144 



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



of approximately 7 mm. of aluminium-foil was amply sufficient 

 to absorb all the /3 rays and the secondary rays excited by 

 them. 



B. Measurements on Reflected Rays from Aluminium. 

 A series of measurements was also made on the secondary 

 radiation produced at the front side of sheets of aluminium-foil 

 when /3 and 7 rays were allowed to fall on them ; and from 

 these the critical depth of the /3-ray effect has been deter- 

 mined. The arrangement of the apparatus was the same as 

 for the measurements on the reflected radiations from tin and 

 lead. The values of the saturation-currents in the chamber 

 found for the different thicknesses of aluminium-foil at the 

 bottom are given in Table XIV., and the maximum ionizations 

 due to the /3 rays have been deduced from these tables and 

 their corresponding curves. These maximum currents are 

 given in Table XV., and the curve in fig. 15 plotted from 



Table XIV. 

 Ionization by /3 reflected Secondary Rays from Aluminium. 





Saturation Current (arbitrary scale), w 



th 





different thicknesses of Aluminium-foil. 



Current 

 in magnet 

















(amperes). 



I. 



II. 



III. 



IV. 



V. 





mm. 



mm. 



mm. 



mm. 



mm. 





0-0065 



0-026 



0-065 



0-280 



0963 



00 



1106-0 



11260 



1140-0 



11760 



1181-0 



0-8 



709-8 



716-5 









0-85 







684-3 



710-0 



723-9 



2-5 



127-1 



128-9 



116-5 



125-8 



170-4 



4-5 



66-8 



58-6 



65-7 



59-2 



62-1 



6-5 



57'4 



52-0 



555 



49-5 



52-9 



15-0 



57-7 



55-2 



58-0 



54-3 



53-2 



23-0 



56-1 



54-5 



56-6 



53-4 



52-5 



30-0 



i 





53-4 



54-7 



520 



51-5 



Table XV. 



Thickness of 



aluminium-foil 



over bottom 



of chamber. 



Max. (5+(3 

 secondary + 7 + y 



secondary + 

 natural ionization. 



y + 7 secondary 

 -j- natural 

 ionization. 



Max. j3+/3 

 secondary 

 ionization. 



mm. 

 0-0065 

 026 

 0-065 



0-280 

 0-963 



1106 

 1126 

 1140 

 1176 

 1181 



57-1 

 53-8 

 56-2 

 52-3 

 52-5 



1048-9 

 1072-2 

 • 1083 8 

 1123-7 

 1128-5 



