150 RATE OF EMISSION OF ENERGY [CH. 
shown (section 79) that the current reaches half its maximum 
value for a thickness of 0°11 gr. per sq. cm. 
On account of the difference in the penetrating power of the a 
and @ rays, the ratio of the ionization currents produced by them 
depends on the thickness of the radio-active layer under examina- 
tion. The following comparative values of the current due to the 
a and 8 rays were obtained for very thin layers of active matter’. A 
weight of 1/10 gramme of fine powder, consisting of uranium oxide, 
thorium oxide, or radium chloride of activity 2000, was spread as 
uniformly as possible over an area of 80 sq. cms. The saturation 
current was observed between parallel plates 5°7 cms. apart. This 
distance was sufficient to absorb most of the a rays from the active 
substances. A layer of aluminium ‘009 cm. thick absorbed all 
the a rays. 

Current due | Current due B 
to a rays to 6 rays Ratio currents e 


Uranium ... 1 1 “0074 
Thorium 1 oii “0020 
Radium sy ee 2000 1350 0033 





In the above table the saturation current due to the a and 
8 rays of uranium is, in each case, taken as unity. The third 
column gives the ratio of the currents observed for equal weights 
of substance. The results are only approximate in character, for 
the ionization due to a given weight of substance depends on its 
fineness of division. In all cases, the current due to the § rays is 
small compared with that due to the « rays, being greatest for 
uranium and least for thorium. As the thickness of layer increases, 
8 
the ratio of currents - steadily increases to a constant value. 
103. Comparison of the energy radiated by the a and 
B rays. It has not yet been found possible to measure directly 
the energy of the a and @ rays. A comparison of the energy 
radiated in the two forms of rays can, however, be made indirectly 
by two distinct methods. 
1 Rutherford and Grier, Phil. Mag. Sept. 1902. 
Re rn 
