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 tinfoil connected to earth. The tube C is supported centrally on ebonite rods at each end. The capacity is given approximately by the formula
where b is the internal diameter of D, a the external diameter of C, and l the length of the tubes.
The following method can be used in some cases with advantage. While a testing vessel is in connection with the electrometer, a sample of uranium is placed on the lower plate A. Let d2 and d1 be the number of divisions passed over per second by the needle with and without the standard capacity in connection.
C + C1 d1
Then ------ = ------ ,
C d2
d2