the air be sufficiently warmed as to become conducting, the body is
rendered more intensely incandescent in a large bulb, obviously
because, under otherwise equal conditions of test, more energy may be
given off from the body when the bulb is large.
At very high degrees of exhaustion, when the matter in the bulb
becomes "radiant," a large bulb has still an advantage, but a
comparatively slight one, over the small bulb.
Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.
These observations were the result of a number of experiments, of
which one, showing the effect of the size of the bulb at a high degree
of exhaustion, may be described and shown here, as it presents a
feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4
inches diameter were taken, and in the centre of each was mounted an
equal length of an ordinary incandescent lamp filament of uniform
thickness. In each bulb the piece of filament was fastened to the
leading-in wire of platinum, contained in a glass stem sealed in the
bulb; care being taken, of course, to make everything as nearly alike
as possible. On each glass stem in the inside of the bulb was slipped
a highly polished tube made of aluminium sheet, which fitted the stem
and was held on it by spring pressure. The function of this aluminium
tube will be explained subsequently. In each bulb an equal length of
filament protruded above the metal tube. It is sufficient to say now
that under these conditions equal lengths of filament of the same
thickness--in other words, bodies of equal bulk--were brought to
incandescence. The three bulbs were sealed to a glass tube, which was
connected to a Sprengel pump. When a high vacuum had been reached, the
glass tube carrying the bulbs was sealed off. A current was then
turned on successively on each bulb, and it was found that the
filaments came to about the same brightness, and, if anything, the
smallest bulb, which was placed midway between the two larger ones,
may have been slightly brighter. This result was expected, for when
either of the bulbs was connected to the coil the luminosity spread
through the other two, hence the three bulbs constituted really one
vessel. When all the three bulbs were connected in multiple arc to the
coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only
came to redness. The bulbs were then sealed off and separately tried.
The brightness of the filaments was now such as would have been
expected on the supposition that the energy given off was
proportionate to the surface of the bulb, this surface in each case
representing one of the coatings of a condenser. Accordingly, time
was less difference between the largest and the middle sized than
between the latter and the smallest bulb.
An interesting observation was made in this experiment. The three
bulbs were suspended from a straight bare wire connected to a terminal
of the coil, the largest bulb being placed at the end of the wire, at
some distance from it the smallest bulb, and an equal distance from
the latter the middle-sized one. The carbons glowed then in both the
larger bulbs about as expected, but the smallest did not get its share
by far. This observation led me to exchange the position of the bulbs,
and I then observed that whichever of the bulbs was in the middle it
was by far less bright than it was in any other position. This
mystifying result was, of course, found to be due to the electrostatic
action between the bulbs. When they were placed at a considerable
distance, or when they were attached to the corners of an equilateral
triangle of copper wire, they glowed about in the order determined by
their surfaces.
As to the shape of the vessel, it is also of some importance,
especially at high degrees of exhaustion. Of all the possible
constructions, it seems that a spherical globe with the refractory
body mounted in its centre is the best to employ. In experience it has
been demonstrated that in such a globe a refractory body of a given
bulk is more easily brought to incandescence than when otherwise
shaped bulbs are used. There is also an advantage in giving to the
incandescent body the shape of a sphere, for self-evident reasons. In
any case the body should be mounted in the centre, where the atoms
rebounding from the glass collide. This object is best attained in
the spherical bulb; but it is also attained in a cylindrical vessel
with one or two straight filaments coinciding with its axis, and
possibly also in parabolical or spherical bulbs with the refractory
body or bodies placed in the focus or foci of the same; though the
latter is not probable, as the electrified atoms should in all cases
rebound normally from the surface they strike, unless the speed were
excessive, in which case they _would_ probably follow the general law
of reflection. No matter what shape the vessel may have, if the
exhaustion be low, a filament mounted in the globe is brought to the
same degree of incandescence in all parts; but if the exhaustion be
high and the bulb be spherical or pear-shaped, as usual, focal points
form and the filament is heated to a higher degree at or near such
points.
To illustrate the effect, I have here two small bulbs which are alike,
only one is exhausted to a low and the other to a very high degree.
When connected to the coil, the filament in the former glows uniformly
throughout all its length; whereas in the latter, that portion of the
filament which is in the centre of the bulb glows far more intensely
than the rest. A curious point is that the phenomenon occurs even if
two filaments are mounted in a bulb, each being connected to one
terminal of the coil, and, what is still more curious, if they be very
near together, provided the vacuum be very high. I noted in
experiments with such bulbs that the filaments would give way usually
at a certain point, and in the first trials I attributed it to a
defect in the carbon. But when the phenomenon occurred many times in
succession I recognized its real cause.
In order to bring a refractory body inclosed in a bulb to
incandescence, it is desirable, on account of economy, that all the
energy supplied to the bulb from the source should reach without loss
the body to be heated; from there, and from nowhere else, it should be
radiated. It is, of course, out of the question to reach this
theoretical result, but it is possible by a proper construction of the
illuminating device to approximate it more or less.
For many reasons, the refractory body is placed in the centre of the
bulb, and it is usually supported on a glass stem containing the
leading-in wire. As the potential of this wire is alternated, the
rarefied gas surrounding the stem is acted upon inductively, and the
glass stem is violently bombarded and heated. In this manner by far
the greater portion of the energy supplied to the bulb--especially
when exceedingly high frequencies are used--may be lost for the
purpose contemplated. To obviate this loss, or at least to reduce it
to a minimum, I usually screen the rarefied gas surrounding the stem
from the inductive action of the leading-in wire by providing the stem
with a tube or coating of conducting material. It seems beyond doubt
that the best among metals to employ for this purpose is aluminium, on
account of its many remarkable properties. Its only fault is that it
is easily fusible, and, therefore, its distance from the incandescing
body should be properly estimated. Usually, a thin tube, of a diameter
somewhat smaller than that of the glass stem, is made of the finest
aluminium sheet, and slipped on the stem. The tube is conveniently
prepared by wrapping around a rod fastened in a lathe a piece of
aluminium sheet of the proper size, grasping the sheet firmly with
clean chamois leather or blotting paper, and spinning the rod very
fast. The sheet is wound tightly around the rod, and a highly polished
tube of one or three layers of the sheet is obtained. When slipped on
the stem, the pressure is generally sufficient to prevent it from
slipping off, but, for safety, the lower edge of the sheet may be
turned inside. The upper inside corner of the sheet--that is, the one
which is nearest to the refractory incandescent body--should be cut
out diagonally, as it often happens that, in consequence of the
intense heat, this corner turns toward the inside and comes very near
to, or in contact with, the wire, or filament, supporting the
refractory body. The greater part of the energy supplied to the bulb
is then used up in heating the metal tube, and the bulb is rendered
useless for the purpose. The aluminium sheet should project above the
glass stem more or less--one inch or so--or else, if the glass be too
close to the incandescing body, it may be strongly heated and become
more or less conducting, whereupon it may be ruptured, or may, by its
conductivity, establish a good electrical connection between the metal
tube and the leading-in wire, in which case, again, most of the energy
will be lost in heating the former. Perhaps the best way is to make
the top of the glass tube, for about an inch, of a much smaller
diameter. To still further reduce the danger arising from the heating
of the glass stem, and also with the view of preventing an electrical
connection between the metal tube and the electrode, I preferably wrap
the stem with several layers of thin mica, which extends at least as
far as the metal tube. In some bulbs I have also used an outside
insulating cover.
The preceding remarks are only made to aid the experimenter in the
first trials, for the difficulties which he encounters he may soon
find means to overcome in his own way.
To illustrate the effect of the screen, and the advantage of using it,
I have here two bulbs of the same size, with their stems, leading-in
wires and incandescent lamp filaments tied to the latter, as nearly
alike as possible. The stem of one bulb is provided with an aluminium
tube, the stem of the other has none. Originally the two bulbs were
joined by a tube which was connected to a Sprengel pump. When a high
vacuum had been reached, first the connecting tube, and then the
bulbs, were sealed off; they are therefore of the same degree of
exhaustion. When they are separately connected to the coil giving a
certain potential, the carbon filament in the bulb provided with the
aluminium screen is rendered highly incandescent, while the filament
in the other bulb may, with the same potential, not even come to
redness, although in reality the latter bulb takes generally more
energy than the former. When they are both connected together to the
terminal, the difference is even more apparent, showing the importance
of the screening. The metal tube placed on the stem containing the
leading-in wire performs really two distinct functions: First: it acts
more or less as an electrostatic screen, thus economizing the energy
supplied to the bulb; and, second, to whatever extent it may fail to
act electrostatically, it acts mechanically, preventing the
bombardment, and consequently intense heating and possible
deterioration of the slender support of the refractory incandescent
body, or of the glass stem containing the leading-in wire. I say
_slender_ support, for it is evident that in order to confine the heat
more completely to the incandescing body its support should be very
thin, so as to carry away the smallest possible amount of heat by
conduction. Of all the supports used I have found an ordinary
incandescent lamp filament to be the best, principally because among
conductors it can withstand the highest degrees of heat.
The effectiveness of the metal tube as an electrostatic screen depends
largely on the degree of exhaustion.
At excessively high degrees of exhaustion--which are reached by using
great care and special means in connection with the Sprengel
pump--when the matter in the globe is in the ultra-radiant state, it
acts most perfectly. The shadow of the upper edge of the tube is then
sharply defined upon the bulb.
At a somewhat lower degree of exhaustion, which is about the ordinary
"non-striking" vacuum, and generally as long as the matter moves
predominantly in straight lines, the screen still does well. In
elucidation of the preceding remark it is necessary to state that what
is a "non-striking" vacuum for a coil operated, as ordinarily, by
impulses, or currents, of low-frequency, is not, by far, so when the
coil is operated by currents of very high frequency. In such case the
discharge may pass with great freedom through the rarefied gas through
which a low-frequency discharge may not pass, even though the
potential be much higher. At ordinary atmospheric pressures just the
reverse rule holds good: the higher the frequency, the less the spark
discharge is able to jump between the terminals, especially if they
are knobs or spheres of some size.
Finally, at very low degrees of exhaustion, when the gas is well
conducting, the metal tube not only does not act as an electrostatic
screen, but even is a drawback, aiding to a considerable extent the
dissipation of the energy laterally from the leading-in wire. This, of
course, is to be expected. In this case, namely, the metal tube is in
good electrical connection with the leading-in wire, and most of the
bombardment is directed upon the tube. As long as the electrical
connection is not good, the conducting tube is always of some
advantage, for although it may not greatly economize energy, still it
protects the support of the refractory button, and is a means for
concentrating more energy upon the same.
To whatever extent the aluminium tube performs the function of a
screen, its usefulness is therefore limited to very high degrees of
exhaustion when it is insulated from the electrode--that is, when the
gas as a whole is non-conducting, and the molecules, or atoms, act as
independent carriers of electric charges.
In addition to acting as a more or less effective screen, in the true
meaning of the word, the conducting tube or coating may also act, by
reason of its conductivity, as a sort of equalizer or dampener of the
bombardment against the stem. To be explicit, I assume the action as
follows: Suppose a rhythmical bombardment to occur against the
conducting tube by reason of its imperfect action as a screen, it
certainly must happen that some molecules, or atoms, strike the tube
sooner than others. Those which come first in contact with it give up
their superfluous charge, and the tube is electrified, the
electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment for two reasons: first,
the charge given up by the atoms spreads over a great area, and hence
the electric density at any point is small, and the atoms are repelled
with less energy than they would be if they would strike against a
good insulator: secondly, as the tube is electrified by the atoms
which first come in contact with it, the progress of the following
atoms against the tube is more or less checked by the repulsion which
the electrified tube must exert upon the similarly electrified atoms.
This repulsion may perhaps be sufficient to prevent a large portion of
the atoms from striking the tube, but at any rate it must diminish the
energy of their impact. It is clear that when the exhaustion is very
low, and the rarefied gas well conducting, neither of the above
effects can occur, and, on the other hand, the fewer the atoms, with
the greater freedom they move; in other words, the higher the degree
of exhaustion, up to a limit, the more telling will be both the
effects.
What I have just said may afford an explanation of the phenomenon
observed by Prof. Crookes, namely, that a discharge through a bulb is
established with much greater facility when an insulator than when a
conductor is present in the same. In my opinion, the conductor acts as
a dampener of the motion of the atoms in the two ways pointed out;
hence, to cause a visible discharge to pass through the bulb, a much
higher potential is needed if a conductor, especially of much surface,
be present.
For the sake of clearness of some of the remarks before made, I must
now refer to Figs. 18, 19 and 20, which illustrate various
arrangements with a type of bulb most generally used.
[Illustration: FIG. 18.--BULB WITH MICA TUBE AND ALUMINIUM SCREEN.]
[Illustration: FIG. 19.--IMPROVED BULB WITH SOCKET AND SCREEN.]
Fig. 18 is a section through a spherical bulb L, with the glass stem
s, containing the leading-in wire w; which has a lamp filament l
fastened to it, serving to support the refractory button m in the
centre. M is a sheet of thin mica wound in several layers around the
stem s, and a is the aluminium tube.
Fig. 19 illustrates such a bulb in a somewhat more advanced stage of
perfection. A metallic tube S is fastened by means of some cement to
the neck of the tube. In the tube is screwed a plug P, of insulating
material, in the centre of which is fastened a metallic terminal t,
for the connection to the leading-in wire w. This terminal must be
well insulated from the metal tube S, therefore, if the cement used is
conducting--and most generally it is sufficiently so--the space
between the plug P and the neck of the bulb should be filled with some
good insulating material, as mica powder.
[Illustration: FIG. 20.--BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.]
Fig. 20 shows a bulb made for experimental purposes. In this bulb the
aluminium tube is provided with an external connection, which serves
to investigate the effect of the tube under various conditions. It is
referred to chiefly to suggest a line of experiment followed.
Since the bombardment against the stem containing the leading-in wire
is due to the inductive action of the latter upon the rarefied gas, it
is of advantage to reduce this action as far as practicable by
employing a very thin wire, surrounded by a very thick insulation of
glass or other material, and by making the wire passing through the
rarefied gas as short as practicable. To combine these features I
employ a large tube T (Fig. 21), which protrudes into the bulb to some
distance, and carries on the top a very short glass stem s, into which
is sealed the leading-in wire w, and I protect the top of the glass
stem against the heat by a small, aluminium tube a and a layer of mica
underneath the same, as usual. The wire w, passing through the large
tube to the outside of the bulb, should be well insulated--with a
glass tube, for instance--and the space between ought to be filled out
with some excellent insulator. Among many insulating powders I have
tried, I have found that mica powder is the best to employ. If this
precaution is not taken, the tube T, protruding into the bulb, will
surely be cracked in consequence of the heating by the brushes which
are apt to form in the upper part of the tube, near the exhausted
globe, especially if the vacuum be excellent, and therefore the
potential necessary to operate the lamp very high.
Fig. 22 illustrates a similar arrangement, with a large tube T
protruding in to the part of the bulb containing the refractors button
m. In this case the wire leading from the outside into the bulb is
omitted, the energy required being supplied through condenser coatings
CC. The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge
might avoid passing through the wire w, which connects the inside
condenser coating to the incandescent button m. The molecular
bombardment against the glass stem in the bulb is a source of great
trouble. As illustration I will cite a phenomenon only too frequently
and unwillingly observed. A bulb, preferably a large one, may be
taken, and a good conducting body, such as a piece of carbon, may be
mounted in it upon a platinum wire sealed in the glass stem. The bulb
may be exhausted to a fairly high degree, nearly to the point when
phosphorescence begins to appear.
[Illustration: FIG. 21.--IMPROVED BULB WITH NON-CONDUCTING BUTTON.]
[Illustration: FIG. 22.--TYPE OF BULB WITHOUT LEADING-IN WIRE.]
When the bulb is connected with the coil, the piece of carbon, if
small, may become highly incandescent at first, but its brightness
immediately diminishes, and then the discharge may break through the
glass somewhere in the middle of the stem, in the form of bright
sparks, in spite of the fact that the platinum wire is in good
electrical connection with the rarefied gas through the piece of
carbon or metal at the top. The first sparks are singularly bright,
recalling those drawn from a clear surface of mercury. But, as they
heat the glass rapidly, they, of course, lose their brightness, and
cease when the glass at the ruptured place becomes incandescent, or
generally sufficiently hot to conduct. When observed for the first
time the phenomenon must appear very curious, and shows in a striking
manner how radically different alternate currents, or impulses, of
high frequency behave, as compared with steady currents, or currents
of low frequency. With such currents--namely, the latter--the
phenomenon would of course not occur. When frequencies such as are
obtained by mechanical means are used, I think that the rupture of the
glass is more or less the consequence of the bombardment, which warms
it up and impairs its insulating power; but with frequencies
obtainable with condensers I have no doubt that the glass may give way
without previous heating. Although this appears most singular at
first, it is in reality what we might expect to occur. The energy
supplied to the wire leading into the bulb is given off partly by
direct action through the carbon button, and partly by inductive
action through the glass surrounding the wire. The case is thus
analogous to that in which a condenser shunted by a conductor of low
resistance is connected to a source of alternating currents. As long
as the frequencies are low, the conductor gets the most, and the
condenser is perfectly safe: but when the frequency becomes excessive,
the _role_ of the conductor may become quite insignificant. In the
latter case the difference of potential at the terminals of the
condenser may become so great as to rupture the dielectric,
notwithstanding the fact that the terminals are joined by a conductor
of low resistance.
[Illustration: FIG. 23.--EFFECT PRODUCED BY A RUBY DROP.]
It is, of course, not necessary, when it is desired to produce the
incandescence of a body inclosed in a bulb by means of these currents,
that the body should be a conductor, for even a perfect non-conductor
may be quite as readily heated. For this purpose it is sufficient to
surround a conducting electrode with a non-conducting material, as,
for instance, in the bulb described before in Fig. 21, in which a thin
incandescent lamp filament is coated with a non-conductor, and
supports a button of the same material on the top. At the start the
bombardment goes on by inductive action through the non-conductor,
until the same is sufficiently heated to become conducting, when the
bombardment continues in the ordinary way.
A different arrangement used in some of the bulbs constructed is
illustrated in Fig. 23. In this instance a non-conductor m is
mounted in a piece of common arc light carbon so as to project some
small distance above the latter. The carbon piece is connected to the
leading-in wire passing through a glass stem, which is wrapped with
several layers of mica. An aluminium tube a is employed as usual for
screening. It is so arranged that it reaches very nearly as high as
the carbon and only the non-conductor m projects a little above it.
The bombardment goes at first against the upper surface of carbon, the
lower parts being protected by the aluminium tube. As soon, however,
as the non-conductor m is heated it is rendered good conducting, and
then it becomes the centre of the bombardment, being most exposed to
the same.