APPENDIX D. 

 MISCELLANEOUS PRACTICAL APPLICATIONS.* 



28. The Morse telegraph is an arrangement for signalling 

 between distant stations as follows : An insulated wire leads from 

 one station to the other and back. The ground is generally used 

 instead of a return wire. An electric current from a battery or 

 other source is sent intermittently through this circuit by operat- 

 ing at one station a key which makes and breaks the circuit. 

 This current excites an electromagnet at the other station, and 

 the armature of this electromagnet makes a graphical record on 

 a moving strip of paper, or produces sound signals which are 

 interpreted by the operator at the receiving station. 



Relays and sounders. A fairly strong electric current is 

 required to operate the instrument which produces the signals at 

 a telegraph receiving station, and it is not desirable to send so 

 strong a current over a long line because of the great number of 

 voltaic cells that would be required. This difficulty is obviated 

 by the use of the relay. The current in the line flows through 



Fig. 25. 



many turns of fine wire which are wound upon an electromagnet 

 at the receiving station. This magnet actuates a very light lever 

 and this lever is arranged to open and close what is called a local 



* Many of the practical applications of electricity and magnetism have been described 

 in the foregoing chapters. 



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