FIG. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF EXHAUSTION.

The apparatus is illustrated in a drawing shown in Fig 80. 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 arrange-
ment overcomes the imperfections and troubles which often arise from the use of the stopcock on the reservoir and
the connection of the latter with the fall tube.
The pump is connected through a U-shaped tube to a very large reservoir R1. Especial care was taken in fitting the
grinding surfaces of the stoppers p and p1 and both of these and the mercury caps above them were made excep-
tionally 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 connec-
tions g and g1--one for a small bulb b, usually containing caustic potash, and the other for the receiver r, to be
exhausted.
The reservoir Rt was connected by means of a rubber tube to a slightly larger reservoir R2, each of the two
reservoirs being provided with a stopcock Ct and C2, respectively. The reservoir R2 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 C2 closed, so as to form a Torricellian vacuum in it when raised, it could be lifted so high that the mercury
in reservoir R1 would stand a little above stopcock C1; and when this stopcock was closed and the reservoir R
descended, so as to form a Torricellian vacuum in reservoir R1 it could be lowered so far as to completely empty
the latter, the mercury filling the reservoir R2 up to a little above stopcock C2.
The capacity of the pump and of the connections was taken as small as possible relatively to the volume of reser-
voir, R1 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 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
moisture off 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 C1 being open, and all
other connections closed, the reservoir R2 was raised so far that the mercury filled the reservoir R1 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 R2 was lowered, the experimenter keeping the mercury at about the same level. The reservoir
R1 was balanced by a long spring which facilitated the operation, and the friction of the parts was generally suf-
ficient to keep it almost in any position. When the Sprengel pump had done its work, the reservoir R2 was further
lowered and the mercury descended in R1 and filled R2, whereupon stopcock C2 was closed. The air adhering to the
walls of R1 and that absorbed by the mercury was carried off, and to free the mercury of all air the reservoir R2 was
for a long time worked up and down. During this process some air, which would gather below stopcock C2, was
expelled from R2 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 R2 when it was lowered,
the caustic potash was resorted to. The reservoir R2 was now again raised until the mercury in R1 stood above
stopcock C1. 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 R2 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 R2 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 R2 was lowered, and upon
reservoir R1 being emptied the receiver r was quickly sealed up.
When a new bulb was put on, the mercury was always raised above stopcock C1 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 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

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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, in-
compressible, 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 conden-
sers 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 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 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 to take a large hollow metallic sphere and fill it with a feet
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

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be cut in the middle as not, for practically the heating is due to the rarefied gas. Here the bright 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 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 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.