the Electrical Conduction produced by it. 141 



sufficient to very approximately saturate the gas even for the 

 greatest distance apart of 1*5 cm. 



It will be observed that the rate of leak increases nearly 

 proportionally to the distance between the plates for short 

 distances, but for air at atmospheric pressure increases very 

 slowly with the distance when the distances are large. 



If there were no appreciable absorption of the radiation by 

 the gas, the ionization would be approximately uniform 

 between the plates, provided the diameter of the uranium 

 surface was large compared with the greatest distance between 

 the plates. The saturation rate of leak would in that case 

 vary as the distance. If there is a large absorption of the 

 radiation by the gas. the ionization will be greatest near the 

 uranium and will fall off rapidly with the distance. The 

 saturation rate of leak will thus increase at first with the 

 distance, and then tend to a constant value when the radiation 

 is completely absorbed between the plates. 



The results given in the previous table allow us to deter- 

 mine the absorption coefficient of air at various pressures. 

 My attention was first drawn to the rapid absorption of the 

 radiation by experiments of this kind. 



The number of ions produced between two parallel plates 

 distant d apart is equal to 



Jo 

 (1-e- 



pq 



i. e., to 



I 



assuming the ionization and the absorption are proportional 

 to the pressure. The notation is the same as that used 

 in § 10. 



For the pressure p the saturation rate of leak between the 

 plates is thus proportional to 1 — e~ pk ° d . 



If p and d are varied so that p x d is a constant, the rate 

 of leak should be a constant. This is approximately true as 

 the numbers previously given (see fig. 8) show. It must, 

 however, be borne in mind that the conditions, on which the 

 calculations are based, are only approximately fulfilled in 

 practice, for we have assumed the uranium surface to be 

 infinite in extent and that the saturation is complete. 



The variation of the rate of leak with distance agrees fairly 

 closely with the theory. When p\ d is small the rate of leak 

 is nearly proportional to the distance between the plates and 

 the pressure of the gas. When p\ d is large the rate of leak 

 varies very slowly with the distance, 



