Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . he contact of aconductor in any circuit with the earth, permitting an escapeof current if another ground exists. 313. The Telegraph.—The telegraph instrument is anapparatus for transmitting signals by the aid of an electriccurrent. It consists ofthe line, the transmitteror key, the receiver orsounder, the relay andthe battery. The linebetween two stations isgenerally a single ironwire with a ground re-turn. The transmitterkey is depicted in It consists of a brass lever with
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . he contact of aconductor in any circuit with the earth, permitting an escapeof current if another ground exists. 313. The Telegraph.—The telegraph instrument is anapparatus for transmitting signals by the aid of an electriccurrent. It consists ofthe line, the transmitteror key, the receiver orsounder, the relay andthe battery. The linebetween two stations isgenerally a single ironwire with a ground re-turn. The transmitterkey is depicted in It consists of a brass lever with its axis . Fi^ Sounder. pivoted at AA, carrying a platinum contact point, B, whichis brought into contact by depressing the knob against theaction of the spring, S. On depressing the lever by the knob,N, the two platinum points, B, P, connected to the line byposts C and D, are brought into contact and complete thecircuit. When not in use the circuit is closed by the switch,K. The sounder, Fig. 299, consists of a brass lever with itsaxis pivoted at C, and carries an iron armature, A, of the. 330 PRACTICAL ELECTRICITY, electromagnet, M, which is connected in circuit by twobinding posts, P, P. A clicking sound is produced at each make V of the circuit and a spring pulls the armature backon break of circuit. In long lines the current, due to thehigh resistance, may be so small as to render the clicks of thesounder inaudible, when a relay is used. A relay, Fig. 300,is a small switch operated by an electromagnet wound withmany turns of tine wire. The magnet, M, is inserted in themain circuit by the posts, 3 and 4, and the platinum contactpoints, S, S, are brought in contact when the armature, L, ofthe magnet is attracted against the action of the spring, switch, S, S, is inserted in a local circuit with a batteryand sounder by the posts, 1 and 2, Thus a very weak cur-rent through the relay brings into action a strong local currentwhich operates the sounder. 31
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