynamo upon the weakening or subsidence
of the extra current. This process is repeated in rapid succession, and
in this manner I have maintained oscillation with as low as 50 volts, or
even less, across the gap. But conversion on this plan is not to be
recommended on account of the too heavy currents through the gap and
consequent heating of the electrodes; besides, the frequencies obtained
in this way are low, owing to the high self-induction necessarily
associated with the circuit. It is very desirable to have the E. M. F.
as high as possible, first, in order to increase the economy of the
conversion, and, secondly, to obtain high frequencies. The difference of
potential in this electric oscillation is, of course, the equivalent of
the stretching force in the mechanical vibration of the spring. To
obtain very rapid vibration in a circuit of some inertia, a great
stretching force or difference of potential is necessary. Incidentally,
when the E. M. F. is very great, the condenser which is usually employed
in connection with the circuit need but have a small capacity, and many
other advantages are gained. With a view of raising the E. M. F. to a
many times greater value than obtainable from ordinary distribution
circuits, a rotating transformer _g_ is used, as indicated at II_a_,
Fig. 165, or else a separate high potential machine is driven by means
of a motor operated from the generator G. The latter plan is in fact
preferable, as changes are easier made. The connections from the high
tension winding are quite similar to those in branch I_a_ with the
exception that a condenser C, which should be adjustable, is connected
to the high tension circuit. Usually, also, an adjustable self-induction
coil in series with the circuit has been employed in these experiments.
When the tension of the currents is very high, the magnet ordinarily
used in connection with the discharger is of comparatively small value,
as it is quite easy to adjust the dimensions of the circuit so that
oscillation is maintained. The employment of a steady E. M. F. in the
high frequency conversion affords some advantages over the employment of
alternating E. M. F., as the adjustments are much simpler and the action
can be easier controlled. But unfortunately one is limited by the
obtainable potential difference. The winding also breaks down easily in
consequence of the sparks which form between the sections of the
armature or commutator when a vigorous oscillation takes place. Besides,
these transformers are expensive to build. It has been found by
experience that it is best to follow the plan illustrated at III_a_. In
this arrangement a rotating transformer _g_, is employed to convert the
low tension direct currents into low frequency alternating currents,
preferably also of small tension. The tension of the currents is then
raised in a stationary transformer T. The secondary S of this
transformer is connected to an adjustable condenser C which discharges
through the gap or discharger _d d_, placed in either of the ways
indicated, through the primary P of a disruptive discharge coil, the
high frequency current being obtained from the secondary S of this coil,
as described on previous occasions. This will undoubtedly be found the
cheapest and most convenient way of converting direct currents.
The three branches of the circuit A represent the usual cases met in
practice when alternating currents are converted. In Fig. 1_b_ a
condenser C, generally of large capacity, is connected to the circuit L
containing the devices _l l_, _m m_. The devices _m m_ are supposed to
be of high self-induction so as to bring the frequency of the circuit
more or less to that of the dynamo. In this instance the discharger _d
d_ should best have a number of makes and breaks per second equal to
twice the frequency of the dynamo. If not so, then it should have at
least a number equal to a multiple or even fraction of the dynamo
frequency. It should be observed, referring to I_b_, that the conversion
to a high potential is also effected when the discharger _d d_, which is
shown in the sketch, is omitted. But the effects which are produced by
currents which rise instantly to high values, as in a disruptive
discharge, are entirely different from those produced by dynamo currents
which rise and fall harmonically. So, for instance, there might be in a
given case a number of makes and breaks at _d d_ equal to just twice the
frequency of the dynamo, or in other words, there may be the same number
of fundamental oscillations as would be produced without the discharge
gap, and there might even not be any quicker superimposed vibration; yet
the differences of potential at the various points of the circuit, the
impedance and other phenomena, dependent upon the rate of change, will
bear no similarity in the two cases. Thus, when working with currents
discharging disruptively, the element chiefly to be considered is not
the frequency, as a student might be apt to believe, but the rate of
change per unit of time. With low frequencies in a certain measure the
same effects may be obtained as with high frequencies, provided the rate
of change is sufficiently great. So if a low frequency current is raised
to a potential of, say, 75,000 volts, and the high tension current
passed through a series of high resistance lamp filaments, the
importance of the rarefied gas surrounding the filament is clearly
noted, as will be seen later; or, if a low frequency current of several
thousand amperes is passed through a metal bar, striking phenomena of
impedance are observed, just as with currents of high frequencies. But
it is, of course, evident that with low frequency currents it is
impossible to obtain such rates of change per unit of time as with high
frequencies, hence the effects produced by the latter are much more
prominent. It is deemed advisable to make the preceding remarks,
inasmuch as many more recently described effects have been unwittingly
identified with high frequencies. Frequency alone in reality does not
mean anything, except when an undisturbed harmonic oscillation is
considered.
In the branch III_b_ a similar disposition to that in I_b_ is
illustrated, with the difference that the currents discharging through
the gap _d d_ are used to induce currents in the secondary S of a
transformer T. In such case the secondary should be provided with an
adjustable condenser for the purpose of tuning it to the primary.
II_b_ illustrates a plan of alternate current high frequency conversion
which is most frequently used and which is found to be most convenient.
This plan has been dwelt upon in detail on previous occasions and need
not be described here.
Some of these results were obtained by the use of a high frequency
alternator. A description of such machines will be found in my original
paper before the American Institute of Electrical Engineers, and in
periodicals of that period, notably in THE ELECTRICAL ENGINEER of March
18, 1891.
I will now proceed with the experiments.
ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.
The first class of effects I intend to show you are effects produced by
electrostatic force. It is the force which governs the the motion of the
atoms, which causes them to collide and develop the life-sustaining
energy of heat and light, and which causes them to aggregate in an
infinite variety of ways, according to Nature's fanciful designs, and to
form all these wondrous structures we perceive around us; it is, in
fact, if our present views be true, the most important force for us to
consider in Nature. As the term _electrostatic_ might imply a steady
electric condition, it should be remarked, that in these experiments the
force is not constant, but varies at a rate which may be considered
moderate, about one million times a second, or thereabouts. This enables
me to produce many effects which are not producible with an unvarying
force.
When two conducting bodies are insulated and electrified, we say that an
electrostatic force is acting between them. This force manifests itself
in attractions, repulsions and stresses in the bodies and space or
medium without. So great may be the strain exerted in the air, or
whatever separates the two conducting bodies, that it may break down,
and we observe sparks or bundles of light or streamers, as they are
called. These streamers form abundantly when the force through the air
is rapidly varying. I will illustrate this action of electrostatic force
in a novel experiment in which I will employ the induction coil before
referred to. The coil is contained in a trough filled with oil, and
placed under the table. The two ends of the secondary wire pass through
the two thick columns of hard rubber which protrude to some height above
the table. It is necessary to insulate the ends or terminals of the
secondary heavily with hard rubber, because even dry wood is by far too
poor an insulator for these currents of enormous potential differences.
On one of the terminals of the coil, I have placed a large sphere of
sheet brass, which is connected to a larger insulated brass plate, in
order to enable me to perform the experiments under conditions, which,
as you will see, are more suitable for this experiment. I now set the
coil to work and approach the free terminal with a metallic object held
in my hand, this simply to avoid burns. As I approach the metallic
object to a distance of eight or ten inches, a torrent of furious sparks
breaks forth from the end of the secondary wire, which passes through
the rubber column. The sparks cease when the metal in my hand touches
the wire. My arm is now traversed by a powerful electric current,
vibrating at about the rate of one million times a second. All around me
the electrostatic force makes itself felt, and the air molecules and
particles of dust flying about are acted upon and are hammering
violently against my body. So great is this agitation of the particles,
that when the lights are turned out you may see streams of feeble light
appear on some parts of my body. When such a streamer breaks out on any
part of the body, it produces a sensation like the pricking of a needle.
Were the potentials sufficiently high and the frequency of the vibration
rather low, the skin would probably be ruptured under the tremendous
strain, and the blood would rush out with great force in the form of
fine spray or jet so thin as to be invisible, just as oil will when
placed on the positive terminal of a Holtz machine. The breaking through
of the skin though it may seem impossible at first, would perhaps occur,
by reason of the tissues under the skin being incomparably better
conducting. This, at least, appears plausible, judging from some
observations.
[Illustration: FIG. 169.]
I can make these streams of light visible to all, by touching with the
metallic object one of the terminals as before, and approaching my free
hand to the brass sphere, which is connected to the second terminal of
the coil. As the hand is approached, the air between it and the sphere,
or in the immediate neighborhood, is more violently agitated, and you
see streams of light now break forth from my finger tips and from the
whole hand (Fig. 169). Were I to approach the hand closer, powerful
sparks would jump from the brass sphere to my hand, which might be
injurious. The streamers offer no particular inconvenience, except that
in the ends of the finger tips a burning sensation is felt. They should
not be confounded with those produced by an influence machine, because
in many respects they behave differently. I have attached the brass
sphere and plate to one of the terminals in order to prevent the
formation of visible streamers on that terminal, also in order to
prevent sparks from jumping at a considerable distance. Besides, the
attachment is favorable for the working of the coil.
The streams of light which you have observed issuing from my hand are
due to a potential of about 200,000 volts, alternating in rather
irregular intervals, sometimes like a million times a second. A
vibration of the same amplitude, but four times as fast, to maintain
which over 3,000,000 volts would be required, would be more than
sufficient to envelop my body in a complete sheet of flame. But this
flame would not burn me up; quite contrarily, the probability is that I
would not be injured in the least. Yet a hundredth part of that energy,
otherwise directed, would be amply sufficient to kill a person.
The amount of energy which may thus be passed into the body of a person
depends on the frequency and potential of the currents, and by making
both of these very great, a vast amount of energy may be passed into the
body without causing any discomfort, except perhaps, in the arm, which
is traversed by a true conduction current. The reason why no pain in the
body is felt, and no injurious effect noted, is that everywhere, if a
current be imagined to flow through the body, the direction of its flow
would be at right angles to the surface; hence the body of the
experimenter offers an enormous section to the current, and the density
is very small, with the exception of the arm, perhaps, where the density
may be considerable. But if only a small fraction of that energy would
be applied in such a way that a current would traverse the body in the
same manner as a low frequency current, a shock would be received which
might be fatal. A direct or low frequency alternating current is fatal,
I think, principally because its distribution through the body is not
uniform, as it must divide itself in minute streamlets of great density,
whereby some organs are vitally injured. That such a process occurs I
have not the least doubt, though no evidence might apparently exist, or
be found upon examination. The surest to injure and destroy life, is a
continuous current, but the most painful is an alternating current of
very low frequency. The expression of these views, which are the result
of long continued experiment and observation, both with steady and
varying currents, is elicited by the interest which is at present taken
in this subject, and by the manifestly erroneous ideas which are daily
propounded in journals on this subject.
I may illustrate an effect of the electrostatic force by another
striking experiment, but before, I must call your attention to one or
two facts. I have said before, that when the medium between two
oppositely electrified bodies is strained beyond a certain limit it
gives way and, stated in popular language, the opposite electric charges
unite and neutralize each other. This breaking down of the medium occurs
principally when the force acting between the bodies is steady, or
varies at a moderate rate. Were the variation sufficiently rapid, such a
destructive break would not occur, no matter how great the force, for
all the energy would be spent in radiation, convection and mechanical
and chemical action. Thus the _spark_ length, or greatest distance which
a _spark_ will jump between the electrified bodies is the smaller, the
greater the variation or time rate of change. But this rule may be taken
to be true only in a general way, when comparing rates which are widely
different.
[Illustration: FIG. 170a.]
[Illustration: FIG. 170b.]
I will show you by an experiment the difference in the effect produced
by a rapidly varying and a steady or moderately varying force. I have
here two large circular brass plates _p p_ (Fig. 170_a_ and Fig.
170_b_), supported on movable insulating stands on the table, connected
to the ends of the secondary of a coil similar to the one used before. I
place the plates ten or twelve inches apart and set the coil to work.
You see the whole space between the plates, nearly two cubic feet,
filled with uniform light, Fig. 170_a_. This light is due to the
streamers you have seen in the first experiment, which are now much more
intense. I have already pointed out the importance of these streamers in
commercial apparatus and their still greater importance in some purely
scientific investigations. Often they are too weak to be visible, but
they always exist, consuming energy and modifying the action of the
apparatus. When intense, as they are at present, they produce ozone in
great quantity, and also, as Professor Crookes has pointed out, nitrous
acid. So quick is the chemical action that if a coil, such as this one,
is worked for a very long time it will make the atmosphere of a small
room unbearable, for the eyes and throat are attacked. But when
moderately produced, the streamers refresh the atmosphere wonderfully,
like a thunder-storm, and exercises unquestionably a beneficial effect.
In this experiment the force acting between the plates changes in
intensity and direction at a very rapid rate. I will now make the rate
of change per unit time much smaller. This I effect by rendering the
discharges through the primary of the induction coil less frequent, and
also by diminishing the rapidity of the vibration in the secondary. The
former result is conveniently secured by lowering the E. M. F. over the
air gap in the primary circuit, the latter by approaching the two brass
plates to a distance of about three or four inches. When the coil is set
to work, you see no streamers or light between the plates, yet the
medium between them is under a tremendous strain. I still further
augment the strain by raising the E. M. F. in the primary circuit, and
soon you see the air give way and the hall is illuminated by a shower of
brilliant and noisy sparks, Fig. 170_b_. These sparks could be produced
also with unvarying force; they have been for many years a familiar
phenomenon, though they were usually obtained from an entirely different
apparatus. In describing these two phenomena so radically different in
appearance, I have advisedly spoken of a "force" acting between the
plates. It would be in accordance with accepted views to say, that there
was an "alternating E. M. F," acting between the plates. This term is
quite proper and applicable in all cases where there is evidence of at
least a possibility of an essential inter-dependence of the electric
state of the plates, or electric action in their neighborhood. But if
the plates were removed to an infinite distance, or if at a finite
distance, there is no probability or necessity whatever for such
dependence. I prefer to use the term "electrostatic force," and to say
that such a force is acting around each plate or electrified insulated
body in general. There is an inconvenience in using this expression as
the term incidentally means a steady electric condition; but a proper
nomenclature will eventually settle this difficulty.
I now return to the experiment to which I have already alluded, and with
which I desire to illustrate a striking effect produced by a rapidly
varying electrostatic force. I attach to the end of the wire, _l_ (Fig.
171), which is in connection with one of the terminals of the secondary
of the induction coil, an exhausted bulb _b_. This bulb contains a thin
carbon filament _f_, which is fastened to a platinum wire _w_, sealed in
the glass and leading outside of the bulb, where it connects to the wire
_l_. The bulb may be exhausted to any degree attainable with ordinary
apparatus. Just a moment before, you have witnessed the breaking down of
the air between the charged brass plates. You know that a plate of
glass, or any other insulating material, would break down in like
manner. Had I therefore a metallic coating attached to the outside of
the bulb, or placed near the same, and were this coating connected to
the other terminal of the coil, you would be prepared to see the glass
give way if the strain were sufficiently increased. Even were the
coating not connected to the other terminal, but to an insulated plate,
still, if you have followed recent developments, you would naturally
expect a rupture of the glass.
[Illustration: FIG. 171.]
[Illustration: FIG. 172a.]
[Illustration: FIG. 172b.]
But it will certainly surprise you to note that under the action of the
varying electrostatic force, the glass gives way when all other bodies
are removed from the bulb. In fact, all the surrounding bodies we
perceive might be removed to an infinite distance without affecting the
result in the slightest. When the coil is set to work, the glass is
invariably broken through at the seal, or other narrow channel, and the
vacuum is quickly impaired. Such a damaging break would not occur with
a steady force, even if the same were many times greater. The break is
due to the agitation of the molecules of the gas within the bulb, and
outside of the same. This agitation, which is generally most violent in
the narrow pointed channel near the seal, causes a heating and rupture
of the glass. This rupture, would, however, not occur, not even with a
varying force, if the medium filling the inside of the bulb, and that
surrounding it, were perfectly homogeneous. The break occurs much
quicker if the top of the bulb is drawn out into a fine fibre. In bulbs
used with these coils such narrow, pointed channels must therefore be
avoided.
When a conducting body is immersed in air, or similar insulating medium,
consisting of, or containing, small freely movable particles capable of
being electrified, and when the electrification of the body is made to
undergo a very rapid change--which is equivalent to saying that the
electrostatic force acting around the body is varying in intensity,--the
small particles are attracted and repelled, and their violent impacts
against the body may cause a mechanical motion of the latter. Phenomena
of this kind are noteworthy, inasmuch as they have not been observed
before with apparatus such as has been commonly in use. If a very light
conducting sphere be suspended on an exceedingly fine wire, and charged
to a steady potential, however high, the sphere will remain at rest.
Even if the potential would be rapidly varying, provided that the small
particles of matter, molecules or atoms, are evenly distributed, no
motion of the sphere should result. But if one side of the conducting
sphere is covered with a thick insulating layer, the impacts of the
particles will cause the sphere to move about, generally in irregular
curves, Fig. 172_a_. In like manner, as I have shown on a previous
occasion, a fan of sheet metal, Fig. 172_b_, covered partially with
insulating material as indicated, and placed upon the terminal of the
coil so as to turn freely on it, is spun around.
All these phenomena you have witnessed and others which will be shown
later, are due to the presence of a medium like air, and would not occur
in a continuous medium. The action of the air may be illustrated still
better by the following experiment. I take a glass tube _t_, Fig. 173,
of about an inch in diameter, which has a platinum wire _w_ sealed in
the lower end, and to which is attached a thin lamp filament _f_. I
connect the wire with the terminal of the coil and set the coil to work.
The platinum wire is now electrified positively and negatively in rapid
succession and the wire and air inside of the tube is rapidly heated by
the impacts of the particles, which may be so violent as to render the
filament incandescent. But if I pour oil in the tube, just as soon as
the wire is covered with the oil, all action apparently ceases and there
is no marked evidence of heating. The reason of this is that the oil is
a practically continuous medium. The displacements in such a continuous
medium are, with these frequencies, to all appearance incomparably
smaller than in air, hence the work performed in such a medium is
insignificant. But oil would behave very differently with frequencies
many times as great, for even though the displacements be small, if the
frequency were much greater, considerable work might be performed in the
oil.
[Illustration: FIG. 173.]
[Illustration: FIG. 174.]