carborundum in the first trials, I attributed to phosphorescence, but
in subsequent experiments it appeared that it was devoid of that
quality. The crystals possess a noteworthy feature. In a bulb provided
with a single electrode in the shape of a small circular metal disc,
for instance, at a certain degree of exhaustion the electrode is
covered with a milky film, which is separated by a dark space from the
glow filling the bulb. When the metal disc is covered with carborundum
crystals, the film is far more intense, and snow-white. This I found
later to be merely an effect of the bright surface of the crystals,
for when an aluminium electrode was highly polished it exhibited more
or less the same phenomenon. I made a number of experiments with the
samples of crystals obtained, principally because it would have been
of special interest to find that they are capable of phosphorescence,
on account of their being conducting. I could not produce
phosphorescence distinctly, but I must remark that a decisive opinion
cannot be formed until other experimenters have gone over the same
ground.
The powder behaved in some experiments as though it contained alumina,
but it did not exhibit with sufficient distinctness the red of the
latter. Its dead color brightens considerably under the molecular
impact, but I am now convinced it does not phosphoresce. Still, the
tests with the powder are not conclusive, because powdered carborundum
probably does not behave like a phosphorescent sulphide, for example,
which could be finely powdered without impairing the phosphorescence,
but rather like powdered ruby or diamond, and therefore it would be
necessary, in order to make a decisive test, to obtain it in a large
lump and polish up the surface.
If the carborundum proves useful in connection with these and similar
experiments, its chief value will be found in the production of
coatings, thin conductors, buttons, or other electrodes capable of
withstanding extremely high degrees of heat.
The production of a small electrode capable of withstanding enormous
temperatures I regard as of the greatest importance in the manufacture
of light. It would enable us to obtain, by means of currents of very
high frequencies, certainly 20 times, if not more, the quantity of
light which is obtained in the present incandescent lamp by the same
expenditure of energy. This estimate may appear to many exaggerated,
but in reality I think it is far from being so. As this statement
might be misunderstood I think it necessary to expose clearly the
problem with which in this line of work we are confronted, and the
manner in which, in my opinion, a solution will be arrived at.
Any one who begins a study of the problem will be apt to think that
what is wanted in a lamp with an electrode is a very high degree of
incandescence of the electrode. There he will be mistaken. The high
incandescence of the button is a necessary evil, but what is really
wanted is the high incandescence of the gas surrounding the button. In
other words, the problem in such a lamp is to bring a mass of gas to
the highest possible incandescence. The higher the incandescence, the
quicker the mean vibration, the greater is the economy of the light
production. But to maintain a mass of gas at a high degree of
incandescence in a glass vessel, it will always be necessary to keep
the incandescent mass away from the glass; that is, to confine it as
much as possible to the central portion of the globe.
In one of the experiments this evening a brush was produced at the end
of a wire. This brush was a flame, a source of heat and light. It did
not emit much perceptible heat, nor did it glow with an intense light;
but is it the less a flame because it does not scorch my hand? Is it
the less a flame because it does not hurt my eye by its brilliancy?
The problem is precisely to produce in the bulb such a flame, much
smaller in size, but incomparably more powerful. Were there means at
hand for producing electric impulses of a sufficiently high frequency,
and for transmitting them, the bulb could be done away with, unless it
were used to protect the electrode, or to economize the energy by
confining the heat. But as such means are not at disposal, it becomes
necessary to place the terminal in a bulb and rarefy the air in the
same. This is done merely to enable the apparatus to perform the work
which it is not capable of performing at ordinary air pressure. In the
bulb we are able to intensify the action to any degree--so far that
the brush emits a powerful light.
The intensity of the light emitted depends principally on the
frequency and potential of the impulses, and on the electric density
of the surface of the electrode. It is of the greatest importance to
employ the smallest possible button, in order to push the density very
far. Under the violent impact of the molecules of the gas surrounding
it, the small electrode is of course brought to an extremely high
temperature, but around it is a mass of highly incandescent gas, a
flame photosphere, many hundred times the volume of the electrode.
With a diamond, carborundum or zirconia button the photosphere can be
as much as one thousand times the volume of the button. Without much
reflecting one would think that in pushing so far the incandescence of
the electrode it would be instantly volatilized. But after a careful
consideration he would find that, theoretically, it should not occur,
and in this fact--which, however, is experimentally demonstrated--lies
principally the future value of such a lamp.
At first, when the bombardment begins, most of the work is performed
on the surface of the button, but when a highly conducting photosphere
is formed the button is comparatively relieved. The higher the
incandescence of the photosphere the more it approaches in
conductivity to that of the electrode, and the more, therefore, the
solid and the gas form one conducting body. The consequence is that
the further is forced the incandescence the more work, comparatively,
is performed on the gas, and the less on the electrode. The formation
of a powerful photosphere is consequently the very means for
protecting the electrode. This protection, of course, is a relative
one, and it should not be thought that by pushing the incandescence
higher the electrode is actually less deteriorated. Still,
theoretically, with extreme frequencies, this result must be reached,
but probably at a temperature too high for most of the refractory
bodies known. Given, then, an electrode which can withstand to a very
high limit the effect of the bombardment and outward strain, it would
be safe no matter how much it is forced beyond that limit. In an
incandescent lamp quite different considerations apply. There the gas
is not at all concerned: the whole of the work is performed on the
filament; and the life of the lamp diminishes so rapidly with the
increase of the degree of incandescence that economical reasons compel
us to work it at a low incandescence. But if an incandescent lamp is
operated with currents of very high frequency, the action of the gas
cannot be neglected, and the rules for the most economical working
must be considerably modified.
In order to bring such a lamp with one or two electrodes to a great
perfection, it is necessary to employ impulses of very high frequency.
The high frequency secures, among others, two chief advantages, which
have a most important bearing upon the economy of the light
production. First, the deterioration of the electrode is reduced by
reason of the fact that we employ a great many small impacts, instead
of a few violent ones, which shatter quickly the structure; secondly,
the formation of a large photosphere is facilitated.
In order to reduce the deterioration of the electrode to the minimum,
it is desirable that the vibration be harmonic, for any suddenness
hastens the process of destruction. An electrode lasts much longer
when kept at incandescence by currents, or impulses, obtained from a
high-frequency alternator, which rise and fall more or less
harmonically, than by impulses obtained from a disruptive discharge
coil. In the latter case there is no doubt that most of the damage is
done by the fundamental sudden discharges.
One of the elements of loss in such a lamp is the bombardment of the
globe. As the potential is very high, the molecules are projected with
great speed; they strike the glass, and usually excite a strong
phosphorescence. The effect produced is very pretty, but for
economical reasons it would be perhaps preferable to prevent, or at
least reduce to the minimum, the bombardment against the globe, as in
such case it is, as a rule, not the object to excite phosphorescence,
and as some loss of energy results from the bombardment. This loss in
the bulb is principally dependent on the potential of the impulses and
on the electric density on the surface of the electrode. In employing
very high frequencies the loss of energy by the bombardment is greatly
reduced, for, first, the potential needed to perform a given amount of
work is much smaller; and, secondly, by producing a highly conducting
photosphere around the electrode, the same result is obtained as
though the electrode were much larger, which is equivalent to a
smaller electric density. But be it by the diminution of the maximum
potential or of the density, the gain is effected in the same manner,
namely, by avoiding violent shocks, which strain the glass much beyond
its limit of elasticity. If the frequency could be brought high
enough, the loss due to the imperfect elasticity of the glass would be
entirely negligible. The loss due to bombardment of the globe may,
however, be reduced by using two electrodes instead of one. In such
case each of the electrodes may be connected to one of the terminals;
or else, if it is preferable to use only one wire, one electrode may
be connected to one terminal and the other to the ground or to an
insulated body of some surface, as, for instance, a shade on the lamp.
In the latter case, unless some judgment is used, one of the
electrodes might glow more intensely than the other.
But on the whole I find it preferable when using such high frequencies
to employ only one electrode and one connecting wire. I am convinced
that the illuminating device of the near future will not require for
its operation more than one lead, and, at any rate, it will have no
leading-in wire, since the energy required can be as well transmitted
through the glass. In experimental bulbs the leading-in wire is most
generally used on account of convenience, as in employing condenser
coatings in the manner indicated in Fig. 22, for example, there is
some difficulty in fitting the parts, but these difficulties would not
exist if a great many bulbs were manufactured; otherwise the energy
can be conveyed through the glass as well as through a wire, and with
these high frequencies the losses are very small. Such illuminating
devices will necessarily involve the use of very high potentials, and
this, in the eyes of practical men, might be an objectionable feature.
Yet, in reality, high potentials are not objectionable--certainly not
in the least as far as the safety of the devices is concerned.
There are two ways of rendering an electric appliance safe. One is to
use low potentials, the other is to determine the dimensions of the
apparatus so that it is safe no matter how high a potential is used.
Of the two the latter seems to me the better way, for then the safety
is absolute, unaffected by any possible combination of circumstances
which might render even a low-potential appliance dangerous to life
and property. But the practical conditions require not only the
judicious determination of the dimensions of the apparatus; they
likewise necessitate the employment of energy of the proper kind. It
is easy, for instance, to construct a transformer capable of giving,
when operated from an ordinary alternate current machine of low
tension, say 50,000 volts, which might be required to light a highly
exhausted phosphorescent tube, so that, in spite of the high
potential, it is perfectly safe, the shock from it producing no
inconvenience. Still, such a transformer would be expensive, and in
itself inefficient; and, besides, what energy was obtained from it
would not be economically used for the production of light. The
economy demands the employment of energy in the form of extremely
rapid vibrations. The problem of producing light has been likened to
that of maintaining a certain high-pitch note by means of a bell. It
should be said a _barely audible_ note; and even these words would not
express it, so wonderful is the sensitiveness of the eye. We may
deliver powerful blows at long intervals, waste a good deal of energy,
and still not get what we want; or we may keep up the note by
delivering frequent gentle taps, and get nearer to the object sought
by the expenditure of much less energy. In the production of light, as
far as the illuminating device is concerned, there can be only one
rule--that is, to use as high frequencies as can be obtained; but the
means for the production and conveyance of impulses of such character
impose, at present at least, great limitations. Once it is decided to
use very high frequencies, the return wire becomes unnecessary, and
all the appliances are simplified. By the use of obvious means the
same result is obtained as though the return wire were used. It is
sufficient for this purpose to bring in contact with the bulb, or
merely in the vicinity of the same, an insulated body of some surface.
The surface need, of course, be the smaller, the higher the frequency
and potential used, and necessarily, also, the higher the economy of
the lamp or other device.
This plan of working has been resorted to on several occasions this
evening. So, for instance, when the incandescence of a button was
produced by grasping the bulb with the hand, the body of the
experimenter merely served to intensify the action. The bulb used was
similar to that illustrated in Fig. 19, and the coil was excited to a
small potential, not sufficient to bring the button to incandescence
when the bulb was hanging from the wire; and incidentally, in order to
perform the experiment in a more suitable manner, the button was taken
so large that a perceptible time had to elapse before, upon grasping
the bulb, it could be rendered incandescent. The contact with the bulb
was, of course, quite unnecessary. It is easy, by using a rather large
bulb with an exceedingly small electrode, to adjust the conditions so
that the latter is brought to bright incandescence by the mere
approach of the experimenter within a few feet of the bulb, and that
the incandescence subsides upon his receding.
[Illustration: FIG. 24.--BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT
OF PROJECTED MATTER.]
In another experiment, when phosphorescence was excited, a similar
bulb was used. Here again, originally, the potential was not
sufficient to excite phosphorescence until the action was
intensified--in this case, however, to present a different feature, by
touching the socket with a metallic object held in the hand. The
electrode in the bulb was a carbon button so large that it could not
be brought to incandescence, and thereby spoil the effect produced by
phosphorescence.
[Illustration: FIG. 25.--IMPROVED EXPERIMENTAL BULB.]
Again, in another of the early experiments, a bulb was used as
illustrated in Fig. 12. In this instance, by touching the bulb with
one or two fingers, one or two shadows of the stem inside were
projected against the glass, the touch of the finger producing the
same result as the application of an external negative electrode under
ordinary circumstances.
In all these experiments the action was intensified by augmenting the
capacity at the end of the lead connected to the terminal. As a rule,
it is not necessary to resort to such means, and would be quite
unnecessary with still higher frequencies; but when it _is_ desired,
the bulb, or tube, can be easily adapted to the purpose.
[Illustration: FIG. 26.--IMPROVED BULB WITH INTENSIFYING REFLECTOR.]
In Fig. 24, for example, an experimental bulb L is shown, which is
provided with a neck n on the top for the application of an external
tinfoil coating, which may be connected to a body of larger surface.
Such a lamp as illustrated in Fig. 25 may also be lighted by
connecting the tinfoil coating on the neck n to the terminal, and the
leading-in wire w to an insulated plate. If the bulb stands in a
socket upright, as shown in the cut, a shade of conducting material
may be slipped in the neck n, and the action thus magnified.
A more perfected arrangement used in some of these bulbs is
illustrated in Fig. 26. In this case the construction of the bulb is
as shown and described before, when reference was made to Fig. 19. A
zinc sheet Z, with a tubular extension T, is slipped over the metallic
socket S. The bulb hangs downward from the terminal t, the zinc sheet
Z, performing the double office of intensifier and reflector. The
reflector is separated from the terminal t by an extension of the
insulating plug P.
[Illustration: FIG. 27.--PHOSPHORESCENT TUBE WITH INTENSIFYING
REFLECTOR.]
A similar disposition with a phosphorescent tube is illustrated in
Fig. 27. The tube T is prepared from two short tubes of a different
diameter, which are sealed on the ends. On the lower end is placed an
outside conducting coating C, which connects to the wire w. The wire
has a hook on the upper end for suspension, and passes through the
centre of the inside tube, which is filled with some good and tightly
packed insulator. On the outside of the upper end of the tube T is
another conducting coating C_1 upon which is slipped a metallic
reflector Z, which should be separated by a thick insulation from the
end of wire w.
The economical use of such a reflector or intensifier would require
that all energy supplied to an air condenser should be recoverable,
or, in other words, that there should not be any losses, neither in
the gaseous medium nor through its action elsewhere. This is far from
being so, but, fortunately, the losses may be reduced to anything
desired. A few remarks are necessary on this subject, in order to make
the experiences gathered in the course of these investigations
perfectly clear.
Suppose a small helix with many well insulated turns, as in experiment
Fig. 17, has one of its ends connected to one of the terminals of the
induction coil, and the other to a metal plate, or, for the sake of
simplicity, a sphere, insulated in space. When the coil is set to
work, the potential of the sphere is alternated, and the small helix
now behaves as though its free end were connected to the other
terminal of the induction coil. If an iron rod be held within the
small helix it is quickly brought to a high temperature, indicating
the passage of a strong current through the helix. How does the
insulated sphere act in this case? It can be a condenser, storing and
returning the energy supplied to it, or it can be a mere sink of
energy, and the conditions of the experiment determine whether it is
more one or the other. The sphere being charged to a high potential,
it acts inductively upon the surrounding air, or whatever gaseous
medium there might be. The molecules, or atoms, which are near the
sphere are of course more attracted, and move through a greater
distance than the farther ones. When the nearest molecules strike the
sphere they are repelled, and collisions occur at all distances within
the inductive action of the sphere. It is now clear that, if the
potential be steady, but little loss of energy can be caused in this
way, for the molecules which are nearest to the sphere, having had an
additional charge imparted to them by contact, are not attracted until
they have parted, if not with all, at least with most of the
additional charge, which can be accomplished only after a great many
collisions. From the fact that with a steady potential there is but
little loss in dry air, one must come to such a conclusion. When the
potential of the sphere, instead of being steady, is alternating, the
conditions are entirely different. In this case a rhythmical
bombardment occurs, no matter whether the molecules after coming in
contact with the sphere lose the imparted charge or not; what is more,
if the charge is not lost, the impacts are only the more violent.
Still if the frequency of the impulses be very small, the loss caused
by the impacts and collisions would not be serious unless the
potential were excessive. But when extremely high frequencies and more
or less high potentials are used, the loss may be very great. The
total energy lost per unit of time is proportionate to the product of
the number of impacts per second, or the frequency and the energy lost
in each impact. But the energy of an impact must be proportionate to
the square of the electric density of the sphere, since the charge
imparted to the molecule is proportionate to that density. I conclude
from this that the total energy lost must be proportionate to the
product of the frequency and the square of the electric density; but
this law needs experimental confirmation. Assuming the preceding
considerations to be true, then, by rapidly alternating the potential
of a body immersed in an insulating gaseous medium, any amount of
energy may be dissipated into space. Most of that energy then, I
believe, is not dissipated in the form of long ether waves, propagated
to considerable distance, as is thought most generally, but is
consumed--in the case of an insulated sphere, for example--in impact
and collisional losses--that is, heat vibrations--on the surface and
in the vicinity of the sphere. To reduce the dissipation it is
necessary to work with a small electric density--the smaller the
higher the frequency.
But since, on the assumption before made, the loss is diminished with
the square of the density, and since currents of very high frequencies
involve considerable waste when transmitted through conductors, it
follows that, on the whole, it is better to employ one wire than two.
Therefore, if motors, lamps, or devices of any kind are perfected,
capable of being advantageously operated by currents of extremely high
frequency, economical reasons will make it advisable to use only one
wire, especially if the distances are great.