THE MEASUREMENT OF RESISTANCE 



215 



The supply voltage should be constant and the readings made 

 as expeditiously as possible. First measure E, the supply volt- 

 age, then Vi and V z , the voltages to ground from leads 1 and 2. 

 When the voltmeter is connected from lead 1 to ground, 

 the voltage E sends a current, /i, through the resistance x z plus 

 the paralell resistance of x l and the voltmeter. Then if R v is 

 the voltmeter resistance, 



FIG. 122. 



Similarly, when the voltmeter is between lead 2 and the ground, 



E 



A, + 



also 



Hence:- 

 and 



As the voltages enter as a ratio, any galvanometer which 

 gives a deflection proportional to the current through it may 

 be used in place of the voltmeter, a suitable series resistance 

 being employed. In this case, the scale readings may be used 

 in place of the voltages. Inspection of the formulae shows that 

 the method is not applicable when one side of the circuit is 

 grounded. The method may be used to measure the insulation 



