﻿for Measurement of Resistances and Capacities. 311 



affecting z,or, if the speed of the secohmmeter remain constant, 

 without affecting zt. An operation of this kind is subse- 

 quently referred to for brevity as one "at constant zt" and 

 it has the property that it in no way affects the values of the 

 commutator leads at which zero galvanometer deflexion 

 requires the satisfaction of the partial equalities. The p=r 

 balance only is referred to below, but the description applies 

 equally to the p = k balance by simple interchange of 

 symbols. 



(1) To obtain the equality p = r, make r — at constant zt 

 by transferring the whole resistance R from the arm BD to 

 CD {e.g. in fig. 3 transfer the galvanometer contact D to the 

 point of junction between R and H). Make also resistance 



p 

 P zero, and consequently p= p — r-. =0. Then p= r = 0, and 



the galvanometer deflexion will be zero only when the com- 

 mutator lead has the particular value S 3 which corresponds 

 to the zt at which the operation is carried out. }\ bile the 

 secohmmeter axle is still in rotation turn the commutator X 

 until the galvanometer deflexion is zero, and fix it in this 

 position. Then when R is transferred back to BD, a deflexion 

 of the galvanometer results which should now be balanced by 

 adjustment of P. The resulting value of P will satisfy the 

 equality p = r, or its equivalent P/Q = R/S. 



(2) At the lead & 3 which corresponds to the equality p — r y 

 the galvanometer deflexions must be entirely independent 

 of k, so long as the variations of k are made at constant zt. 

 In every other position of the commutator a motion of the 

 needle will result from varying I; and this will indeed be in 

 opposite directions according as the lead is greater or less 

 than S 3 . If we arrange that the capacity rl may be trans- 

 ferred by a tapping key from the arm BD to CD (or in fig. 1 

 the battery Contact altered to correspond), so that by tapping 

 the key k will be made zero at constant ct, the commutator 

 portion at which tapping this key gives no motion to the 

 galvanometer-needle is easily ascertained. The balance p = r 

 is obt dned as before by the adjustment of P in this position. 



I Hnd that the results of these incomplete balances in 

 practice may be obtained correct to under -J per cent. 

 Theoretic -illy, the speed should be kept constant during the 

 adjustment of P, since S 3 varies with the speed, but the 

 actual effect ot variations of sj eed on 8 3 and on the balance 

 is very small, and no difficulty arises in practice when the 

 s< cohmmeter is ke t t in rotation by a motor. Method (2) 

 has the slight advantage in this respect, that the correctness 

 of the balance can be tested after P has been adjusted by 



