132 Prof. E. Rutherford on Uranium Radiation and 



The current at atmospheric pressure is in each case taken as 

 unity for comparison, although the actual rates of leak were 

 different for the three gases. Fig. 5 (p. 130) shows these 

 results graphically, where the ordinates represent current 

 and the abscissas pressure. The dotted line shows the position 

 of the curve if the rate of leak varied directly as the pressure. 

 It will be observed that for all three gases the rate of leak 

 first of all increases directly as the pressure, and then 

 increases more slowly as the pressure increases. The differ- 

 ence is least marked in hydrogen and most marked in 

 carbonic acid. In hydrogen the rate of leak is nearly pro- 

 portional to the pressure. 



The relation between the rate of leak and the pressure 

 depends also on the distance between the plates. The following- 

 few numbers are typical of the results obtained. There was 

 a potential-difference of 200 volts between the plates and the 

 rate of leak is given in scale-divisions per mm. 



Pressure. 



Rate of Leak. 



Distance between 

 plates 2*5 mm. 



Distance between 

 plates 15 mm. 



mm. 



187 



376 

 752 

 1 



11 

 21 

 41 



47 

 83 



1,7 i 



For small distances between the plates the rate of leak is 

 more nearly proportional to the pressure than for large dis- 

 tances. 



The differences between the results for various gases and 

 for different distances receive a simple explanation if we 

 consider that the intensity of the radiation falls off rapidly 

 between the plates on account of the absorption in the gas. 

 The tables given for the relation between current and pres- 

 sure, where the distance between the plates is small, show 

 that when the absorption is small, the rate of leak varies 

 directly as the pressure. For small absorption the intensity 

 of the radiation is approximately uniform between the plates, 

 and therefore the ionization of the gas is uniform throughout 

 the volume of the gas between the plates. Since under a 

 saturating electromotive force the rate of leak is proportional 



