130 Mr. L. Schwendler on the General Theory 





and 



m" F7/ 



• (iv.) 



and similar expressions will be obtained for station II. 



, namely 



WW 1 A" 

 E' N" /*" mty'" • • • • 

 and 



t,. ™«* .,.., E"4» 



• (HI"-) 



Rigid fulfilment of the first condition, i. e. D = 0. 



For station I. we have 



D' = 0, 



which equation can only be satisfied by 



A' = 0, 



since the other factor of D' cannot become zero for quantities 

 larger than or smaller than oo. Then, substituting for A' its 

 value, we have 



a'd'-b'(L'+p")=r.O; (V.) 



or balance in station I., when that station is sending and sta- 

 tion II. is at rest, must be rigidly established. 



Therefore if balance in station I. is disturbed, say by 1/ vary- 

 ing or by any other cause* external to I/, we must have means 

 of conveniently reestablishing balance without delay. This, of 

 course, could always be done by altering either all the branches 

 a 1 , V, and d! , or any two of them, or only one of them; but it 

 is clear that so long as the variation of 1/ which disturbs the 

 balance does not exceed certain limits, balance may be regained 

 by altering only one of the three branches available ; and as this 

 will also be more convenient in practice than altering two of the 

 branches, or all three simultaneously, we shall make the suppo- 

 sition that 



"Balance is reestablished by an appropriate readjustment of one 

 of the three available branches >} f. 



* Causes of disturbance to balance external to L' are inappreciable in 

 practice and therefore may be neglected from the beginning. 



t Finally, when the best resistance-arrangement has been found, the 

 resistance of the different branches will be expressed in terms of L ; and 

 therefore to keep the best arrangement when L varies between any two 

 given limits will involve necessarily a simultaneous alteration of the resist- 

 ance of all the branches. 



If, however, the variation of L is small in comparison with L itself, an 

 alteration of one branch for the purpose of reestablishing balance is justi lied, 

 and would be absolutely correct if the variation of L were infinitesimal. 



