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



the curves A and A' representing them are drawn in fig. 16^ 

 These curves and the curves E and E' drawn in fig. 13 are on 

 the same scale. A comparison of the latter, which correspond 

 to a screen of 8*14 mms. of aluminium alone over the top o£ 

 the chamber, with the curves A and A' in fig. 16, makes it 

 clear that the insertion of the sheet of tinfoil beneath the 

 aluminium screen brought the curves representing the two 

 deflexions more nearly into coincidence. The natural con- 

 clusion would be theu, that for a greater thickness of tinfoil 

 below the aluminium, the two curves representing the ioniza- 

 tions for the two deflexions would coincide. To test this 

 conclusion, four sheets of tinfoil, or a thickness of *0784 mm., 

 were placed above the opening of the chamber, and over this 

 the 8*14 mm. of aluminium. The conductivity in the chamber 

 was then measured for different magnetic fields. The results 

 are given in Table XIX., and curve B illustrating them is- 



Table XIX. 



Thickness of aluminium over the top of chamber = 8*14 mms. 

 Thickness of tin over the top of chamber = '0784 mm. 

 Aluminium above tin. 



/3 rays deflected towards 



| /3 rays deflected away from 



chamber. 



chamber. 



Current 





Current 





through 



Saturation 



through 



Saturation 



magnet 



Current. 



magnet 



Current. 



(amperes). 



(amperes). 





o-o 



1600 



o-o 



16-10 



2-5 



1604 



45 



15-69 



4-35 



15-66 



10-0 



15-56 



6-5 



15-66 



150 



15-39 



100 



15-44 



22-0 



1617 



15-0 



15-35 







180 



15-19 







shown in fig. 16. The numbers in the table and the curve 

 both show that with a screen made up in this way the con- 

 ductivities in the chamber were identical for magnetic fields 

 of equal intensity in either direction. This experiment showed 

 clearly that the effect under consideration was due to a pecu- 

 liarity in the secondary radiation emitted by the aluminium. 

 (4) In the fourth experiment the radium protected by the 

 lead cylinder was placed vertically above the ionization- 

 chamber and also above the poles of the magnet in such 



