X.
TELEGRAPHIC APPARATUS.
The easily made but practical apparatus described in this chapter supplies an incentive for learning the Morse telegraphic code, which is used for sending sound signals, and for visible signals transmitted by means of flags, lamps, and heliograph mirrors. Signalling is so interesting, and on occasion can be so useful, that no apology is needed for introducing signalling apparatus into this book.
The apparatus in question is a double-instrument outfit, which enables an operator at either end of the line to cause a “buzzer” or “tapper” to work at the other end when he depresses a key and closes an electric circuit. Each unit consists of three main parts—(1) the transmitting key; (2) the receiving buzzer or tapper; (3) the electric battery.
The principles of an installation are shown in Fig. 33. One unit only is illustrated, but, as the other is an exact duplicate, the working of the system will be followed easily.
[Illustration: Fig. 32.—Morse alphabet]
A wooden lever, L, is pivoted on a support, A. Passing through it at the forward end is a metal bar having at the top a knob, K, which can be grasped conveniently in the fingers; at the other a brass screw, O, which is normally pulled down against the contact, N, by the spiral spring, S. The contact M under K is in connection with the binding post T1 and N with binding post T3; K is joined up to T2, and O to T4.
T3 and T4 are connected with one of the line wires; T1 with the other wire through a battery, B; T3 with the other wire through the buzzer, R. [1]
[Footnote 1: For the buzzer may be substituted the tapper, described on a later page.]
Assuming both keys to be at rest, as in Fig. 33, the two buzzers are evidently in circuit with the line wires, though no current is passing. If the stem of K is depressed to make contact with M, the electric circuit of which the battery, B, forms part is completed, and the buzzer at the other end of the lines comes into action. Since the depression of K raises O off N, the “home” buzzer’s connection with the line wires is broken, to prevent the current being short-circuited. The fact that this buzzer is periodically in circuit, even when the key is being worked, makes it possible for the operator at the other end to attract attention by depressing his key, if he cannot read the signals sent.
[Illustration: Fig.33—Telegraphic apparatus; sending key, buzzer and battery]