The two stations, R and L, may be near each other or in different houses. The _return wire_, R W, passes from the copper of b to the zinc of B. This is important, as the cells must help each other; that is, they are in series. The _line wire_, L W, passes from one station to the other, and the return may be through a wire, R W, or through the earth; but for short lines a return wire is best.
_=460. Operation of Line.=_ Suppose R (right) and L (left) have a line. Fig. 151 shows that R's switch, E, is open, while e is closed. The entire circuit, then, is broken at but one point. As soon as R presses his key, the circuit is closed, and the current from both cells rushes around from B through K, S, L W, s, k, b, R W and back to B. This makes the armatures of S and s come down with a click at the same time. (See Exp. 191.) As soon as the key is raised, the armatures raise, making the up-click. (See § 458.) As soon as R has finished, he closes his switch, E. L then opens e and answers R. Both E and e are closed when the line is not in use, so that either can open his switch at any time and call up the other. Closed circuit cells are used for such lines. On large lines the current from a dynamo is used.
=EXPERIMENT 192. To study the action and use of the "relay" on telegraph lines.=
=461. Directions.= (A) Arrange as in Fig. 152. Place K and D C at one end of the table to represent the sending station. At the other end of the table place E, which is the electromagnet of the relay, and H, the electromagnet of the sounder. Connect the ends of E with K and D C, L W being the line wire, and R W the return. In practice, the return is through the earth. The relay armature, R A, should vibrate towards E every time K is pressed. C is a piece of copper against which R A presses each time it is attracted by E, and this closes what is called the local circuit. Connect the poles of another battery, L B, with C and H, and the other end of coil H with R A. The sounder armature, S A, should be arranged as in Exp. 191. Small springs are shown on the two armatures, and these keep them away from the cores when the circuits are open.
(B) Fasten the parts to a board, and study the connections and action of this home-made outfit.