produced by ft and <y Rays at High Pressures. 177 



to the obliquity and the scattering of the ft rays. The Ur X 

 was placed at the bottom of the ionization-chamber and 

 covered with 01 mm. of aluminium. At a distance of 

 4*3 cm. above the active material was a lead plate, 2*3 mm. 

 thick, with a circular opening 3 cm. in diameter. Over this 

 opening was fastened a sheet of aluminium, O'l mm. thick, 

 to absorb soft scattered radiation. The upper plate was 

 Al foil, and the distance apart was 1*2 cm. The lead plate 

 and aluminium covering were charoed to 1520 volts. 



As the pressure in the vessel was raised the ionization 

 between the plates increased until it reached a maximum 

 and then decreased. The distance between the active 

 material and the bottom plate being 4*3 cm., and the 

 circular opening in the lead and the surface of the active 

 material being both 3 cm. in diameter, the direct distance 

 that the ft rays had to traverse was between 4*3 and 5 cm. 

 in the lower chamber, and between 1*2 and 1*3 cm. between 

 the plates. 



The corresponding problem for a. rays has been given by 

 Rutherford in ' Radio-Activity.' 



The saturation current is proportional to 



X 

 This is a maximum when 



lose ~ = — pX (d 2 — d x ) 



Cl O 



CV 



2 



From this expression 



X=-0076. 



Allowing for the obliquity of the rays, X would be about 

 •007. It should be noticed in fig. 4 that at 80 atmospheres, 

 when the ft rays have passed through a distance equivalent 

 to 350 cm. at normal pressure, there is a considerable ioni- 

 zation effect between the two plates. Substituting X = *0076 

 in the expression for the saturation current and plotting the 

 curve, there is a fair agreement between the theoretical and 

 experimental curves. 



The experiments with Ur X were in progress for over'a 

 fortnight, so that the decay of Ur X was also examined. 

 For any definite pressure the period of Ur X was found to 

 be about twenty-two days. 



AVhen the UrX was covered with "4 mm. of Al the ioni- 

 zation-pressure curve was similar to that obtained when the 

 material was covered with *2 mm. of Al. The absorption 



Phil. Mag. S. 6. Vol. 25. No. 145. Jan. 1913. X 



