If a clean plate is put in the place of A, a small movement of the electrometer needle is always observed. If there is no radio-active substance in the neighbourhood, this effect is due to the small natural ionization of the air. We can correct for this natural leak when necessary.
65. We have often to measure the activity due to the emanations of thorium or radium, or the excited activity produced by those emanations on rods or wires. A convenient apparatus for this purpose is shown in [Fig. 18]. The cylinder B is connected with the battery in the usual way, and the central conductor A with the electrometer. This central rod is insulated from the external cylinder by an ebonite cork, which is divided into two parts by a metal ring CC´ connected to earth. This ring acts the part of a guard-ring, and prevents any conduction leak between B and A. The ebonite is thus only required to insulate satisfactorily for the small rise of potential produced on A during the experiment. In all accurate measurements of current in radio-activity the guard-ring principle should always be used to ensure good insulation. This is easily secured when the ebonite is only required to insulate for a fraction of a volt, instead of for several hundred volts, as is the case when the guard-ring is absent.
Fig. 18.
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,