bulbs with or without internal electrode, in which the radiant matter
was projected against, or focused upon, the body to be rendered
incandescent. Fig. 24 illustrates one of the bulbs used. It consists
of a spherical globe L, provided with a long neck n, on the top, for
increasing the action in some cases by the application of an external
conducting coating. The globe L is blown out on the bottom into a very
small bulb b, which serves to hold it firmly in a socket S of
insulating material into which it is cemented. A fine lamp filament f,
supported on a wire w, passes through the centre of the globe L. The
filament is rendered incandescent in the middle portion, where the
bombardment proceeding from the lower inside surface of the globe is
most intense. The lower portion of the globe, as far as the socket S
reaches, is rendered conducting, either by a tinfoil coating or
otherwise, and the external electrode is connected to a terminal of
the coil.
The arrangement diagrammatically indicated in Fig. 24 was found to be
an inferior one when it was desired to render incandescent a filament
or button supported in the centre of the globe, but it was convenient
when the object was to excite phosphorescence.
In many experiments in which bodies of a different kind were mounted
in the bulb as, for instance, indicated in Fig. 23, some observations
of interest were made.
It was found, among other things, that in such cases, no matter where
the bombardment began, just as soon as a high temperature was reached
there was generally one of the bodies which seemed to take most of the
bombardment upon itself, the other, or others, being thereby relieved.
This quality appeared to depend principally on the point of fusion,
and on the facility with which the body was "evaporated," or,
generally speaking, disintegrated--meaning by the latter term not only
the throwing off of atoms, but likewise of larger lumps. The
observation made was in accordance with generally accepted notions. In
a highly exhausted bulb electricity is carried off from the electrode
by independent carriers, which are partly the atoms, or molecules, of
the residual atmosphere, and partly the atoms, molecules, or lumps
thrown off from the electrode. If the electrode is composed of bodies
of different character, and if one of these is more easily
disintegrated than the others, most of the electricity supplied is
carried off from that body, which is then brought to a higher
temperature than the others, and this the more, as upon an increase of
the temperature the body is still more easily disintegrated.
It seems to me quite probable that a similar process takes place in
the bulb even with a homogeneous electrode, and I think it to be the
principal cause of the disintegration. There is bound to be some
irregularity, even if the surface is highly polished, which, of
course, is impossible with most of the refractory bodies employed as
electrodes. Assume that a point of the electrode gets hotter,
instantly most of the discharge passes through that point, and a
minute patch is probably fused and evaporated. It is now possible that
in consequence of the violent disintegration the spot attacked sinks
in temperature, or that a counter force is created, as in an arc; at
any rate, the local tearing off meets with the limitations incident to
the experiment, whereupon the same process occurs on another place. To
the eye the electrode appears uniformly brilliant, but there are upon
it points constantly shifting and wandering around, of a temperature
far above the mean, and this materially hastens the process of
deterioration. That some such thing occurs, at least when the
electrode is at a lower temperature, sufficient experimental evidence
can be obtained in the following manner: Exhaust a bulb to a very high
degree, so that with a fairly high potential the discharge cannot
pass--that is, not a _luminous_ one, for a weak invisible discharge
occurs always, in all probability. Now raise slowly and carefully the
potential, leaving the primary current on no more than for an instant.
At a certain point, two, three, or half a dozen phosphorescent spots
will appear on the globe. These places of the glass are evidently more
violently bombarded than others, this being due to the unevenly
distributed electric density, necessitated, of course, by sharp
projections, or, generally speaking, irregularities of the electrode.
But the luminous patches are constantly changing in position, which is
especially well observable if one manages to produce very few, and
this indicates that the configuration of the electrode is rapidly
changing.
From experiences of this kind I am led to infer that, in order to be
most durable, the refractory button in the bulb should be in the form
of a sphere with a highly polished surface. Such a small sphere could
be manufactured from a diamond or some other crystal, but a better way
would be to fuse, by the employment of extreme degrees of temperature,
some oxide--as, for instance, zirconia--into a small drop, and then
keep it in the bulb at a temperature somewhat below its point of
fusion.
Interesting and useful results can no doubt be reached in the
direction of extreme degrees of heat. How can such high temperatures
be arrived at? How are the highest degrees of heat reached in nature?
By the impact of stars, by high speeds and collisions. In a collision
any rate of heat generation may be attained. In a chemical process we
are limited. When oxygen and hydrogen combine, they fall,
metaphorically speaking, from a definite height. We cannot go very far
with a blast, nor by confining heat in a furnace, but in an exhausted
bulb we can concentrate any amount of energy upon a minute button.
Leaving practicability out of consideration, this, then, would be the
means which, in my opinion, would enable us to reach the highest
temperature. But a great difficulty when proceeding in this way is
encountered, namely, in most cases the body is carried off before it
can fuse and form a drop. This difficulty exists principally with an
oxide such as zirconia, because it cannot be compressed in so hard a
cake that it would not be carried off quickly. I endeavored repeatedly
to fuse zirconia, placing it in a cup or arc light carbon as indicated
in Fig. 23. It glowed with a most intense light, and the stream of the
particles projected out of the carbon cup was of a vivid white: but
whether it was compressed in a cake or made into a paste with carbon,
it was carried off before it could be fused. The carbon cup containing
the zirconia had to be mounted very low in the neck of a large bulb,
as the heating of the glass by the projected particles of the oxide
was so rapid that in the first trial the bulb was cracked almost in an
instant when the current was turned on. The heating of the glass by
the projected particles was found to be always greater when the carbon
cup contained a body which was rapidly carried off--I presume because
in such cases, with the same potential, higher speeds were reached,
and also because, per unit of time, more matter was projected--that
is, more particles would strike the glass.
The before mentioned difficulty did not exist, however, when the body
mounted in the carbon cup offered great resistance to deterioration.
For instance, when an oxide was first fused in an oxygen blast and
then mounted in the bulb, it melted very readily into a drop.
Generally during the process of fusion magnificent light effects were
noted, of which it would be difficult to give an adequate idea. Fig.
23 is intended to illustrate the effect observed with a ruby drop. At
first one may see a narrow funnel of white light projected against the
top of the globe, where it produces an irregularly outlined
phosphorescent patch. When the point of the ruby fuses the
phosphorescence becomes very powerful; but as the atoms are projected
with much greater speed from the surface of the drop, soon the glass
gets hot and "tired," and now only the outer edge of the patch glows.
In this manner an intensely phosphorescent, sharply defined line, _l_,
corresponding to the outline of the drop, is produced, which spreads
slowly over the globe as the drop gets larger. When the mass begins to
boil, small bubbles and cavities are formed, which cause dark colored
spots to sweep across the globe. The bulb may be turned downward
without fear of the drop falling off, as the mass possesses
considerable viscosity.
I may mention here another feature of some interest, which I believe
to have noted in the course of these experiments, though the
observations do not amount to a certitude. It _appeared_ that under
the molecular impact caused by the rapidly alternating potential the
body was fused and maintained in that state at a lower temperature in
a highly exhausted bulb than was the case at normal pressure and
application of heat in the ordinary way--that is, at least, judging
from the quantity of the light emitted. One of the experiments
performed may be mentioned here by way of illustration. A small piece
of pumice stone was stuck on a platinum wire, and first melted to it
in a gas burner. The wire was next placed between two pieces of
charcoal and a burner applied so as to produce an intense heat,
sufficient to melt down the pumice stone into a small glass-like
button. The platinum wire had to be taken of sufficient thickness to
prevent its melting in the fire. While in the charcoal fire, or when
held in a burner to get a better idea of the degree of heat, the
button glowed with great brilliancy. The wire with the button was then
mounted in a bulb, and upon exhausting the same to a high degree, the
current was turned on slowly so as to prevent the cracking of the
button. The button was heated to the point of fusion, and when it
melted it did not, apparently, glow with the same brilliancy as
before, and this would indicate a lower temperature. Leaving out of
consideration the observer's possible, and even probable, error, the
question is, can a body under these conditions be brought from a solid
to a liquid state with evolution of _less_ light?
When the potential of a body is rapidly alternated it is certain that
the structure is jarred. When the potential is very high, although the
vibrations may be few--say 20,000 per second--the effect upon the
structure may be considerable. Suppose, for example, that a ruby is
melted into a drop by a steady application of energy. When it forms a
drop it will emit visible and invisible waves, which will be in a
definite ratio, and to the eye the drop will appear to be of a certain
brilliancy. Next, suppose we diminish to any degree we choose the
energy steadily supplied, and, instead, supply energy which rises and
falls according to a certain law. Now, when the drop is formed, there
will be emitted from it three different kinds of vibrations--the
ordinary visible, and two kinds of invisible waves: that is, the
ordinary dark waves of all lengths, and, in addition, waves of a well
defined character. The latter would not exist by a steady supply of
the energy; still they help to jar and loosen the structure. If this
really be the case, then the ruby drop will emit relatively less
visible and more invisible waves than before. Thus it would seem that
when a platinum wire, for instance, is fused by currents alternating
with extreme rapidity, it emits at the point of fusion less light and
more invisible radiation than it does when melted by a steady current,
though the total energy used up in the process of fusion is the same
in both cases. Or, to cite another example, a lamp filament is not
capable of withstanding as long with currents of extreme frequency as
it does with steady currents, assuming that it be worked at the same
luminous intensity. This means that for rapidly alternating currents
the filament should be shorter and thicker. The higher the
frequency--that is, the greater the departure from the steady
flow--the worse it would be for the filament. But if the truth of this
remark were demonstrated, it would be erroneous to conclude that such
a refractory button as used in these bulbs would be deteriorated
quicker by currents of extremely high frequency than by steady or low
frequency currents. From experience I may say that just the opposite
holds good: the button withstands the bombardment better with currents
of very high frequency. But this is due to the fact that a high
frequency discharge passes through a rarefied gas with much greater
freedom than a steady or low frequency discharge, and this will say
that with the former we can work with a lower potential or with a less
violent impact. As long, then, as the gas is of no consequence, a
steady or low frequency current is better; but as soon as the action
of the gas is desired and important, high frequencies are preferable.
In the course of these experiments a great many trials were made with
all kinds of carbon buttons. Electrodes made of ordinary carbon
buttons were decidedly more durable when the buttons were obtained by
the application of enormous pressure. Electrodes prepared by
depositing carbon in well known ways did not show up well; they
blackened the globe very quickly. From many experiences I conclude
that lamp filaments obtained in this manner can be advantageously used
only with low potentials and low frequency currents. Some kinds of
carbon withstand so well that, in order to bring them to the point of
fusion, it is necessary to employ very small buttons. In this case the
observation is rendered very difficult on account of the intense heat
produced. Nevertheless there can be no doubt that all kinds of carbon
are fused under the molecular bombardment, but the liquid state must
be one of great instability. Of all the bodies tried there were two
which withstood best--diamond and carborundum. These two showed up
about equally, but the latter was preferable, for many reasons. As it
is more than likely that this body is not yet generally known, I will
venture to call your attention to it.
It has been recently produced by Mr. E.G. Acheson, of Monongahela
City, Pa., U.S.A. It is intended to replace ordinary diamond powder
for polishing precious stones, etc., and I have been informed that it
accomplishes this object quite successfully. I do not know why the
name "carborundum" has been given to it, unless there is something in
the process of its manufacture which justifies this selection. Through
the kindness of the inventor, I obtained a short while ago some
samples which I desired to test in regard to their qualities of
phosphorescence and capability of withstanding high degrees of heat.
Carborundum can be obtained in two forms--in the form of "crystals"
and of powder. The former appear to the naked eye dark colored, but
are very brilliant; the latter is of nearly the same color as ordinary
diamond powder, but very much finer. When viewed under a microscope
the samples of crystals given to me did not appear to have any
definite form, but rather resembled pieces of broken up egg coal of
fine quality. The majority were opaque, but there were some which were
transparent and colored. The crystals are a kind of carbon containing
some impurities; they are extremely hard, and withstand for a long
time even an oxygen blast. When the blast is directed against them
they at first form a cake of some compactness, probably in consequence
of the fusion of impurities they contain. The mass withstands for a
very long time the blast without further fusion; but a slow carrying
off, or burning, occurs, and, finally, a small quantity of a
glass-like residue is left, which, I suppose, is melted alumina. When
compressed strongly they conduct very well, but not as well as
ordinary carbon. The powder, which is obtained from the crystals in
some way, is practically non-conducting. It affords a magnificent
polishing material for stones.
The time has been too short to make a satisfactory study of the
properties of this product, but enough experience has been gained in a
few weeks I have experimented upon it to say that it does possess some
remarkable properties in many respects. It withstands excessively high
degrees of heat, it is little deteriorated by molecular bombardment,
and it does not blacken the globe as ordinary carbon does. The only
difficulty which I have found in its use in connection with these
experiments was to find some binding material which would resist the
heat and the effect of the bombardment as successfully as carborundum
itself does.
I have here a number of bulbs which I have provided with buttons of
carborundum. To make such a button of carborundum crystals I proceed
in the following manner: I take an ordinary lamp filament and dip its
point in tar, or some other thick substance or paint which may be
readily carbonized. I next pass the point of the filament through the
crystals, and then hold it vertically over a hot plate. The tar
softens and forms a drop on the point of the filament, the crystals
adhering to the surface of the drop. By regulating the distance from
the plate the tar is slowly dried out and the button becomes solid. I
then once more dip the button in tar and hold it again over a plate
until the tar is evaporated, leaving only a hard mass which firmly
binds the crystals. When a larger button is required I repeat the
process several times, and I generally also cover the filament a
certain distance below the button with crystals. The button being
mounted in a bulb, when a good vacuum has been reached, first a weak
and then a strong discharge is passed through the bulb to carbonize
the tar and expel all gases, and later it is brought to a very intense
incandescence.
When the powder is used I have found it best to proceed as follows: I
make a thick paint of carborundum and tar, and pass a lamp filament
through the paint. Taking then most of the paint off by rubbing the
filament against a piece of chamois leather, I hold it over a hot
plate until the tar evaporates and the coating becomes firm. I repeat
this process as many times as it is necessary to obtain a certain
thickness of coating. On the point of the coated filament I form a
button in the same manner.
There is no doubt that such a button--properly prepared under great
pressure--of carborundum, especially of powder of the best quality,
will withstand the effect of the bombardment fully as well as anything
we know. The difficulty is that the binding material gives way, and
the carborundum is slowly thrown off after some time. As it does not
seem to blacken the globe in the least, it might be found useful for
coating the filaments of ordinary incandescent lamps, and I think that
it is even possible to produce thin threads or sticks of carborundum
which will replace the ordinary filaments in an incandescent lamp. A
carborundum coating seems to be more durable than other coatings, not
only because the carborundum can withstand high degrees of heat, but
also because it seems to unite with the carbon better than any other
material I have tried. A coating of zirconia or any other oxide, for
instance, is far more quickly destroyed. I prepared buttons of diamond
dust in the same manner as of carborundum, and these came in
durability nearest to those prepared of carborundum, but the binding
paste gave way much more quickly in the diamond buttons: this,
however, I attributed to the size and irregularity of the grains of
the diamond.
It was of interest to find whether carborundum possesses the quality
of phosphorescence. One is, of course, prepared to encounter two
difficulties: first, as regards the rough product, the "crystals,"
they are good conducting, and it is a fact that conductors do not
phosphoresce; second, the powder, being exceedingly fine, would not be
apt to exhibit very prominently this quality, since we know that when
crystals, even such as diamond or ruby, are finely powdered, they lose
the property of phosphorescence to a considerable degree.
The question presents itself here, can a conductor phosphoresce? What
is there in such a body as a metal, for instance, that would deprive
it of the quality of phosphorescence, unless it is that property which
characterizes it as a conductor? for it is a fact that most of the
phosphorescent bodies lose that quality when they are sufficiently
heated to become more or less conducting. Then, if a metal be in a
large measure, or perhaps entirely, deprived of that property, it
should be capable of phosphorescence. Therefore it is quite possible
that at some extremely high frequency, when behaving practically as a
non-conductor, a metal or any other conductor might exhibit the
quality of phosphorescence, even though it be entirely incapable of
phosphorescing under the impact of a low-frequency discharge. There
is, however, another possible way how a conductor might at least
_appear_ to phosphoresce.
Considerable doubt still exists as to what really is phosphorescence,
and as to whether the various phenomena comprised under this head are
due to the same causes. Suppose that in an exhausted bulb, under the
molecular impact, the surface of a piece of metal or other conductor
is rendered strongly luminous, but at the same time it is found that
it remains comparatively cool, would not this luminosity be called
phosphorescence? Now such a result, theoretically at least, is
possible, for it is a mere question of potential or speed. Assume the
potential of the electrode, and consequently the speed of the
projected atoms, to be sufficiently high, the surface of the metal
piece against which the atoms are projected would be rendered highly
incandescent, since the process of heat generation would be
incomparably faster than that of radiating or conducting away from the
surface of the collision. In the eye of the observer a single impact
of the atoms would cause an instantaneous flash, but if the impacts
were repeated with sufficient rapidity they would produce a
continuous impression upon his retina. To him then the surface of the
metal would appear continuously incandescent and of constant luminous
intensity, while in reality the light would be either intermittent or
at least changing periodically in intensity. The metal piece would
rise in temperature until equilibrium was attained--that is until the
energy continuously radiated would equal that intermittently supplied.
But the supplied energy might under such conditions not be sufficient
to bring the body to any more than a very moderate mean temperature,
especially if the frequency of the atomic impacts be very low--just
enough that the fluctuation of the intensity of the light emitted
could not be detected by the eye. The body would now, owing to the
manner in which the energy is supplied, emit a strong light, and yet
be at a comparatively very low mean temperature. How could the
observer call the luminosity thus produced? Even if the analysis of
the light would teach him something definite, still he would probably
rank it under the phenomena of phosphorescence. It is conceivable that
in such a way both conducting and non-conducting bodies may be
maintained at a certain luminous intensity, but the energy required
would very greatly vary with the nature and properties of the bodies.
These and some foregoing remarks of a speculative nature were made
merely to bring out curious features of alternate currents or electric
impulses. By their help we may cause a body to emit _more_ light,
while at a certain mean temperature, than it would emit if brought to
that temperature by a steady supply; and, again, we may bring a body
to the point of fusion, and cause it to emit _less_ light than when
fused by the application of energy in ordinary ways. It all depends on
how we supply the energy, and what kind of vibrations we set up: in
one case the vibrations are more, in the other less, adapted to affect
our sense of vision.