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,