the fantastic inventions of nikola tesla - Exopolitics Hong Kong

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, bv 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 electric- ; al connection between the metal tube and the electrode, T 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.