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 almost in 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 the
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 r 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withdrawn from R_1 except when the pump had reached the highest degree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 15 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 1-20, 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 act 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 claim 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 allowing
barely the light to shine through. A metallic clasp, 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 aluminium 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 not by far
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 a high frequency current,
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 the 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 he
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 to 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 as 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.
[Illustration: FIG. 31.--BULB SHOWING RADIANT LIME STREAM AT LOW
EXHAUSTION.]
I have prepared a bulb to illustrate by an experiment the correctness
of these assertions. In a globe L (Fig. 31) 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. 19, 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. 31) 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.
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.
A thought which naturally presents itself in connection with high
frequency currents, is to make use of their powerful electro-dynamic
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. 32 and Fig. 33.
In Fig. 32 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 aluminium
wire, the ends of which were provided with small spheres t and t_1 of
aluminium, 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 currents 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.
[Illustration: FIG. 32.--ELECTRO-DYNAMIC INDUCTION TUBE.]
[Illustration: FIG. 33--ELECTRO-DYNAMIC INDUCTION LAMP.]
In Fig. 33 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 just a trifle heated 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, was 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. 33 an
aluminium 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 experiences 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, gets 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 ball 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, still, 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 far 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 he works
with condenser charges--and they are the only means up to the present
known for reaching these extreme frequencies--he gets to electromotive
forces of several thousands of volts per turn of the primary. He
cannot multiply the electro-dynamic inductive effect by taking more
turns in the primary, for he arrives at the conclusion that the best
way is to work with one single turn--though he must sometimes depart
from this rule--and he must get along with whatever inductive effect
he can obtain with one turn. But before he has long experimented with
the extreme frequencies required to set up in a small bulb an
electromotive force of several thousands of volts he 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 electro-dynamic
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 condenser to a higher potential, establish electrostatic
alternating fields which acted through the whole extent of a 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.
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 the 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 the 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. 34 and Fig. 35 two such tubes are illustrated
which are prepared for the occasion. In Fig. 34 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 aluminium 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.
[Illustration: FIG. 34.--TUBE WITH FILAMENT RENDERED INCANDESCENT IN
AN ELECTROSTATIC FIELD.]