for Measuring Ionization. 717 



measure the relative ionization due to the /3 and 7 rays of 

 radium for different thicknesses of aluminium, and then to 

 balance these ionizations against that of the standard. 



An examination of the calibration curve shows that the 

 ionization increases very rapidly at first as the cover is 

 opened, and then more slowly, following a curve similar to 

 a saturation one. The short straight part of the curve from 

 to 5 is that due to the natural ionization of the air in 

 between the plates of the standard, as the cover does not 

 begin to expose the uranium oxide until it has opened to 

 5 millimetres. 



If an unknown ionization in the testing-vessel X were to 

 be balanced by that of the standard, when its pointer stood at 

 3-4 on the scale s, then the unknown ionization would be, 

 from the curve, 50 per cent, of that of the standard. 



The calibration curve here shown is only a relative one,, 

 but the absolute values can be obtained if one knows the 

 sensitiveness of the electrometer and the capacity of the 

 system. 



The balance-point of the standard for a given constant 

 ionization in the vessel X is independent of the sensibility of 

 the needle, and a calibration curve once obtained can always 

 be used no matter what the sensitiveness may be. 



This is very convenient when comparing ionizations of 

 widely different strengths ; for a weak intensity a high 

 sensitiveness may be used, while for a strong intensity a 

 balance is more easily obtained with a low sensitiveness. 



If the sensitiveness should for any reason change during 

 the course of the experiment, the results are in no way 

 impaired, but can be compared with one another just as 

 accurately as if it had remained constant. 



In order to compare ionizations over a wide range, it is 

 necessary to have a standard which can itself be adjusted 

 between wide limits. This could be done by making the 

 first part of the standard of a weak radioactive material such 

 as uranium oxide, while the remainder could consist of a 

 strong radioactive substance such as polonium, which would 

 be very convenient, as it gives off only a rays, and no 

 emanation. For the first few centimetres of opening of the 

 cover, the ionization in the vessel S would be chiefly due to 

 the uranium oxide, and be therefore weak, but as the cover 

 opened more and more, the polonium rays would come into 

 evidence and the ionization increase rapidly to a maximum, 

 the uranium and polonium rays blending together without 

 any abrupt change of intensity. 



In this manner a standard could be made, which would 



