4 Morris- AlREY & Spencer, Capacity of a Condenser. 



the prongs of the fork, both of which dipped into a 

 common mercury cup (3) on one side, and into separate 

 insulated mercury cups (i) and (2) on the other. The 

 level of the mercury in these cups was adjusted by raising 

 or lowering two pegs, which were partially immersed in 

 the mercury, so that when the prongs of the fork were 

 excited and approaching each other, the lower rider was 

 lifted out of the cup (2), and the upper rider descended 

 into the cup (i). Similarly, when the prongs were 

 separating, contact was made in cup (2), and broken in 

 cup (i). The terminals of the condenser were connected 

 to the cups (3) and (2) ; the one connected to (2) being 

 also connected through a galvanometer to one pole of a 

 cell, the other pole of which was connected to the cup (i). 

 According as contact was made in cup (i) or cup (2), the 

 condenser was charged or short-circuited. 



When the fork was set into vibration, a steady deflec- 

 tion of the galvanometer needle was obtained, which could 

 be adjusted to a convenient amount by varying the 

 galvanometer shunt. The condenser and key were now 

 replaced by a standard resistance box, which was adjusted 

 until the deflection obtained was the same as that obtained 

 with the condenser. The value of n, which was in the 

 first place determined by comparison with standard tuning 

 forks, and found to be 123*4, was from time to time 

 verified by means of the results obtained with a standard 

 condenser. 



As the capacity of the riders, which were insulated 

 from the fork, was very small compared with the capacities 

 to be determined, no correction for the key was necessary. 

 Some trouble was experienced at first in establishing 

 good contacts at the mercury cups, owing to the action of 

 the mercury on the metal riders, the resulting galva- 

 nometer deflections being unsteady. After copper, brass, 



