86 METHODS OF MEASUREMENT [CH. 
63. The capacity of the electrometer and its connections 
cannot be measured by any of the commutator methods used for 
the determination of small capacities, for in such cases the needle 
does not move, and the capacity measured is not that of the 
electrometer system when in actual use. The value of the capacity 
may, however, be determined by the method of mixtures. 
Let C =capacity of electrometer and connections. 
C, =capacity of a standard condenser. 
The electrometer and its connections are charged to a potential 
V, by a battery, and the deflection d, 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 V. be the 
potential of the system, and d, the new deflection. 

Then Cy (C ee) Va, 
CeCe 
(OR fae 4 
ale (C5 
and U =U, ena 3 
A simple standard capacity for this purpose can be constructed 
of two concentric brass tubes the diameters of which can be 
accurately measured. The external cylinder D (Fig. 18) is mounted 
Earth 


Fig. 18. 
on a wooden base, which is covered with a sheet of metal or tin-foil 
connected to earth. The tube C is supported centrally on ebonite 
