=66.= For measurements of radio-activity with an electrometer, a steady source of E.M.F. of at least 300 volts is necessary. This is best obtained by a battery of small cells simply made by immersing strips of lead in dilute sulphuric acid, or by a battery of small accumulators of the usual construction. Small accumulators of capacity about one-half ampere-hour can now be obtained at a moderate price, and are more constant and require less attention than simple lead cells. In order to measure currents over a wide range, a graduated series of capacities is required. The capacity of an electrometer and testing apparatus is usually about 50 electrostatic units or ·000056 microfarads. Subdivided condensers of mica are constructed in which capacities varying from ·001 to ·2 microfarads are provided. With such a condenser, another extra capacity is required to bridge over the gap between the capacity of the electrometer and the lowest capacity of the condenser. This capacity of value about 200 electrostatic units can readily be made by using parallel plates or still better concentric cylinders. With this series of capacities, currents may be measured between 3 × 10⁻¹⁴ and 3 × 10⁻⁸ amperes—a range of over one million. Still larger currents can be measured if the sensibility of the electrometer is reduced, or if larger capacities are available. In a room devoted to electrometer measurements of radio-activity, it is desirable to have no radio-active matter present except that to be tested. The room should also be as free from dust as possible. The presence of a large quantity of dust in the air (see section 31) is a very disturbing factor in all radio-active measurements. A larger E.M.F. is required to produce saturation on account of the diffusion of the ions to the dust particles. The presence of dust in the air also leads to uncertainty in the distribution of excited activity in an electric field (see section 181). =67. Measurement of Current.= In order to determine the current in the electrometer circuit by measuring the rate of movement of the needle, it is necessary to know both the capacity of the circuit and the sensibility of the electrometer. Let _C_ = capacity of electrometer and its connections in E.S. units, _d_ = number of divisions of the scale passed over per second, _D_ = sensibility of the electrometer measured in scale divisions for 1 volt P.D. between the quadrants. The current _i_ is given by the product of the capacity of the system and the rate of rise of potential. Thus _Cd_ _i_ = ----- E.S. units, 300_D_ _Cd_ = ----------- amperes. 9 × 10¹¹ _D_ Suppose, for example,