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ELEMENTARY LESSONS ON [CHAP. xn. 



ZC) one pole of which is put to earth, and the other com- 

 municates with a key K. This key is arranged (like that 

 in Fig. 1 60), so that when it is depressed, so as to send 

 a signal through the line, it quits contact with the 

 receiving instrument at its own end. The current 

 flowing through the line passes through K' and enters a 



Fig. 1 60. 



receiving instrument G' at the distant end, where it pro- 

 duces a signal, and returns by the earth to the battery 

 whence it started. A similar battery and key at the 

 distant end suffice to transmit signals in the opposite 

 direction to G when K is not depressed. The diagram 

 is drawn as if G were a simple galvanometer ; but the 

 arrangement would perfectly suit the Morse instrument, 

 in which it is only required at either end to send long 

 and short currents without reversing the direction. 



425. The Morse Instrument. The most widely 

 used instrument at the present day is the Morse. The 

 Morse instrument consists essentially of an electro- 

 magnet, which, when a current passes through its coils, 

 draws down an armature for a short or a long time. 



