C1 = capacity of a standard condenser.
The electrometer and its connections are charged to a potential V1 by a battery, and the deflection d1 of the needle is noted. By means of an insulated key, the capacity of the standard condenser is added in parallel with the electrometer system. Let V2 be the potential of the system, and d2 the new deflection.
Then
CV1 = (C + C1) V2,
C + C1 V1 d1
-------- = ----- = -----
C V2 d2
d2
and C = C1 --------
d1 – d2