128 Mr. L. Sehwendler on the General Theory 



The resistances which are similarly situated in both the sta- 

 tions will be designated by the same letters ; and to indicate the 

 station to which they belong, each letter will have one accent in 

 station I. and two accents in station II. 



Further, if a relation between the resistances of one station 

 has to hold good between those of the other station also, the let- 

 ters will be used without any accents. 



The great practical advantage of the bridge method, it will be 

 clear at once, is that any kind of receiving-instrument which has 

 been used for single working may also be employed for duplex 

 telegraphy. This fact must always be of great consideration for 

 any administration that contemplates the general introduction 

 of duplex telegraphy. 



General expressions for the two functions J) and S. 



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

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



Byjo' we understand the force which acts on the receiving- 

 instrument g 1 of station I. when that station is sending alone 

 (station II. at rest). 



p 1 y in our particular case, is therefore proportional to the cur- 

 rent which passes through the galvanometer in a "Wheatstone's 

 bridge when balance is not rigidly established ; thus 



A' 

 p' ocE'- 



N' 

 where 



, i = a'd'-b , {L' + p»)=a , d t -b ! c t , 

 and 



N' ==y {V + d') {a 1 + c') -{-f {g'(a' + b' + J + d') + {c> + d 1 ) [a! + V) } 



Further, by P ; is understood the force which acts on the re- 

 ceiving-instrument in station I. when station II. is sending 

 alone : single signals. 



This force in our particular case is proportional to the current 

 which passes through the receiving-instrument of station I. when 

 station II. is sending alone ; and we have consequently 



F cc C W, 



where C /f is the current which enters the line at point 2 when 

 station II. alone is sending, C"pl the part of this current C" 

 which arrives actually at point 1 (on account of leakage between 

 points 2 and 1, a part of C is lost), and C"/^' that part of the 

 current C'pJ which ultimately produces the signal [single signal) 

 in station I. The current C'V arriving at point 1 branches off 



