§n. 



Apparatus. 

 The apparatus used in these experiments is shown in fig. 

 i. The radium contained in a small conical hole cut in a 

 piece of Al was covered by a sheet of Cu foil "002 cm. thick. 

 The ^ rays passed up through a conical hole cut in a block 

 of wood, portions of the block being removed as shown to 

 allow of the introduction of the screens in different positions 

 as at A, B, C. The ionization chamber was hemispherical 

 and made of wood, with the inner surface covered with very 

 thin Al foil. The electrode connecting to the electrometer 



ToElej:trometer. 



Batteru. 



was in the form of a circular ring of wire, suitably protected 

 by the usual methods. The hemispherical chamber rested 

 upon a circular plate of Pb, above which was laid a sheet 

 of Al. A circular hole cut centrally in the Pb and Al plates 

 enabled the screen to be placed in the position A . In this 

 position practically all the emergent scattered radiation was 

 able to produce its effect to the same extent as the rays in 

 the main stream, all rays having the same length of path 

 in which to produce ionization, and the complications of sec- 

 ondary effects being reduced by having the walls of the cham- 

 ber wood. 



If we may for the present neglect any alteration in speed 

 of the scattered radiation and consider the original stream 

 of rays more or less homogeneous, the current may be taken 

 approximately as a measure of the number of ;8 particles 



