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.
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