532 Mr. L. Schwendlcr on the General Theory 



may be taken as constant, for their variation with temperature 

 is exceedingly small ; and in case of accident (i. e. a coil break- 

 ing or becoming shunted) nothing short of actual repair couk 

 help. Further, o, supposing the differential instrument to be 

 properly designed and mechanically well executed, may be 

 taken as a perfectly 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, R, and X. 



Of these three quantities the variation of K may become 

 largest ; for K 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 R, since it contains the internal resistance of the com- 

 pensation battery. 



X, the ratio of the two electromotive forces in one and 

 the same station, though also liable to change, will yei 

 vary very little. The electromotive force of a well-preparec 

 galvanic battery, especially when the battery is worked by 

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



With respect to the variation of the three quantities K, R 

 and X, the function S may therefore be expressed in three dif- 

 ferent forms. 



Bi = eq -pTf SK when K alone varies. 



B 2 = eq— ~ SR when R, i. e. a, alone varies. 

 RK 



S 3 =^E__ 8X when X, i. e. E or e or both, alone vary. 

 R 



These three different disturbances of balance may act singly 

 or conjointly ; and it is clear that they are independent of each 



* It appears that changes which have been observed to take place in the 

 electromotive force of a Minotto or Leclanche's battery are generally ap- 

 parent only, not real. Such changes are generally quite within the limit? 

 of observation-errors ; and if they are large, they are then generally due to 

 the incorrectness of the method employed for measuring the electromotive 

 force, 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. 



