400 ELEMENTARY LESSONS ON [CHAP. xn. 



divided at R into two branches, which go, by A and 

 B respectively, the one to the line, the other through 

 a certain resistance P to the earth. If the ratio 

 between the resistances in the arms RA and RB is 

 equal to the ratio of the resistances of the line and of 

 P, then, by the principle of Wheatstone's Bridge, no 

 current will pass through M. So M does not show any 

 currents sent from R ; but M' will show them, for the 

 current on arriving at C will divide into two parts, part 

 flowing round to the earth by R', the other part flowing 



through M' and producing a signal. If, while this is 

 going on, the operator at the distant R' depresses his 

 key and sends an equal current in the opposite direction, 

 the flow through the line will cease ; but M will now 

 show a signal, because, although no current flows 

 through the line, the current in the branch RA will 

 now flow down through M, as if it had come from the 

 distant R', so, whether the operator at R be signalling 

 or not, M will respond to signals sent from R'. 



The Diplex method of working consists in sending 

 two messages at once through a wire in the same direc- 

 tion. To do this it is needful to employ instruments 

 which work only with currents in one given direction. 

 The method involves the use of " relays " in which the 

 armatures are themselves permanently magnetised (or 

 " polarised "), and which therefore respond only to 

 currents in one direction. 



The Quadruplex method of working combines the 



