THE MEASUREMENT OF RESISTANCE 217 



through the resistances a + A% and part through b + x lt and 

 back to the battery via the ground connection. 

 The total resistance encountered will be 



R = Ro + - 





a + x, ' b + x l 

 R G , a and 6 are negligible with respect to x l and x 2 , so 



x.x, 

 R = - very approximately. 



x l + x z 



/ + b\ 



* = K tnrv 



If the voltage of the battery B is E, then 



E 



R 



I a = ^ = KD 



where K is the current necessary for unit deflection of the galva- 

 nometer and D is the deflection. 

 Then 



K E 

 = KD 



E (a + b\ 

 *' " KD\ a > 



E /a + 



Xi ^KDVT 



Changes in circuit conditions, such as throwing on or off ap- 

 pliances not perfectly insulated, will change x l and x 2 and shift 

 the neutral point. This may introduce difficulties in the execu- 

 tion of the test. 



Other Methods of Measuring Resistance to Ground. 

 The current which flows to ground from one of the line wires 

 may be measured by the method given on page 94 for meas- 

 uring the current in a permanently closed circuit. The con- 

 nections are shown in Fig. 124. By varying r 2 the P.D. between 



