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_ [Illustration: 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 $$ C = \frac {l} {2 \log_e \frac {b} {a}} $$, 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_ and _C_ = _C₁_ --------- _d₁_ − _d₂_ 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.