372 



ELECTRICAL MEASUREMENTS 



capacity, on discharge, of a piece of rubber-covered wire when 

 subjected to different times of charging. Specifications for 

 rubber-covered wires commonly call for a charging period of 

 10 seconds. 



The Zeleny Discharge Key. In order to obtain the free charge 

 capacity of a condenser with an imperfect dielectric, the con- 

 denser must be disconnected from the galvanometer after it has 

 parted with its free charge and before any appreciable portion 

 of the absorbed charge has been given up. This may be accom- 

 plished by the Zeleny discharge key, 8 shown diagrammatically 

 in Fig. 219. In this key there are three flexible leaves, LI, L 2 , and 

 L 3 . As shown, the condenser C is being charged from the bat- 

 tery B. When the key is depressed the battery circuit is broken 



Li a 



L 3 



c 



J-ll- 



c d 



FIG. 219. Diagram for Zeleny discharge key. 



at a, and the discharge circuit completed at b. On continuing 

 the depression, mechanical contact is made at c, and the dis- 

 charge circuit broken at d. The period of delay before the gal- 

 vanometer is taken out of circuit is controlled by varying the 

 distance s, by turning the milled head e. The key must be kept 

 depressed until the first elongation has been completed. 



In using this key one starts with the distance s large (5 mm.) 

 and, maintaining as nearly as may be a constant velocity of 

 tapping, successive throws of the galvanometer needle are ob- 

 served as s is diminished. The result will be as shown in Fig. 

 220. The deflections fall off regularly until the point is reached 

 where sufficient time has not been allowed for the condenser to 

 part with its free charge. After this, the deflections rapidly 

 decrease and the ordinary variations in the velocity of tapping 



