=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? 2. Draw a diagram of an electric bell circuit at your home. Give the location of the electric bell, the electric generator and the push-button. Show the connecting wires, and explain briefly how the circuit is operated. 3. Represent some other electric circuit, naming the generator and other devices in the circuit. 4. Look up the work of Volta and Galvani and write a statement of the electrical discoveries and inventions made by them. (2) THE VOLTAIC CELL AND ITS ACTION