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