111] METHODS OF MEASUREMENT 73 
Now for an electroscope with a volume of 1000 c.c., 1 was equal 
to about 1:9 x 10-” amperes. Substituting the values given above 
q = 17 ions per cubic centimetre per second. 
With suitable precautions an electroscope can thus readily 
measure an ionization current corresponding to the production of 
1 ion per cubic centimetre per second. 
The great advantage of an apparatus of this kind les in the 
fact that the current measured is due to the ionization inside the 
vessel and is not influenced by the ionization of the external air or 
by electrostatic disturbances. Such an apparatus 1s very convenient 
for investigating the very penetrating radiations from the radio- 
elements, since these rays pass readily through the walls of the 
electroscope. When the electroscope is placed on a lead plate 3 or 
4 mms. thick, the ionization in the electroscope, due to a radio- 
active body placed under the lead, is due entirely to the very 
penetrating rays, since the other two types of rays are completely 
absorbed in the lead plate. 
53. A modified form of electroscope, which promises to be of 
great utility for measuring currents even more minute than those 
to be observed with the type of instrument already described, has 
recently been devised by C. T. R. Wilson’. The construction of the 
apparatus is shown in Fig. 12. 

Fig. 12. 
1 Proc. Camb. Phil. Soc. Vol. 12, Part 11. 1903. 
