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 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 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 units, d = number of divisions of the scale passed over per second, D = sensibility of the electrometer measured in scale divisions for 1 volt 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  i = Cd/(300D)  units, = Cd/(9 × 10^{11}D) amperes.

Suppose, for example,

C = 50, d = 5, D = 1000; then  i = 2·8 × 10^{-13} amperes.

Since the electrometer can readily measure a current corresponding to a movement of half a scale division per second, we see that an electrometer can measure a current of 3 × 10^{-14}