en it begins to rotate slowly, and the hands are held at a
proper distance, it is impossible to make even the slightest motion
without producing a visible effect upon the brush. A metal plate
connected to the other terminal of the coil affects it at a great
distance, slowing down the rotation often to one turn a second.

I am firmly convinced that such a brush, when we learn how to produce
it properly, will prove a valuable aid in the investigation of the
nature of the forces acting in an electrostatic or magnetic field. If
there is any motion which is measurable going on in the space, such a
brush ought to reveal it. It is, so to speak, a beam of light,
frictionless, devoid of inertia.

I think that it may find practical applications in telegraphy. With
such a brush it would be possible to send dispatches across the
Atlantic, for instance, with any speed, since its sensitiveness may be
so great that the slightest changes will affect it. If it were
possible to make the stream more intense and very narrow, its
deflections could be easily photographed.

I have been interested to find whether there is a rotation of the
stream itself, or whether there is simply a stress traveling around in
the bulb. For this purpose I mounted a light mica fan so that its
vanes were in the path of the brush. If the stream itself was rotating
the fan would be spun around. I could produce no distinct rotation of
the fan, although I tried the experiment repeatedly; but as the fan
exerted a noticeable influence on the stream, and the apparent
rotation of the latter was, in this case, never quite satisfactory,
the experiment did not appear to be conclusive.

I have been unable to produce the phenomenon with the disruptive
discharge coil, although every other of these phenomena can be well
produced by it--many, in fact, much better than with coils operated
from an alternator.

It may be possible to produce the brush by impulses of one direction,
or even by a steady potential, in which case it would be still more
sensitive to magnetic influence.

In operating an induction coil with rapidly alternating currents, we
realize with astonishment, for the first time, the great importance
of the relation of capacity, self-induction and frequency as regards
the general result. The effects of capacity are the most striking, for
in these experiments, since the self-induction and frequency both are
high, the critical capacity is very small, and need be