Question of the Homogeneity of y-liai/s. 



747 



Experiments were now conducted in the middle of the 

 laboratory and the apparatus was set up so as to reduce 

 secondary radiation effects to a minimum. Fig. 14 shows 

 the disposition. The radium, mounted in a block of wood 2, 





Fig 



14. 









1) 



f~l 







4 



(Zi 







4 



3 



1 



3 





2 





i 



vj 



was placed on a wooden table 5. Over the radium were 

 placed absorption plates of lead or of copper 1, and on the 

 top of 1 was placed the electroscope. Two circular screens 

 of metal could be placed either round the electroscope 4, or 

 round the wooden stand and absorption screens 3. Through- 

 out the experiments 4 was of lead and the electroscope o£ 

 brass. 5 could be changed by lining the table with some 

 other material than wood, and 3 could be varied in nature 

 and in thickness by placing plates and circular screens of 

 various bodies in position. The results are summarized in 

 Table IV. (p. 748). 



For different thicknesses of 1 (lead) the value of the 

 differences in leaks A and B, C and D, E and F varies, but 

 in general it was about 5 to 10 per cent, when experiments 

 were conducted in the centre of the laboratory and 10 per 

 cent, and more when the electroscope was held in position by 

 a stand of wood. Leaks H, G, B, and D are the smallest 

 and correspond to the ionization due to the absorption of the 

 primary y-rays only. A and C are the greatest, and corre- 

 spond to the maximum amount of secondary radiation 

 obtainable by the disposition employed, plus that due to the 

 primary y-rays. The values of E and F depend on whether 

 the thickness of 3 (lead) is great enough to ensure perfect 

 E = F = G if this thickness be 2 cm., but 



