398 



ELECTRICAL MEASUREMENTS 



telephone he is using. This gives rise to a charging current in 

 the instrument which may be sufficient to prevent an exact 

 balance being obtained. The trouble may be eliminated by 

 bringing the telephone and the observer to the same (earth) 



FIG. 231. Diagram of bridge for determining electrolytic conductivities. 



potential by the Wagner Earth Connection 19 which is shown 

 in Fig. 232. 



The adjustable auxiliary circuit efg is similar in its makeup 



to the bridge circuit adc. It is 

 earthed at / and by varying its 

 component parts the impedances 

 of the sections ef and fg can be 

 adjusted and the potentials of e 

 and g altered in reference to the 

 earth. 



With the switch s open the 

 bridge is balanced as well as pos- 

 sible, using the telephone T\. 

 Then s is closed and the impe- 

 dance of the auxiliary circuit 

 adjusted until the sound in T 2 is 

 a minimum. This means that 

 d and consequently T\ are con- 

 tinuously at practically the same 



potential as /, which is earthed. Therefore, there can be no in- 

 ductive action between the telephone T\ and the observer. After 

 this adjustment has been made, s is opened and the final balance 

 is obtained by adjusting the bridge proper. 



FIG. 232. Diagram for Wagner earth 

 connection. 



