Electricity for Boys | Page 9

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 copper plates, and exteriorly from the copper to the zinc plates through the conductors (B and E).
A few years afterwards Volta devised what is known as the voltaic pile (Fig. 20).
VOLTAIC PILE--HOW MADE.--This is made of alternate discs of copper and zinc with a piece of cardboard of corresponding size between each zinc and copper plate. The cardboard discs are moistened with acidulated water. The bottom disc of copper has a strip which connects with a cup of acid, and one wire terminal (A) runs therefrom. The upper disc, which is of zinc, is also connected, by a strip, with a cup of acid from which extends the other terminal wire (B).
[Illustration: Fig. 20. VOLTAIC ELECTRICITY]
Plus and Minus Signs.--It will be noted that the positive or copper disc has the plus sign (+) while the zinc disc has the minus (-) sign. These signs denote the positive and the negative sides of the current.
The liquid in the cells, or