INDUCTANCE AND CAPACITY 



371 



section on condensers (page 357) will show that this assumption 

 is strictly true only for air condensers charged or discharged 

 through a negligible resistance. If the capacity is determined 

 from the discharge deflection, both the first rush of current due 

 to the free charge, and the gradually decreasing current due to 

 the liberation of the absorbed charge, are active in producing 

 the deflection, and the galvanometer needle is acted on by two 

 forces a sudden blow due to the passage of the free charge and 

 a long-continued and gradually diminishing push due to the 

 absorbed charge. Therefore, when there is considerable absorp- 

 tion, the apparent capacity as determined by this simple method 



.25 



- .20 







.05 



10 20 30 40 50 60 70 80 90 100 110 120 

 Time of Charging In Seconds 



FIG. 218. Showing effect of time of charging on the apparent capacity of 

 sample of rubber-covered wire. 



is dependent on the period of the galvanometer employed. In 

 working with the discharge deflection, any delay after the con- 

 denser is disconnected from the battery and before it is con- 

 nected to the galvanometer will cause an error if the time of 

 charging has not been sufficiently long for the dielectric to be- 

 come saturated, for absorption goes on during this delay, thus 

 reducing the free charge. 



In industrial testing the direct deflection method is very com- 

 monly applied to cables, but it is obvious that to obtain results 

 which are of value as a basis of comparison between samples 

 which are nominally the same, some definite procedure must be 

 adopted. 



This point is emphasized by Fig. 218, which shows the apparent 



