a Rat/s, 



ionization 



133 



Different Types of BeU. 



in row III. the deduced maximum ionization due to the 

 ft and ft secondary radiations for the same screens of tinfoil 

 sheets. 



Table III. 



Remarks. 



Saturation Current. 



Thickness of tin 1 

 over the top I 

 of chamber. J 



mm. 

 00 



mm. mm. 

 0-0196 0039 



mm. 

 0-0784 



mm. 

 01568 



mm. | mm. 

 0-3136 1-254 



mm. 

 1-96 



mm. 

 3136 



I. Max. /3+/31 

 secondary 



+ 7 + 7 L 

 secondary j 

 + natural | 

 ionization. J 



360-0 



343-2 287-5 



2300 



113-8 



67-8 



17-2 



14-4 



12-37 



II. y + y secon- j 

 dary -j- natu- i- 

 ral ionization. J 



737 



16-9 17-8 



17-7 



17-7 



17-6 



15-4 



141 



1 2-37 



III. 8 + 8 j 

 secondary I 

 ionization. J 



352-63 



326-3 2697 2123 



96-1 



50-2 1 -8 



0-3 



00 



The ordinates of the curve in fig. 6 are the values of the 

 maximum ft and ft secondary ionization- taken from row III. 

 of the ahove table. The curve, as will In- seen, is drawn 

 with an initial rise, although no values were obtained from 

 which the position of the highest point could be determined. 

 Some observations to be given later, however, on the deter- 

 mination of the maximum thickness of aluminium necessary 

 to absorb all the ft and ft secondary radiations, show clearly 

 that for aluminium the ionization in the chamber due to the 

 ft rays rose and fell as the thickness of aluminium was in- 

 creased. The inference was therefore drawn that for all 

 metals this rise and fall in the conductivity due to # rays 



Striking a wall of the chamber would occur, and would be 



made manifest if thin enough -licet- of the metal were 



used. 



From the regular manner in which the curve in fig. 6 

 falls away, it Is clear that in the experiments with tinfoil the 

 thickest screen used was amply sufficient to absorb all the 

 ft rays, and the secondary rays excited by them. An exami- 

 nation of the curve, moreover, makes it evident that even a 

 thickness of 2*5 millimetres of tinfoil was amply sufficient 

 for that purpose. 



