368 ELECTRICAL MEASUREMENTS 



Eliminating Q B gives 



M + Ro + N - 



Qc = 



+ P + 



N(N + P) 



- (M + N) 



Qo. 



(5) 



The values of Q c from (2) and (5) may be equated, thus eliminat- 

 ing Qo, and the resulting equation solved for C. 



FIG. 216. Commutator used by Rosa and Dorsey in applying Maxwell's 

 method for the absolute measurement of electrostatic capacity. 



C = 



(6) 



With very small capacities it is necessary to use a high value of 

 n (500 cycles per second), and high voltages (100 to 200 volts) 

 in order to obtain sufficient sensitiveness. 



