THE MEASUREMENT OF RESISTANCE 



209 



E 



Rv+X 



If the power may be shut off, this method lends itself to the 

 determination of the insulation resistance between the conductors 

 of a two-wire distribution circuit and the ground; that is, the 

 water and gas pipes. Such tests are necessary when investigat- 

 ing the wiring of buildings. The circuit may be opened by re- 

 moving the main fuses, and the necessary e.m.f . obtained by using 

 either the supply voltage or a portable battery of dry cells. 



Loss of Charge Method. The loss of charge method of 

 measuring insulation resistance is based on the following 

 considerations. 



If a perfect condenser is charged and then discharged throtigh 



t_ 



a resistance, it can be shown that V t = F c c# where VQ 



is the initial P.D. of the plates, V t the P.D. at any subsequent 

 time t, C and R the capacity of the 

 condenser and resistance of the circuit. 

 Solving this for R, 



t W.4343 



R = 



C log, 



c log 10 



CO N 120 150 180 

 Time -Sec. 



FIG. 118. Illustrating 

 fall of potential in loss of 

 charge method for measur- 

 ing insulation resistance. 



The relation of its units is such that if 

 C is in microfarads, and t in seconds, 

 R will be given in megohms. If R be 

 large, several hundred or thousand 

 megohms, the time of discharge will be sufficiently prolonged, 

 so that it is possible to follow the variation of V t with an elec- 

 trometer or electrostatic voltmeter. From the above it is seen 

 that it is possible to measure a large resistance by discharging 

 through it a condenser of known capacity and noting the P.D. 

 at the beginning and end of a time t. 



The curve illustrates the phenomena for the case where 

 R = 1,000 megohms, C = 0.1 microfarad, V Q = 500 volts. 



If the resistance be exceedingly high, the P.D. of the condenser 

 may fall so slowly that in any reasonable time, V t may not 



14 



