462. The Relay replaces the sounder in the line wire circuit, and its coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. Owing to the large resistance of long telegraph lines, the current is weak when it reaches a distant station, and not strong enough to work an ordinary sounder. The current passes back from the relay to the sending station through the earth. The relay armature acts as an automatic key to open and close the local circuit, which includes also a battery and sounder. The line current does not enter the sounder. (See "Things A Boy Should Know About Electricity.")
EXPERIMENT 193. To study the action of a two-pole telegraph instrument.
463. Directions. (A) Arrange as in [Fig. 153]. Connect the two coils to the connecting plates, as described in [§ 408]. Join a strip of copper Cu with wire 2 leading from D C, and join the zinc of D C to M. The ends of wires 1 and 3 should be near Cu but they must not touch it. If Cu be slightly curved so that its ends are raised above the table, the ends of wires 1 and 3 may be put directly under the ends of Cu; each half of Cu can then be used as a key. Two armatures, A and B, should be held as shown. D C can be placed at one side, of course, its terminals being joined to M and Cu.
(B) Press first one end and then the other of Cu, so that the current will pass through H or E at will.
(C) Paste pieces of paper to the armatures, the left one being marked with a dot, and the other with a dash. The one who sends the message can make dots or dashes at the instrument by pressing the proper key. This form of instrument can be easily made by boys, and the messages are more easily read by the eye than by the ear, as in regular sounders.
EXPERIMENT 194. To study the action of a simple "single needle telegraph instrument."