14 M‘Ciettanp— The Energy of Secondary Radiation. 
formerly ovcupied by the end of the tube A. The same pencil of rays that 
produced the ionisation measured in the first position of A was therefore used 
to produce secondary radiation from the plate P. The distance OC’ was kept 
the same as CR, although not necessary for calculation-purposes, in order to 
ensure that the ionisation-chamber may really be similar in the two cases. 
In one case the pencil of rays falling on the end of the tube A diverges from 
the point &; and in the other case the rays diverge from all points of the 
plate P; it is therefore better to have the distances CC’ and CR always equal, 
so that the pencil of rays entering the tube A may be equally effective in 
the two cases. SS isa lead screen to protect the tube A’ from direct radiation. 
In all observations corrections were applied for conductivity in the ionisation- 
chamber due to spontaneous ionisation or defective screening. 
The secondary radiation is emitted in all directions from the plate, but 
not equally in all directions. An accurate determination was made of the 
ionisation produced in A’ when 6 was 45°; and a separate investigation determined 
how the intensity of the radiation varied with @; it will obviously, from symmetry, 
be constant while @ is constant. This separate investigation consisted in moving 
A’ in one plane, changing @ and keeping CC’ constant, and measuring the current 
for the different values of 0. 
We give the numbers observed in one of many concordant determinations. 
The ionisation produced by the primary B rays falling on A, the plate P 
being removed, gave a saturation-current measured by 
246 scale-divisions per minute, 
the capacity charged being :199 microfarads. When the plate P was in position, 
the saturation-current in A’ was measured by 
58 scale-divisions per minute, 
the capacity charged being ‘01 microfarads. The area of the end of the 
ionisation tube A’ was 7(8°75) sq. cm., and CC’ was 20 cm. 
Supposing for the moment that the secondary radiation from the plate P 
were the same in all directions, the ratio of the total energy of the secondary 
radiation to that of the primary would be 
58x 01 2m (20) 
246 x 199 ~ (3-75) 
= 0%. 
A large number of observations similar to the above were made with 
the distance CC’ (and CR), varying in different experiments from 30 cm. to 
15cm. ‘They gave very concordant results, the mean of all the observations 
giving for the ratio the value °68. 
