Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
Fig. 14 shows the brush as it first appears in a bulb provided with a
conducting terminal; but, as in such a bulb it very soon
disappears--often after a few minutes--I will confine myself to the
description of the phenomenon as seen in a bulb without conducting
electrode. It is observed under the following conditions:

When the globe L (Figs. 12 and 13) is exhausted to a very high
degree, generally the bulb is not excited upon connecting the wire w
(Fig. 12) or the tinfoil coating of the bulb (Fig. 13) to the terminal
of the induction coil. To excite it, it is usually sufficient to grasp
the globe L with the hand. An intense phosphorescence then spreads at
first over the globe, but soon gives place to a white, misty light.
Shortly afterward one may notice that the luminosity is unevenly
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