82 METHODS OF MEASUREMENT [CH. 
deep) mm the brass plate A. In order to avoid breaking the 
battery connection every time the plate A is removed, the wire 
To Electrometer 



UI 
Active Material 
Fig. 16. 
from the battery is permanently connected to the metal block V 
resting on the ebonite support. In this arrangement there is no 
possibility of a conduction leak from the plate A to B, since the 
earth-connected vessel V intervenes. 
An apparatus of this kind is very convenient for testing the 
absorption of the radiations by solid screens, as well as for making 
comparative studies of the activity of different bodies. Unless 
very active preparations of radium are employed, a battery of 
300 volts is sufficient to ensure saturation when the plates are not 
more than 5 centimetres apart. If substances are bemg tested which 
give off a radio-active emanation, the effect of the emanation can 
be eliminated by passing a steady current of air from a gas bag 
between the plates. This removes the emanation as fast as it is 
produced. 
Tf 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 1s due to the 
small natural ionization of the air. We can always correct for this 
natural leak when necessary. 
60. It is often required to measure the activity due to the 
emanations of thorium or radium or the excited activity produced 
