1S76.] L. Schwendler — On the General Theory of Duplex Telegraphy. 3 



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

 some others which will he understood as the investigation proceeds. 



General expressions for the two functions " D" and " 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' = A! m — B' n! 

 where A, and B' are the currents which pass through the two coils a' and V 

 respectively, when Station I is sending and Station II is at rest ; m' and n 

 are the forces exerted hy 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,p' = o. 



Further P' = &' m' 



where ST is the current which passes through the coil a' when Station II is 

 sending and Station I is at rest (single signals). 



Further Q' = y' m -f- g~' n' 



where y', and 3' are the currents which pass through the coils a', and b' 

 respectively when both stations are sending simultaneously (Duplex signals). 



The compensation circuit, and the line circuit in each station being 

 electrically independent of each other, we have 



g;' = B' 

 invariably without condition. 



If we further presuppose that depressing of the key K does not alter 

 the complex resistance of the station, a condition which, for the regularity 

 of signals, we are obliged to assume here as well as in the two preceding 

 methods, it will be clear that 



y'=y' + <&' 

 Substituting these values for y' and g' in the expression for Q', we get : 

 p' = A' m' — B' n' 

 P'=g' m ' 

 Q'=(A' + S') m' + B' n' 



The signs of the terms may be again contained in the currents, while m' and 

 n' are taken as absolute numbers. We must only remember that A' m', and 

 B' n' must be invariably of opposite sign. Arbitrarily we will call the cur- 

 rent A positive when the negative pole of the line battery is to earth. 



Now we have again two different modes of connecting up the line 

 batteries, viz. : — 



1st. The same poles of the line batteries are connected to earth in the 

 two stations : 



