On the General Theory of Duplex Telegraphy. 527 



The compensation method has two principal defects which 

 the two preceding methods do not possess. 



First. The success of working a line dupliciter by the com- 

 pensation method will clearly depend on the possibility of 

 being able to close and open simultaneously two different con- 

 tacts (4 and 5). The mechanical difficulty of doing so suffi- 

 ciently accurately was pointed out by Dr. Werner Siemens, 

 and in fact constitutes one of the reasons which led him to 

 propose the differential method. 



Secondly. The balance in each station may be disturbed 

 directly by a variation of the electrical condition (internal re- 

 sistance and electromotive force) of the two batteries (E and 

 e) employed. 



In the preceding two methods the variation of the internal 

 resistance of the signalling- battery can only be felt indirectly 

 by affecting the balance of the distant station, while the varia- 

 tion of electromotive force has no effect at all. Hence a given 

 variation in the battery or batteries must necessarily produce 

 a greater disturbance of balance in the compensation method 

 than in the two preceding ones. We know that even so-called 

 constant galvanic batteries, doing work, alter their electrical 

 conditions perceptibly, especially their internal resistance; and 

 consequently this defect weighs most decidedly against the 

 compensation method. In all other respects the compensation 

 method has the same defects as the differential method, and in 

 addition some others which will be understood as the investi- 

 gation proceeds. 



General expression for the two functions " D " and u S." 



To obtain the functions D and S, we have to develop the 

 general expressions for the forces p, P, and Q, say (for station I.) 



p f = A!m r -Wn f , 



where A! and B / are the currents which pass through the two 

 coils a' and V respectively when station I. is sending and sta- 

 tion II. is at rest ; m' and n' are the forces exerted by the two 

 coils a' and V respectively on one and the same magnetic pole 

 when a unit of current passes through them. At balance in 

 station I. 



y 



Further, 



^-0. 



:■--.'- 



where <&! is the current which passes through the coil a' when 

 station II. is sending and station I. is at rest (single signals). 

 Further, 



