distributed in the globe, and after passing the current for some time
the bulb appears as in Fig. 15. From this stage the phenomenon will
gradually pass to that indicated in Fig. 16, after some minutes,
hours, days or weeks, according as the bulb is worked. Warming the
bulb or increasing the potential hastens the transit.
[Illustration: FIG. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING
BRUSH.]
When the brush assumes the form indicated in Fig. 16, it maybe brought
to a state of extreme sensitiveness to electrostatic and magnetic
influence. The bulb hanging straight down from a wire, and all objects
being remote from it, the approach of the observer at a few paces from
the bulb will cause the brush to fly to the opposite side, and if he
walks around the bulb it will always keep on the opposite side. It may
begin to spin around the terminal long before it reaches that
sensitive stage. When it begins to turn around principally, but also
before, it is affected by a magnet, and at a certain stage it is
susceptible to magnetic influence to an astonishing degree. A small
permanent magnet, with its poles at a distance of no more than two
centimetres, will affect it visibly at a distance of two metres,
slowing down or accelerating the rotation according to how it is held
relatively to the brush. I think I have observed that at the stage
when it is most sensitive to magnetic, it is not most sensitive to
electrostatic, influence. My explanation is, that the electrostatic
attraction between the brush and the glass of the bulb, which retards
the rotation, grows much quicker than the magnetic influence when the
intensity of the stream is increased.
When the bulb hangs with the globe L down, the rotation is always
clockwise. In the southern hemisphere it would occur in the opposite
direction and on the equator the brush should not turn at all. The
rotation may be reversed by a magnet kept at some distance. The brush
rotates best, seemingly, when it is at right angles to the lines of
force of the earth. It very likely rotates, when at its maximum speed,
in synchronism with the alternations, say 10,000 times a second. The
rotation can be slowed down or accelerated by the approach or receding
of the observer, or any conducting body, but it cannot be reversed by
putting the bulb in any position. When it is in the state of the
highest sensitiveness and the potential or frequency be varied the
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