138 



Mr. V. E. Pound on the Absorption of the 



The curve B, drawn for a thickness of '723 mm. of lead, 

 indicates that ft rays which were deflected by a field cor- 

 responding to about 6 amperes, penetrated this thickness of 

 lead, while the coincidence of the curves C and C drawn from 

 the values corresponding both to the upward and downward 

 deflexions of the ft rays when *964 mm. was the thickness 

 of the lead screen, shows clearly that the ft and also the 

 ft secondary rays could not pass through this thickness of 

 lead. 



As is fully explained in Section III. A, the maximum 

 value of the conductivities in the chamber due to ft and ft 

 secondary rays for the different thicknesses of the screens 

 can readily be deduced from the tables given above. These 

 deduced values are given in row III. of Table VIII., and a 

 curve representing them is shown in fig. 10. From the 





Table VIII. 









Remarks. 



Saturation Current. 



Thickness of lead over! 

 the top of chamber. J 



mm. 

 



mm. 

 •241 



mm. 



•482 



mm. 

 •723 



mm. 

 •964 



I. Max. 13 +(3 secondary 1 

 -fy-fy secondary + I 

 natural ionization. J 



383-2 



50-9 



25-8 



17-5 



15-6 



II. y-fy secondary -f 1 

 natural ionization. J 



8-4 



194 



17-7 



16-5 



15-6 



III. Max. 8+/3 secon-] 

 dary ionization. J 



374-8 



31-5 



8-1 



1-0 







curve it is evident that a screen of lead *9 mm. in thickness 

 completely absorbed all of the ft and the ft secondary radiations 

 excited in the lead, including the most penetrating. 



B. Measurements on Reflected Rays from Lead. 



In this set of measurements the arrangement of the 

 apparatus was the same as when the measurements on the 

 reflected rays from tin were taken, the radium being placed 

 vertically above the ionization-chamber. Different thick- 

 nesses of lead were placed over the opening at the bottom, 

 while the single sheet of aluminium-foil, '0065 mm. in 

 thickness, covered the top. As before, the saturation- 

 currents in the chamber were taken as the magnetic field 

 deflected the ft rays farther and farther from the opening of 



