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.
This method has the advantage that the relative capacities are expressed in terms of the motion of the needle under the actual conditions of measurement.
69. Steady deflection method. The methods of measurement previously described depend upon the rate of angular movement of a suspended gold-leaf or of an electrometer needle. The galvanometer can only be employed for measurements with intensely active matter. A need, however, has long been felt for a method in which ordinary ionization currents can be measured by means of a steady deflection of an electrometer needle. This is especially the case, where measurements have to be made with active substances whose activity alters rapidly in the course of a few minutes.
This can obviously be secured if the electrometer system (one pair of quadrants being earthed) is connected to earth through a suitable high resistance. A steady deflection of the electrometer needle will be obtained