etd io 
84 METHODS OF MEASUREMENT [CH. 
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 of parallel plates 
or still better of concentric cylinders. With this series of capacities, 
currents may be measured between 3 x 10~ and 3 x 107° amperes 
—a range of over one million times. 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 im all radio-active 
measurements. A larger E.M.F. 1s 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 dis- 
tribution of excited activity in an electric field (see section 171). 
62. 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 ES. units. 
d = number of divisions of the scale passed over per second. 
D =sensibility of the electrometer measured in scale divi- 
sions for 1 volt p.D. between the quadrants. 
The current 7 is given by the product of the capacity of the 
system and the rate of rise of potential. 
Cd raaeenie 
= Sn Es! units 
300 D 
Cd 
mn R02) 
Suppose, for example, 
C= 50) id — a9 oD) Os 
Thus a 
amperes. 
Then 4=2°8 x 10-8 amperes. 
