554 Mr. C. G. Barkla on Energy of 
The energy of secondary radiation passing through the 
secondary electroscope was therefore 
il i 5 t 
> 2” Om ale 
the summation being over all elements of the aperture and all 
the corpuscles which contribute to the radiation through that 
aperture. 
The effect of radiation from the air in the space GHJ K 
is approximately the same as if all the corpuscles in that 
space radiated from the central point O. The angle subtended 
by aperture S at any corpuscle in the outer space shown in 
the figure depends on the position of the corpuscle, varying 
from » just outside the space G HJ K to zero at the outer 
boundaries of TU V W. 
A calculation giving a result which can only be regarded 
as approximate, showed the total effect to be equivalent to 
that of all the corpuscles in a volume of air equal to about 
194 ¢.c. radiating from O. 
Therefore the energy of secondary radiation passing through 
electroscope M 
3 if 
167 ots 730 
Ii being the total energy of secondary radiation from 194 c.c. 
of air. 
The intensities of the primary and secondary beams were 
compared by using apertures of various sizes for the primary 
and secondary electroscopes, and comparing the areas of the 
two apertures which gave the same rate of leak in similar 
electroscopes. The intensities of radiation were then inversely 
as the areas of the apertures. As the electroscopes were not 
exactly similar they were standardized by finding the rate of 
leak produced in each by a beam of given cross section and 
intensity. The proportionality between the ionization pro- 
duced in a given electroscope and the area of the aperture 
through which the beam was admitted was verified, so that 
local effects, as of a small portion of the beam being inter- 
cepted by the gold-leaf itself, were negligible. | 
The result was 

Intensity of Sec. Beam at 8 , 
. ='00000816 
Intensity of Prim. Beam at R 
.*. E=*00000816 x 230 x {Energy of Primary Beam of same 
cross section (5? cm.) } 
Now the length of the primary beam passing through 

