th this rod, or conductor (K), joined to a
main conductor (L), which has a terminating knob (M). On each side and
close to the terminal end of each leather collector (H) is a fork-shaped
collector (N). These two collectors are also connected electrically with
the conductor (K). When the disc is turned electricity is generated by
the leather flaps and accumulated by the collectors (N), after which it
is ready to be discharged at the knob (M).

In order to collect the electricity thus generated a vessel called a
Leyden jar is used.

LEYDEN JAR.--This is shown in Fig. 18. The jar (A) is of glass coated
exteriorly at its lower end with tinfoil (B), which extends up a little
more than halfway from the bottom. This jar has a wooden cover or top
(C), provided centrally with a hole (D). The jar is designed to receive
within it a tripod and standard (E) of lead. Within this lead standard
is fitted a metal rod (F), which projects upwardly through the hole (D),
its upper end having thereon a terminal knob (G). A sliding cork (H) on
the rod (F) serves as a means to close the jar when not in use. When in
use this cork is raised so the rod may not come into contact,
electrically, with the cover (C).

The jar is half filled with sulphuric acid (I), after which, in order
to charge the jar, the knob (G) is brought into contact with the knob
(M) of the friction generator (Fig. 17).

VOLTAIC OR GALVANIC ELECTRICITY.--The second method of generating
electricity is by chemical means, so called, because a liquid is used as
one of the agents.

[Illustration: _Fig. 18._ LEYDEN JAR]

Galvani, in 1790, made the experiments which led to the generation of
electricity by means of liquids and metals. The first battery was called
the "crown of cups," shown in Fig. 19, and consisting of a row of glass
cups (A), containing salt water. These cups were electrically connected
by means of bent metal strips (B), each strip having at one end a copper
plate (C), and at the other end a zinc plate (D). The first plate in the
cup at one end is connected with the last plate in the cup at the other
end by a conductor (E) to make a complete circuit.

[Illustration: _Fig. 19._ GALVANIC ELECTRICITY. CROWN OF CUPS]

THE CELL AND BATTERY.--From the foregoing it will be seen that within
each cup the current flows from the zinc to the copper plates, and
exteriorly from the copper to the zinc plates through the conductors (B
and E).

A few years afterwards Volt