II[ DISTRIBUTION OF ELECTRICITY ON MOVING CONDUCTORS
135
duction without resistance. For very thin shells these dis-
turbing influences disappear.
=
I know of no previous experiments which might serve to
illustrate the effects investigated. Hence I have performed
the following one. Above a plate of mirror glass, of relatively
high conductivity (by a different method had been found to
be 4 seconds), a needle 10 cm. long was suspended by a wire
and allowed to execute torsional vibrations; the moment of
inertia of the needle was sufficiently increased by means of
added weights, and at its ends it carried two horizontal brass
plates, each 3 cm. long and 2 cm. broad. Their distance a
from the glass plate could be varied. When the needle was
electrically charged the brass plates acted on the opposing
glass surface as condensers; the bound electricity was com-
pelled to follow the motion of the needle, and ought, according
to the preceding, to damp the vibration of the needle. Now
such a damping actually showed itself. The needle was con-
nected with a Leyden jar, of which the sparking distance was
0.5 mm., whilst a was 2 mm. The needle was found to return
to its position of rest without further oscillation, though pre-
viously it had vibrated freely; even when a was increased to
35 mm., the increase of the damping at the instant of charging
was perceptible to the naked eye. And when I charged the
needle by a battery of only 50 Daniell cells, while a was
2 mm., I obtained an increase of damping which could be
easily perceived by mirror and scale. It was impossible to
submit the experiment to an exact computation, but by making
some simplifying assumptions I was able to convince myself
that theory led to a value of the logarithmic decrement of the
order of magnitude of that observed.
As we have shown, we possess, in a conductor rotating
under the influence of external forces, a body at the surface
of which the potential has different values, which it again
resumes after a slight disturbance. Hence if we connect two
points of the surface by a conductor, a current flows through
the connection; if we connect the points with two conductors,
these may as often as we please be raised to different potentials.
If we use metallic discs as the rotating bodies the differences
of potential obtainable by means of possible velocities of rota-
tion are indefinitely small; but if we use very bad conductors