of Duplex Telegraphy. ]33 



Rigid fulfilment of the second condition, i. e. S = 0. 

 The general expression for S' was 



E'V E'// 



s'=^V*'-^+-V>'. . • .(iv.) 



Remembering that by equation (VII.) 



iv'+t3'=f, 



we know that ^=0' ; and substituting further for a' its value, 

 the general expression for S' becomes 



E'W E7>' JEW EV ,\ ,, , TVM 



S' = 



and this form of S' shows at once that it is perfectly immaterial 

 for duplex working by the bridge method whether the same or 

 opposite poles of the two signalling-batteries be put to line*; for 

 in both cases equation (IV.) becomes 



S'=^>'-B'J. ..... (IV.) 



Further, it will be seen that the right-hand member of equa- 



A' 

 tion (IV.) can be transformed t into E'~n, which is equal to^', 



or we have generally 



i. e. the difference of forces by which duplex and single signals in 



sending simultaneously, and again when the station for which /3 is to be 

 measured is sending alone. Then clearly, if these two deflections are equal, 

 w-\-(*> must be equal to/. If the two deflections are not equal, then alter 

 w until they become equal. After the determination is made the galvano- 

 scope is short-circuited. 



* In practice, however, I prefer to put the same (namely the positive) 

 poles to the line, as then defective insulation will not be felt so much. 



t We have 



/ & 



^r mk — An 



b 



g_ E&A 



mk — An 



AN 



EA 



