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.
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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
