Potts — Rowland? s Nero Method. 



93 



ing- coil in the cross-connection, in place of the galvanometer 

 in the direct-current method of use. The adjustment of the 

 bridge thus used depends upon the fact that there will be no 

 deflection of the electro-dynamometer if the phase difference 

 of the current in the fixed coils and those in the hanging coil 

 is 90°. 



Fig. 1 is the arrangement used. Let R 1? R 2 , R 3 and R 4 be 

 the resistances of the different arms and r that of the cross- 

 connection. Let Cn be the current in the arm n of the 

 bridge, and C 5 in the cross-connection. 



If we apply at a and b a direct electromotive force E, we 

 shall have the following expressions for the currents : 



C' =E 



RR-RR 



c , =E R 4 (R,+R 3 )+r(R 3 +R 4 ) 



where 



A=r(R 3 +R 4 )(R 1 +R 2 )+R 1 R 2 (R 3 +R,)+R 3 R 4 (R I +R 2 ) 



(0 



(2) 



If in place of using a direct current we apply to the ter- 

 minals a, b, a simple alternating e.m.f. E^, we shall get 

 the corresponding quantities by the following substitutions : 



and if we place in the arm 1 a coil of self-induction L and 

 a capacity c, we must substitute for R 15 R\, where 



R, 



:R,-H6L— T - 

 oc 



and, if I is the self-induction of the hanging coils of the 

 electro-dynamometer, we must replace r by r', where 



r'=r-\-ibl 



