374 



ELECTRICAL MEASUREMENTS 



thoroughly insulated from the shaft and from each other by 

 bushings and washers of hard rubber. 7 is a toothed wheel insu- 

 lated from A and B and from the shaft and serves merely as a 

 support for the brush as it passes from A to B, thus preventing 

 mechanical shock and undue wear. The brushes 1 and 3 make 

 contact with the continuous portions of A and B] as the com- 

 mutator revolves 2 is alternately connected to A and to B and 

 the condenser is rapidly charged and discharged. At their 



FIG. 221. Fleming and Clinton commutator for use in comparing capacities 



peripheries the wheels A,. B, and / are separated by air spaces. 

 Means for accurately determining the speed must be provided. 



If the condenser has a capacity of C microfarads and is dis- 

 charged through the galvanometer n times per second, the vol- 

 tage of the battery being V, the quantity displaced through the 

 galvanometer in 1 sec., or the average current, will be 



nCV 



K is the constant of the galvanometer and DI the deflection. To 

 determine K a steady current from the same battery may be 

 sent through the instrument and regulated by the insertion of a 



