Fig. 218.—Diagram of an electric bell circuit.
240. The Electric Circuit.—The entire conducting path along which a current of electricity flows is called an electric circuit. In the case of a voltaic cell, the circuit includes not only the wires connecting the plates but also the plates themselves and the liquid between them. When some device or apparatus is to receive current from the cell, it is attached to the plates and wires so that the device is a part of the electric circuit. Separating the circuit at any point is called breaking or opening the circuit, while connecting the ends of an open circuit is called making or closing the circuit. A device for opening and closing a circuit is called a key or switch. The electric circuit used in ringing a door bell is familiar to most boys and girls. This circuit is open most of the time. It is closed by pressing the push-button at the door, and the flow of current through the electric bell causes the latter to ring. Such a circuit is represented in Fig. 218. Here C is the voltaic cell, the two lines representing the plates of the cell. A cross-section view of the push-button (P), shows how the circuit is closed, (B) is the bell. Wherever current electricity is used the device in which it is employed forms a part of an electric circuit extending back to some electric generator. This generator must be able to continually produce an E.M.F., or a difference of potential between its terminals, in order that the movement of electricity may be continuous.
Important Topics
(a) Electric generators: (1) voltaic cell uses chemical energy; (2) dynamo uses mechanical energy.
(b) Electric circuits: (1) open, (2) closed, (3) key and switch.
(c) Voltaic and galvanic electricity (names).
(d) Galvanoscope, uses.
Exercises
1. In what two ways are static and current electricity alike? In what two different?