CHAPTER V. 
RATE OF EMISSION OF ENERGY. 
102. Comparison of the ionization produced by the «a 
and 8 rays. With unscreened active material the ionization 
produced between two parallel plates, placed as in Fig. 16, is mainly 
due to the a rays. On account of the slight penetrating power of 
the @ rays, the current due to them practically reaches a maximum 
with a small thickness of radio-active material. The following 
saturation currents were observed! for different thicknesses of 
uranium oxide between parallel plates sufficiently far apart for al] 
the a rays to be absorbed in the gas between them. 
Surface of uranium oxide 38 sq. cms. 


| Weight of uranium oxide Saturation current 
| in grammes per sq. cm. | in ampéres per sq. cm. 
of surface | of surface 
0036 [ey ee Om 
0096 32x 1078 
70189 4:0 x 107} 
0350 44x 10718 
0955 4-7 x 10738 



| 

The current has reached about half its maximum value for 
a weight of oxide ‘0055 gr. per sq.cm. If the a rays are cut off 
by a metallic screen, the ionization is then mainly due to the 
8 rays, since the ionization produced by the y rays is small in 
comparison. For the @ rays from uranium oxide it has been 
1 Rutherford and McClung, Phil. Trans. A. p. 25, 1901. 
