excited by the ft Rays of Radium. 597 



the magnitude of the excited 7 ray effect is proportional to 



B -\LT X 



n 2 



e -H.ro 



The distance i\ was always made about 90 cm., and the 

 difference between r\ and r% was never more than 10 cm. 

 It was therefore sufficient to calculate the magnitude of 

 the effect as above, using the distance of the balance position 

 from the centre of A, without taking into account the shape 

 or dimensions of the ionization chamber. 



In this way the amounts of 7 radiation excited in a few 

 materials were measured, the face of the ionization chamber 

 being a lead plate 1*7 mm. thick. The amount of excited 

 radiation from a plate of metallic uranium, containing about 

 87 per cent, of uranium, is taken as 100. 



The balance was adjusted to an accuracy of about 1 part 

 in 300, and therefore the measurements of the amount of 

 excited radiation could be made with an accuracy of 3 or 4 

 per cent. It was therefore possible to determine approxi- 

 mately the absorption coefficients of the 7 rays excited in 

 different materials. Iron was used as the absorbing substance, 

 as plates of the necessary thickness were readily obtainable, 

 and because it does not cut out the softer radiations too 

 rapidly. 



The amount of excited radiation was first measured when 

 A was covered with the lead plate only, and then with the 

 addition of an iron plate 1*3 cm, thick, The absprption in 

 this plate gives the absorption coefficient ^ within the 

 range from to 1*3 cm. 



A further iron plate of the same thickness was then placed 

 over the chamber, and the amounts of excited radiation 

 again measured. The absorption in this second plate gives 

 the absorption coefficient /a 2 , within a range from 1*3 cm. 

 to 2'6 cm. of iron. The results are given in the following- 

 table : — 



Material. 



Relative amount 

 of excited rays. 



fi l (cm.) . 



—2 

 ju 2 (cm.) . 





100 

 92 

 82 

 79 

 75 



12 

 1-3 

 1-4 

 1-5 



1-8 



•8 



•8 

 •9 

 •95 

 1-0 



Lead 



Tin 



Zinc 



Aluminium 



Phil. Mao. S. 6. Vol. 24. No. 142. Oct. 1912. 2 R 



