111] METHODS OF MEASUREMENT 83 
by those emanations on rods or wires. A convenient apparatus for 
this purpose is shown in Fig. 17. The cylinder B is connected with 
: ery CIN 
Earth 

~*~ Earth 
Fig. 17. 
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 msulate 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. 
61. 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 accumu- 
lators 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 1s 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 
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