THE MEASUREMENT OF RESISTANCE 185 



value. Therefore, the galvanometer should have a resistance 

 equal to the parallel resistance of the bridge arms between its 



niinals. 



The effect of a departure from this best value of the galvanome- 

 ter resistance may be seen from the following: Let the actual 

 resistance of the galvanometer be n times that of the ideal 

 instrument; that is, let 



fn/r i v\f\r i D\ 



(5) 



K * a M + N + X +P 



1.2 I 



0.8 



0.4 



Ordinates 



Max. Obta 



Deflection Obta ned 



liable 



Dert. 



_ 2 



n+1 



1C 



20 



24 



1 ii. 105. Showing effect of change of galvanometer resistance on the 

 sensitivity of a Wheatstone bridge when a Thomson galvanometer is 

 used. 



Substitution of the value of R G in (3A) gives as the corresponding 

 value of the deflection, 



'Dr.- KI*<\.\ - MP) Vn 



The maximum deflection will be attained when n 1. Con- 

 sequently the ratio of the actual to the maximum possible de- 

 flection of the galvanometer is 



D 2 Vn 



Z) = rT+1 



The values of this ratio are plotted in Fig. 105. 



Best Position for the Galvanometer. If the galvanometer 

 be of fixed resistance and if a definite e.m.f. be impressed at 

 the bridge terminals the magnitude of the galvanometer current 

 may be considerably influenced by the relative positions of the 

 galvanometer and battery. 



