THE MEASUREMENT OF RESISTANCE 



207 



Absorption Effects. When the battery is first applied to the 

 cable there will be a sudden rush of current due to the charging 

 of the cable electrostatically. Therefore it is absolutely necessary 

 to have the key K 2 in the position shown when the circuit is 

 made, in order to prevent possible injury to the galvanometer. 

 After the static charging, a current flows into the cable, rapidly 

 diminishing to a nearly constant value. This current furnishes 

 the " absorbed" charge and includes the current which actually 

 flows through the insulation. The first portion diminishes 

 toward zero, while the latter tends to become constant. If 



FHJ. 110. Showing the effect of time of electrification on the galvanometer 

 deflection when measuring insulation resistance. 



tho switch C 2 were now thrown to the discharge position (dotted), 

 Ci reversed (to keep the deflections in the same direction), and 

 the deflection observed, it would be found that at first there is 

 a sudden rush due to the condenser discharge of the cable. 

 This is followed by a current which gradually diminishes toward 

 zero, this latter being due to the gradual working out of the ab- 

 sorbed charge. The various phenomena are illustrated by the 

 curves shown in Fig. 116. 



From the curves it is seen that the apparent resistance of the 

 insulating covering is a function of the time of electrification and 

 that it is necessary to state this time when quoting values of the 

 insulation resistance; otherwise they possess no meaning. It 

 is customary to calculate the resistance at the end of 1 min. 



