160 



ELECTRICAL MEASUREMENTS 



Position. II 



Position / 



may conveniently be done by a commutator with mercury con- 

 tacts such as is shown in Fig. 86. The parts L are insulating 

 strips of ebonite; the other parts of the rocker are of copper; 

 two middle arcs are in electrical connection. 



The parts of the commutator are so large that the resistance 



of the circuit, and therefore the bat- 

 tery current, is not appreciably altered 

 by the interchange. 



Referring to Fig. 87, N is a shunt. 

 A good resistance box may be used; 

 in practice it is applied to the larger 

 of the two resistances, S, X. By it, 

 the resistance between 3 and 4 is re- 

 duced to equality with that between 



1 and 2. The resistance of one of the galvanometer circuits is 

 adjusted by means of g and n; trial determines which one should 

 be varied. 



As S and X are supposed to be nearly equal, the galvanometer 

 is made as nearly differential as convenient but need not be 

 exactly adjusted. 



AC AC 



FIG. 86. Commutator 

 for interchanging the galva- 

 nometer coils. 



Rh I ' Rh 



Position I Position JI 



FIG. 87. Showing connections for both positions of the commutator in 

 Kohlrausch method. 



The connections for the two positions of the commutator are 

 shown in Fig. 87. 



In order to find the conditions necessary for balance, the galva- 

 nometer deflections for the two positions of the rocker must be 



