In Fig. 12 the bulb comprises an incandescent lamp globe L, in the
neck of which is sealed a barometer tube b, the end of which is blown
out to form a small sphere s. This sphere should be sealed as closely
as possible in the centre of the large globe. Before sealing, a thin
tube t, of aluminium sheet, may be slipped in the barometer tube, but
it is not important to employ it.

The small hollow sphere s is filled with some conducting powder, and a
wire w is cemented in the neck for the purpose of connecting the
conducting powder with the generator.

[Illustration: FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.]

The construction shown in Fig. 13 was chosen in order to remove from
the brush any conducting body which might possibly affect it. The bulb
consists in this case of a lamp globe L, which has a neck n, provided
with a tube b and small sphere s, sealed to it, so that two entirely
independent compartments are formed, as indicated in the drawing. When
the bulb is in use, the neck n is provided with a tinfoil coating,
which is connected to the generator and acts inductively upon the
moderately rarefied and highly conducting gas inclosed in the neck.
From there the current passes through the tube b into the small sphere
s to act by induction upon the gas contained in the globe L.

It is of advantage to make the tube t very thick, the hole through it
very small, and to blow the sphere s very thin. It is of the greatest
importance that the sphere s be placed in the centre of the globe L.

[Illustration: FIG. 14.--FORMS AND PHASES OF THE ROTATING BRUSH.]