The Story of Electricity | Page 5

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with it when the exciting rod is withdrawn, it remains as a negative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 is then lifted by the handle and laid upon the
cake. It touches the electrified surface at a few points, takes a minute
charge from these by contact. The rest of it, however, is insulated from
the resin by the air. In the main, therefore, the negative charge of the
resin is free to induce an opposite or positive charge on the lower
surface and a negative charge on the upper surface of the plate. By
touching this upper surface with the finger, as shown in figure 8, the
negative charge will escape through the body to the ground or "earth,"
as it is technically called, and the positive charge will remain on the
plate. We can withdraw it by lifting the plate, and prove its existence
by drawing a spark from it with the knuckle. The process can be
repeated as long as the negative charge continues on the resin.
These tiny sparks from the electrophorus, or the bigger discharges of an
electrical machine, can be stored in a simple apparatus called a Leyden

jar, which was discovered by accident. One day Cuneus, a pupil of
Muschenbroeck, professor in the University of Leyden, was trying to
charge some water in a glass bottle by connecting it with a chain to the
sparkling knob of an electrical machine. Holding the bottle in one hand,
he undid the chain with the other, and received a violent shock which
cast the bottle on the floor. Muschenbroeck, eager to verify the
phenomenon, repeated the experiment, with a still more lively and
convincing result. His. nerves were shaken for two days, and he
afterwards protested that he would not suffer another shock for the
whole kingdom of France.
The Leyden jar is illustrated in figure 9, and consists in general of a
glass bottle partly coated inside and out with tinfoil F, and having a
brass knob K connecting with its internal coat. When the charged plate
or conductor of the electrophorus touches the knob the inner foil takes a
positive charge, which induces a negative charge in the outer foil
through the glass. The corresponding positive charge induced at the
same time escapes through the hand to the ground or "earth." The inner
coating is now positively and the outer coating negatively electrified,
and these two opposite charges bind or hold each other by mutual
attraction. The bottle will therefore continue charged for a long time; in
short, until it is purposely discharged or the two electricities combine
by leakage over the surface of the glass.
To discharge the jar we need only connect the two foils by a conductor,
and thus allow the separated charges to combine. This should be done
by joining the OUTER to the INNER coat with a stout wire, or, better
still, the discharging tongs T, as shown in the figure. Otherwise, if the
tongs are first applied to the inner coat, the operator will receive the
charge through his arms and chest in the manner of Cuneus and
Muschenbroeck.
Leyden jars