capacity were increased. Absorption always exists more or less, but
generally it is small and of no consequence as long as the frequencies
are not very great. In using extremely high frequencies, and,
necessarily in such case, also high potentials, the absorption--or,
what is here meant more particularly by this term, the loss of energy
due to the presence of a gaseous medium--is an important factor to be
considered, as the energy absorbed in the air condenser may be any
fraction of the supplied energy. This would seem to make it very
difficult to tell from the measured or computed capacity of an air
condenser its actual capacity or vibration period, especially if the
condenser is of very small surface and is charged to a very high
potential. As many important results are dependent upon the
correctness of the estimation of the vibration period, this subject
demands the most careful scrutiny of other investigators. To reduce
the probable error as much as possible in experiments of the kind
alluded to, it is advisable to use spheres or plates of large surface,
so as to make the density exceedingly small. Otherwise, when it is
practicable, an oil condenser should be used in preference. In oil or
other liquid dielectrics there are seemingly no such losses as in
gaseous media. It being impossible to exclude entirely the gas in
condensers with solid dielectrics, such condensers should be immersed
in oil, for economical reasons if nothing else; they can then be
strained to the utmost and will remain cool. In Leyden jars the loss
due to air is comparatively small, as the tinfoil coatings are large,
close together, and the charged surfaces not directly exposed; but
when the potentials are very high, the loss may be more or less
considerable at, or near, the upper edge of the foil, where the air is
principally acted upon. If the jar be immersed in boiled-out oil, it
will be capable of performing four times the amount of work which it
can for any length of time when used in the ordinary way, and the loss
will be inappreciable.
It should not be thought that the loss in heat in an air condenser is
necessarily associated with the formation of _visible_ streams or
brushes. If a small electrode, inclosed in an unexhausted bulb, is
connected to one of the terminals of the coil, streams can be seen to
issue from the electrode and the air in the bulb is heated; if,
instead of a small electrode, a large sphere is inclosed in the bulb,
no streams are observed, still the air is heated.
Nor should it be thought that the temperature of an air condenser
would give even an approximate idea of the loss in heat incurred, as
in such case heat must be given off much more quickly, since there is,
in addition to the ordinary radiation, a very active carrying away of
heat by independent carriers going on, and since not only the
apparatus, but the air at some distance from it is heated in
consequence of the collisions which must occur.
Owing to this, in experiments with such a coil, a rise of temperature
can be distinctly observed only when the body connected to the coil is
very small. But with apparatus on a larger scale, even a body of
considerable bulk would be heated, as, for instance, the body of a
person; and I think that skilled physicians might make observations of
utility in such experiments, which, if the apparatus were judiciously
designed, would not present the slightest danger.
A question of some interest, principally to meteorologists, presents
itself here. How does the earth behave? The earth is an air condenser,
but is it a perfect or a very imperfect one--a mere sink of energy?
There can be little doubt that to such small disturbance as might be
caused in an experiment the earth behaves as an almost perfect
condenser. But it might be different when its charge is set in
vibration by some sudden disturbance occurring in the heavens. In such
case, as before stated, probably only little of the energy of the
vibrations set up would be lost into space in the form of long ether
radiations, but most of the energy, I think, would spend itself in
molecular impacts and collisions, and pass off into space in the form
of short heat, and possibly light, waves. As both the frequency of the
vibrations of the charge and the potential are in all probability
excessive, the energy converted into heat may be considerable. Since
the density must be unevenly distributed, either in consequence of the
irregularity of the earth's surface, or on account of the condition of
the atmosphere in various places, the effect produced would
accordingly vary from place to place. Considerable variations in the
temperature and pressure of the atmosphere may in this manner be
caused at any point of the surface of the earth. The variations may be
gradual or very sudden, according to the nature of the general
disturbance, and may produce rain and storms, or locally modify the
weather in any way.
From the remarks before made one may see what an important factor of
loss the air in the neighborhood of a charged surface becomes when the
electric density is great and the frequency of the impulses excessive.
But the action as explained implies that the air is insulating--that
is, that it is composed of independent carriers immersed in an
insulating medium. This is the case only when the air is at something
like ordinary or greater, or at extremely small, pressure. When the
air is slightly rarefied and conducting, then true conduction losses
occur also. In such case, of course, considerable energy may be
dissipated into space even with a steady potential, or with impulses
of low frequency, if the density is very great.
When the gas is at very low pressure, an electrode is heated more
because higher speeds can be reached. If the gas around the electrode
is strongly compressed, the displacements, and consequently the
speeds, are very small, and the heating is insignificant. But if in
such case the frequency could be sufficiently increased, the electrode
would be brought to a high temperature as well as if the gas were at
very low pressure; in fact, exhausting the bulb is only necessary
because we cannot produce (and possibly not convey) currents of the
required frequency.
Returning to the subject of electrode lamps, it is obviously of
advantage in such a lamp to confine as much as possible the heat to
the electrode by preventing the circulation of the gas in the bulb. If
a very small bulb be taken, it would confine the heat better than a
large one, but it might not be of sufficient capacity to be operated
from the coil, or, if so, the glass might get too hot. A simple way to
improve in this direction is to employ a globe of the required size,
but to place a small bulb, the diameter of which is properly
estimated, over the refractory button contained in the globe. This
arrangement is illustrated in Fig. 28.
[Illustration: FIG. 28.--LAMP WITH AUXILIARY BULB FOR CONFINING THE
ACTION TO THE CENTRE.]
The globe L has in this case a large neck n, allowing the small bulb b
to slip through. Otherwise the construction is the same as shown in
Fig. 18, for example. The small bulb is conveniently supported upon
the stem s, carrying the refractory button m. It is separated from the
aluminium tube a by several layers of mica M, in order to prevent the
cracking of the neck by the rapid heating of the aluminium tube upon a
sudden turning on of the current. The inside bulb should be as small
as possible when it is desired to obtain light only by incandescence
of the electrode. If it is desired to produce phosphorescence, the
bulb should be larger, else it would be apt to get too hot, and the
phosphorescence would cease. In this arrangement usually only the
small bulb shows phosphorescence, as there is practically no
bombardment against the outer globe. In some of these bulbs
constructed as illustrated in Fig. 28 the small tube was coated with
phosphorescent paint, and beautiful effects were obtained. Instead of
making the inside bulb large, in order to avoid undue heating, it
answers the purpose to make the electrode m larger. In this case the
bombardment is weakened by reason of the smaller electric density.
Many bulbs were constructed on the plan illustrated in Fig. 29. Here a
small bulb b, containing the refractory button m, upon being exhausted
to a very high degree was sealed in a large globe L, which was then
moderately exhausted and sealed off. The principal advantage of this
construction was that it allowed of reaching extremely high vacua,
and, at the same time use a large bulb. It was found, in the course of
experiences with bulbs such as illustrated in Fig. 29, that it was
well to make the stem s near the seal at e very thick, and the
leading-in wire w thin, as it occurred sometimes that the stem at e
was heated and the bulb was cracked. Often the outer globe L was
exhausted only just enough to allow the discharge to pass through, and
the space between the bulbs appeared crimson, producing a curious
effect. In some cases, when the exhaustion in globe L was very low,
and the air good conducting, it was found necessary, in order to bring
the button m to high incandescence, to place, preferably on the upper
part of the neck of the globe, a tinfoil coating which was connected
to an insulated body, to the ground, or to the other terminal of the
coil, as the highly conducting air weakened the effect somewhat,
probably by being acted upon inductively from the wire w, where it
entered the bulb at e. Another difficulty--which, however, is always
present when the refractory button is mounted in a very small
bulb--existed in the construction illustrated in Fig. 29, namely, the
vacuum in the bulb b would be impaired in a comparatively short time.
[Illustration: FIG. 29.--LAMP WITH INDEPENDENT AUXILIARY BULB.]
The chief idea in the two last described constructions was to confine
the heat to the central portion of the globe by preventing the
exchange of air. An advantage is secured, but owing to the heating of
the inside bulb and slow evaporation of the glass the vacuum is hard
to maintain, even if the construction illustrated in Fig. 28 be
chosen, in which both bulbs communicate.
But by far the better way--the ideal way--would be to reach
sufficiently high frequencies. The higher the frequency the slower
would be the exchange of the air, and I think that a frequency may be
reached at which there would be no exchange whatever of the air
molecules around the terminal. We would then produce a flame in which
there would be no carrying away of material, and a queer flame it
would be, for it would be rigid! With such high frequencies the
inertia of the particles would come into play. As the brush, or flame,
would gain rigidity in virtue of the inertia of the particles, the
exchange of the latter would be prevented. This would necessarily
occur, for, the number of the impulses being augmented, the potential
energy of each would diminish, so that finally only atomic vibrations
could be set up, and the motion of translation through measurable
space would cease. Thus an ordinary gas burner connected to a source
of rapidly alternating potential might have its efficiency augmented
to a certain limit, and this for two reasons--because of the
additional vibration imparted, and because of a slowing down of the
process of carrying off. But the renewal being rendered difficult, and
renewal being necessary to maintain the _burner_, a continued increase
of the frequency of the impulses, assuming they could be transmitted
to and impressed upon the flame, would result in the "extinction" of
the latter, meaning by this term only the cessation of the chemical
process.
I think, however, that in the case of an electrode immersed in a fluid
insulating medium, and surrounded by independent carriers of electric
charges, which can be acted upon inductively, a sufficiently high
frequency of the impulses would probably result in a gravitation of
the gas all around toward the electrode. For this it would be only
necessary to assume that the independent bodies are irregularly
shaped; they would then turn toward the electrode their side of the
greatest electric density, and this would be a position in which the
fluid resistance to approach would be smaller than that offered to the
receding.
The general opinion, I do not doubt, is that it is out of the question
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 knocks 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 1 inch in
diameter and 1 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 different
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 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
insulated 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 a 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 moving matter. In a gas the speed may be 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 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 presently 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 discarded 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. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF
EXHAUSTION.]
The apparatus is illustrated in a drawing shown in Fig. 30. 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 connection 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 mercury in 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 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.