Measurements of Inductance. 525 



zero at the start, so that c l5 c 2 , c 3 cannot all vanish. The 

 equations for x, y, z, u are as above, except that the constant 

 S Lf " 



in u is — 



*'Q 



The cubic in D does not factorize : none 



of the constants vanish, and they vary in sign, so that it is 

 possible for the current in any arm to be reversed; the 

 proportionality between the constants no longer obtains, so 

 that the reversal in the various arms is not simultaneous. 

 A numerical example is given below. 



§ 4. Mven's procedure (Phil. Mag. vol. xxiv., Sept. 1887) 

 only differs in putting K in parallel with a non-inductive 

 portion p of the arm P instead of in the conjugate arm S. 

 The remarks made in the previous paragraph apply here also. 

 [Niven recommends that a non-inductive resistance s be put 

 in parallel with the coil L (of resistance r), in case p would 

 otherwise have to be inconveniently large, and adds that 

 this has the effect of apparently reducing the inductance L, 



2 



the factor being 



This statement is quite correct 



(r + sf 



for the circumstances to which it is meant to be applied, 

 but must not be applied generally except with the proviso 

 that the total current transmitted through r and s is to be 

 the same as was previously transmitted through r only. In 

 that case the truth of the statement is self-evident, since the 



energy in the magnetic field is — times the square of the 

 current. ] 



§ 5. W. Stroud's method is superior to the two preceding, 

 since it attains the same end without sacrificing the con- 

 tinuous balance. He interposes a fifth resistance T between 



the battery and condenser on one side and the arms R, S on 

 the other, and the adjustment of T annuls the kick on making 

 or breaking the battery connexion. The discussion in § 2 

 is applicable perfectly, provided we replace the R and S there 



TP TO 



mentioned by the expressions B, + T+-~- and S + T+ ~ 



