INDUCTANCE AND CAPACITY 369 



The capacity as determined includes that of the commutator 

 and the leads to the condenser under measurement. The correc- 

 tion due to these capacities is determined by a separate measure- 

 ment, the leads being disconnected from the condenser without 

 altering their position more than is absolutely necessary. 



The commutator used by Rosa and Dorsey is shown in Fig. 

 216. The 16 phosphor-bronze contact pieces, Si, Sz, etc., are 

 carried by an ebonite disc and to each one of the pieces is con- 

 nected its corresponding section of the brush ring, T\ t T 2 , etc. 

 This ring is sectionalized to reduce the capacity of the commutator 

 and to allow the guard ring to be charged and discharged syn- 

 chronously with the main condenser. One terminal of the con- 

 denser is connected to the copper brush J5 3 . The brushes B\ 

 and B 2 correspond to b and c in Fig. 215. 



The condenser is charged when $ 3 touches B\, is discharged 

 when it touches B 2 , and so on for each contact piece. The guard 

 ring is charged and discharged in a similar manner by the other 

 side of the commutator (B 6 , B 5 , #4). It will be noted that the 

 brushes are air-insulated between contacts, thus avoiding the 

 possibility of leakage across the commutator from brush to 

 brush. The usual speed of the commutator is from 1,200 to 

 1,500 revolutions per minute. 



Direct- deflection Method for Comparing Capacities. The 

 most obvious method for comparing the capacities of two con- 

 densers is to charge the condensers from the same battery and then 

 to determine the relative quantities which they have accumulated 

 by discharging them in turn through the same ballistic galva- 

 nometer. To carry out this test in its simplest form, the con- 

 nections shown in Fig. 217 may be used. 



FIG. 217. Connections for direct deflection method for comparing 

 capacities. 



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