MISCELLANEOUS PRACTICAL APPLICATIONS. 



325 



south pole 55 of the permanent magnet to the soft iron cores 

 NN^ which stand upon the north pole of the permanent magnet. 

 When a current flows in a certain di- 

 rection through the coils of the elec- 

 tromagnet TWVj one of its poles, N v 

 for example, becomes a strong north 

 pole and attracts the lever a. When 

 the current is reversed, the other pole 

 TV becomes a strong north pole and 

 attracts the lever a. Thus, the lever 

 a is pulled towards N^ or towards N 

 according to the direction of the cur- 

 rent which flows through the coils of 

 the instrument, and a local circuit con- 

 nected, as shown in Fig. 28^, may thus be opened and closed at 

 will by repeated reversals of the current through the winding 

 of the electromagnet NN r 



The ordinary relay is usually called the neutral relay to dis- 

 tinguish it from the polarized relay. 



N-POLE 

 STEEL MAGNET 



0' 



LOCAL 

 CIRCUIT 



Fig. 28b. 



30. Diplex telegraphy. The sending of two messages in the 

 same direction over one line wire simultaneously is known as 

 diplex telegraphy. This is accomplished as follows : At the send- 

 ing station are two keys. One of these keys is arranged to vary 

 the strength of the current in the line (never actually breaking 

 the circuit) by throwing a number of voltaic cells in and out of 

 circuit as it is operated. The other key is arranged to reverse 

 the direction of the line current as it is operated, the line current 

 being in one direction while this key is down, and in the other 



