﻿>) 



for Measurement of Resistance* and Capacities. 315 



become in this case as follows : — (a) either/, or P and Q, must 

 be negligible compared with the combinations, and (b) 



/R_K\ QR ____ /P_K\ 



These are the relations corresponding to balance at 8 = i 

 independent of the speed, and we see as before that they can 

 only have practical utility if we arbitrarily give some of the 

 bridge variables negligible magnitudes with respect to the 

 others. Two practical cases exist : — 



(1) Make F large, and either /, or P and Q (or both), 

 small, compared with the combinations; and the result. of the 

 balance is 



R_K 

 8 ~ H" 



(2) Make P and Q, and also F, small compared with the 

 combinations, and the result of the balance is 



P K 



Q = H (U > 



In (1) the necessary conditions and the resulting balance 

 are exactly the same as in the series case, and the method of 

 practical balancing described above is thus applicable to both 

 classes of combinations. Moreover, under these conditions 

 the form of the differential equation for the current is exactly 

 the same as that of equation (5), and the methods of pp. 309 tV 

 311 for determining the other partial equalities are also equally 

 available. 



In electrochemical work parallel combinations usually 

 occur in the determination of the S.I.C. of leaking dielec- 

 trics ; they then invariably are of very high impedance. 

 and the conditions (1) would be difficult to satisfy in practice 

 (unless an electrometer is used in place of the galvanometer 

 in fig. 3). In such cases a better simplification of the bridge 

 system is got by satisfying (2). The balance at 8 = ^ bv 

 adjustment of P will then give the equality (17), and the 

 complete balance is attained by subsequent adjustment of R 

 at any other commutator position. This makes a very simple 

 method in such cases, since the conditions (2) will usually 

 be quite sufficiently satisfied in practice when P, Q, and l 1 ' 

 have any ordinary values, in consequence of the high impe- 

 dances of the combinations concerned. 



Method independent of Resistances of Battery and Galva- 

 nometer Arms. — It is possible to devise methods by which 



