Electricity for Boys | Page 9

(B) wound around it. The current, through the wire (B), as shown
by the darts (D), moves in one direction, and the induced current in the
core (C) travels in the opposite direction, as shown by the darts (D).
[Illustration: Fig. 16. DIRECTION OF INDUCTION CURRENT]
PARALLEL WIRES.--In like manner, if two wires (A, B, Fig. 16) are
parallel with each other, and a current of electricity passes along the
wire (A) in one direction, the induced current in the wire (B) will move
in the opposite direction.
These fundamental principles should be thoroughly understood and
mastered.
CHAPTER IV
FRICTIONAL, VOLTAIC OR GALVANIC, AND
ELECTRO-MAGNETIC ELECTRICITY

THREE ELECTRICAL SOURCES.--It has been found that there are
three kinds of electricity, or, to be more accurate, there are three ways
to generate it. These will now be described.
When man first began experimenting, he produced a current by
frictional means, and collected the electricity in a bottle or jar.
Electricity, so stored, could be drawn from the jar, by attaching thereto
suitable connection. This could be effected only in one way, and that
was by discharging the entire accumulation instantaneously. At that
time they knew of no means whereby the current could be made to flow
from the jar as from a battery or cell.
FRICTIONAL ELECTRICITY.--With a view of explaining the
principles involved, we show in Fig. 17 a machine for producing
electricity by friction.
[Illustration: Fig. 17. FRICTION-ELECTRICITY MACHINE]
This is made up as follows: A represents the base, having thereon a flat
member (B), on which is mounted a pair of parallel posts or standards
(C, C), which are connected at the top by a cross piece (D). Between
these two posts is a glass disc (E), mounted upon a shaft (F), which
passes through the posts, this shaft having at one end a crank (G). Two
leather collecting surfaces (H, H), which are in contact with the glass
disc (E), are held in position by arms (I, J), the arm (I) being supported
by the cross piece (D), and the arm (J) held by the base piece (B). A rod
(K), U-shaped in form, passes over the structure here thus described, its
ends being secured to the base (B). The arms (I, J) are both electrically
connected with 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