222 Mr. W. Wilson on the Effect of Pressure on 



experimental points are marked on it. The curve agrees 

 much better than would be expected considering the number 

 of assumptions and the uncertain values of the constants. 



The current was tested for saturation at different pressures 

 by noting the time taken by the leaf to move over the various 

 divisions of the scale. The same time was taken to fall 

 through a division at 190 volts as at 120 volts, except in the 

 case of the last point at 40 atmospheres, when the time was 

 slightly longer at the latter reading, i. e.>. the current was not 

 quite saturated. The saturation current was further tested 

 for by taking a few readings with the rays about twice as 

 intense. On making one reading fit on the original curve, 

 all the others on being reduced in the same ratio also fell 

 on the curve. These points are shown by squares in the 

 diagram. 



We can from this formula deduce the relative values of 

 the ionization due to the secondary and primary rays in the 

 vessel at atmospheric pressure. The value found is 6' 6, i. e. 



B(l — e _Aa ) 



— — -r -. This is greatly in excess of that obtained by 



McLennan (loc. cit.*), who found that in aluminium vessels 

 the ionization produced by the secondary is half that due to 

 the primary rays. This discrepancy cannot be accounted for 

 by differences in the sizes of the vessels used, since it can be 

 easily worked out approximately that, assuming the above 

 formula to be correct and the amount of secondary radiation 

 to be proportional to the exposed surface, and the ionization 

 produced by the primary to be proportional to the volume, 

 the ratio of the ionization produced by the primary rays to 

 that produced by the secondary should be of the same order 

 as that given above. 



Experiments on the Natural Ionization. 



We are now in a position to proceed with the experiments 

 on the natural ionization, which were conducted as follows : — 



The air was admitted till the required pressure was reached, 

 which was recorded on a Sch'afer and Budenberg gauge. 

 The apparatus was then left until the gold-leaf came to 

 rest, when the electroscope was charged, and the time taken 

 for the image of the leaf to move over a certain number of 

 scale-divisions in the microscope eyepiece was noted. From 

 this the number of divisions per minute moved over by the 

 image was calculated and the ionization is expressed in these 

 units. 



The first set of experiments was performed with the gas 



