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 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_{1} = capacity of a standard condenser.

The electrometer and its connections are charged to a potential V_{1} by a battery, and the deflection d_{1} 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_{2} be the potential of the system, and d_{2} the new deflection.

Then CV_{1} = (C + C_{1}) V_{2}, (C + C_{1})/C = V_{1}/V_{2} = d_{1}/d_{2},

and C = C_{1} d_{2}/(d_{1} - d_{2}).

Fig. 19.

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. 19) is mounted 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 rods at each end. The capacity is given approximately by the formula

C = l/(2 log_{e}(b/a)),