1876.] L. Schwendler — On the General Theory of Duplex Telegraphy. ^ 



become clear further on. We know now that both these modes of adjust- 

 ment are convenient and practicable, and contain immediate balance without 

 special conditions. In fact in this respect the compensation method is pre- 

 ferable to the differential method where immediate balance by varying 

 resistances could only be ohtained when varying the four branches simul- 

 taneously, according to a fixed relation 



Rapid approximation of the two functions S and D towards zero. 

 On account of f— w + ft we have 



where 



A = It \/a — K X v v J 



Now suppose A = o, then this equation may be disturbed by K, R, A, v, a, 

 or o varying ; a and o are wire resistances which may be taken as constant, 

 for their variation with temperature is exceedingly small, and in case of 

 accident, i. <?., a coil breaking or becoming shunted, nothing short of actual 

 repair could help. Further v, supposing the differential instrument to be 

 properly designed and mechanically well executed, may be taken as a per- 

 fectly constant quantity which certainly, as long as the coils or their 

 armatures are not moved on purpose, does not alter of its own accord. 



The quantities left, which by variation may affect the balance equation, 

 are K, It, and X, 



Of these three quantities the variation of K may become largest, for 

 _ZT does not only contain the line resistance, which is highly variable, but 

 K includes also the internal resistance of both the line batteries, which, even 

 for the best known form of galvanic battery, is by no means a constant 

 quantity. The variation of the internal resistance of the line battery in each 

 station produces of course the greatest disturbance of balance in that station. 



The next quantity most liable to change of its own accord is clearly It, 

 since it contains the internal resistance of the compensation battery. 



X, the ratio of the two E. M. f's. in one and the same station, though 

 being also liable to change, will however vary very little. The e. m. e. of 

 a well prepared galvanic battery, especially when the battery is worked by 

 weak currents, is far more constant than is generally believed.* 



* It appears that changes which have been observed to take place in the e. m. p. of 

 a Minotto or Leclanche's battery are generally apparent only, not real. Such changes 

 are generally quite within the limits of observation errors, and if they are large they are 

 then generally due to the incorrectness of the method employed for measuring the e. m. f., 

 or to cells actually having become exhausted. It appears that this mysterious force in 

 each cell either exists in its full vigour, or not at all, there seems to be no continuous 

 change in either direction. 



