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
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