﻿3 1 6 Secoli mmeter Measurement of Resistances and Capacities. 



the incomplete balance R/S = K/H may be adjusted to, even 

 when no restrictions are placed on the resistances F and /. 

 Unless F is moderately large, however (say 10000 co in the 

 case experimented on above), the corresponding lead is liable 

 to vary a good deal with the speed, and this may in some 

 cases be a disadvantage sufficient to counterbalance the 

 advantage of the generality o£ the method. The following 

 method, which I have tested experimentally and found to 

 work well, is applicable to both series and parallel combina- 

 tions. Arrange (<?. g. by means of a Morse tapping-key T 

 not shown in the figures) that the end B (figs. 1 & 3) of the 

 combination R H may be transferred to the point C, thus 

 making the arm BD infinite, and CD to consist of SK and 

 11H in parallel with each other. Give the adjustable resist- 

 ances P and Q any values small compared with that of R, 

 and, holding down the key T, turn the commutator X until 

 the galvanometer deflexion becomes zero. Then, releasing 

 the key T, thus reproducing the original Wheatstone's bridge 

 arrangement, balance the resulting galvanometer deflexion 

 by the adjustment of R. The balance thus obtained should 

 be tested by again tapping the key T, and the process if 

 necessary repeated (this will not be necessary unless F is very 

 .small). When no deflexion exists whether the key is tapped 

 or not, the balance will correspond accurately to the equality 



f.|. ...... .m 



This may be proved as follows : — When the key T is tapped, 

 and the combinations RH and SK put in parallel with each 

 other, the unreversed current through the galvanometer 

 branch is easily seen, when P and Q are of negligible magni- 

 tude, to be given by the expression 



r ,_ Q B 



/' R'S' ' 



* + R' + S' 



where R' and S' denote the operators (3) or (15) corresponding 

 to the combinations. Also when P and Q are small in the 

 Wheatstone-bridge equation (2), it reduces to 



r _PS / -QR' E 



which is the expression for the unreversed current through 

 the galvanometer branch when the key T is not down. 



