electricity has the power of disturbing or decomposing the neutral
state of a neighbouring conductor, and attracting the unlike while
it repels the like induced charge. Hence, too, it is that the
electrified amber or sealing-wax is able to attract a light straw
or pithball. The effect supplies a simple way of developing a
large amount of electricity from a small initial charge. For if in
figure 6 the positive side of the ball be connected for a moment
to earth by a conductor, its positive charge will escape, leaving
the negative on the ball, and as there is no longer an equal
positive charge to recombine with it when the exciting rod is
withdrawn, it remains as a negative charge on the ball. Similarly,
if we separate the two balls in figure 7, we gain two equal
charges--one positive, the other negative. These processes have
only to be repeated by a machine in order to develop very strong
charges from a feeble source.

Faraday saw that the intervening air played a part in this action
at a distance, and proved conclusively that the value of the
induction depended on the nature of the medium between the induced
and the inducing charge. He showed, for example, that the
induction through an intervening cake of sulphur is greater than
through an equal thickness of air. This property of the medium is
termed its INDUCTIVE CAPACITY.

The Electrophorus, or carrier of electricity, is a simple device
for developing and conveying a charge on the principle of
induction. It consists, as shown in figure 8, of a metal plate B
having an insulating handle of glass H, and a flat cake of resin
or ebonite R. If the resin is laid on a table and briskly rubbed
with cat's fur it becomes negatively electrified. The brass plate