THE MEASUREMENT OF RESISTANCE 



211 



To apply this method to the determination of insulation re- 

 sistance, the connections shown in Fig. 119 are required. 



Care should be taken that the connections are such that the 

 current necessary to charge the wiring does not pass through the 

 galvanometer; the connection from KI to the core should be short. 

 Air lines should be used when possible. 



It is first necessary to determine the capacity of the cable C, for 

 as a matter of convenience the cable is charged and allowed to 

 leak through its own insulation resistance. The measurement is 

 made by the direct deflection method; K\ is placed in the mid- 

 position; by throwing A' 3 to the right the condenser C\ is charged 



FIG. 119. Connections for measuring insulation resistance by loss of 

 charge method. 



through the galvanometer, and the elongation noted. When 

 the key is against the left-hand stop the condenser is discharged. 

 The key KI is now thrown to the position shown, thus making 

 sure that the cable is discharged. The elongation, Z) , on charg- 

 ing the cable is now taken by throwing KI to the right; several 

 observations should be made; between them the cable should be 

 short-circuited long enough to ensure its thorough discharge. 

 The elongation D is used both in computing the capacity C 

 and the resistance R. To determine D t charge the cable, and at 

 a noted minute and second, place KI in the mid-position, thus 



