CHAP, xii.] ELECTRICITY AND MAGNETISM. 399 



communication through the rear end of the key with the 

 relay or receiving instrument, Depressing the key breaks 

 this communication, and by putting the line wire in com- 

 munication with the main battery transmits a current 

 through the line. 



427. Faults in Telegraph Lines. Faults may 

 occur in telegraph lines from several causes : either from 

 the breakage of the wires or conductors, or from the 

 breakage of the insulators, thereby short-circuiting the 

 current through the earth before it reaches the distant 

 station, or, as in overhead wires, by two conducting 

 wires touching one another. Various modes for testing 

 the existence and position of faults are known to telegraph 

 engineers ; they depend upon accurate measurements of 

 resistance or of capacity. Thus, if a telegraph cable part 

 in mid-ocean it is possible to calculate the distance from 

 the shore end to the broken end by comparing the resist- 

 ance that the cable is known to offer per mile with the 

 resistance offered by the length up to the fault, and divid- 

 ing the latter by the former. 



428. Duplex Telegraphy. There are two distinct 

 methods of arranging telegraphic apparatus so as to 

 transmit two messages through one wire, one from each 

 end, at the same time. The first of these, known as 

 the differential method, involves the use of instruments 

 wound with differential coils, and is applicable to special 

 cases. The second method of duplex working, known 

 as the WheatstonJs Bridge Method, is capable of much 

 more general application. The diagram of Fig. 163 

 will explain the general principle. The first require- 

 ment in duplex working is that the instrument at each 

 end shall only move in response to signals from the 

 other, end, so that an operator at R may be able to 

 signal to the distant instrument M' without his own 

 instrument M being affected, M being all the while in 

 circuit and able to receive signals from the distant 

 operator at R'. To accomplish this the circuit is 



