134 Prof. E. Rutherford on M 



ramum 



Radiation and 



between the curves (fig. 5) for air and hydrogen is due to the 

 greater absorption of the radiation by the air. The less the 

 absorption of the gas, the more nearly is the rate of leak pro- 

 portional to the pressure. For carbonic acid the rate of 

 leak decreases far more slowly with the pressure than for 

 hydrogen; this is due partly to the much greater value of the 

 absorption in carbonic acid and partly to the fact that 

 100 volts between the plates was not sufficient to saturate the 

 gas. 



If we take the rate of leak between two parallel plates 

 some distance from the source of radiation, we obtain the 

 somewhat surprising result that the rate of leak increases at 

 first with diminution of pressure, although a saturating elec- 

 tromotive force is applied. 



The arrangement used was very similar to that in fig. 3. 

 The rate of leak was taken between the plates A and B, which 

 were 2 cm. apart, and the plate A was about 1*5 cm. from 

 the uranium surface. The following table gives the results 

 obtained : — 



Pressure. 



Current. 



mm. 

 760 



1 



645 



1-45 



525 



2 



380 



2-2 



295 



2-05 



180 



1-6 



100 



104 



49 



•58 



The current at atmospheric pressure is taken as unity. 

 The results are represented graphically in Rg. 6. 



The rate of leak reaches a maximum at a pressure of less 

 than half an atmosphere, and then decreases, and at a pressure 

 of 100 mm. the rate of leak is still greater than at atmo- 

 spheric pressure. 



This result is readily explained by the great absorption of 

 the radiation at atmospheric pressure and the diminution of 

 absorption with pressure. 



Let d t = distance of plate A from the uranium. 



^2— V V B „ 



