112. Operation of Simple Line. Suppose two boys, R (right) and L (left) have a line. Fig. 111 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. As soon as the key is raised, the armatures lift and make the up-click. As soon as R has finished, he closes his switch E. As the armatures are then held down, L knows that R has finished, so he opens his switch 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 must be used for such lines. On very large lines dynamos are used to furnish the current.
113. The Relay. 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. To get around this, relays are used; these are very delicate instruments that replace the sounder in the line wire circuit. Their coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. The relay armature merely acts as an automatic key to open and close a local circuit which includes a battery and sounder. The line current does not enter the sounder; it passes back from the relay to the sending station through the earth.
Fig. 112.
Fig. 112 gives an idea of simple relay connections. The key K, and cell D C, represent a distant sending station. E is the electromagnet of the relay, and R A is its armature. L W and R W represent the line and return wires. R A will vibrate toward E every time K is pressed, and close the local circuit, which includes a local battery, L B, and a sounder. It is evident that as soon as K is pressed the sounder will work with a good strong click, as the local battery can be made as strong as desired.
Fig. 113 shows a regular instrument which opens and closes the local circuit at the top of the armature.
Fig. 113.
114. Ink Writing Registers are frequently used instead of sounders. Fig. 114 shows a writing register that starts itself promptly at the opening of the circuit, and stops automatically as soon as the circuit returns to its normal condition. A strip of narrow paper is slowly pulled from the reel by the machine, a mark being made upon it every time the armature of an inclosed electromagnet is attracted. When the circuit is simply closed for an instant, a short line, representing a dot, is made.
Registers are built both single pen and double pen. In the latter case, as the record of one wire is made with a fine pen, and the other with a coarse pen, they can always be identified. The record being blocked out upon white tape in solid black color, in a series of clean-cut dots and dashes, it can be read at a glance, and as it is indelible, it may be read years afterward. Registers are made for local circuits, for use in connection with relays, or for direct use on main lines, as is usually desirable in fire-alarm circuits.