﻿364 Prof. E. Rutherford on the Mass and Velocity of 



The experimental ratio is 1*21. From the agreement between 

 the experimental and the theoretical ratios, it could be con- 

 cluded with confidence that the mass of the a particle from 

 actinium is the same as that from radium C. This, however 

 has been experimentally verified by measuring the electric 

 deflexion of the a rays from actinium B. 



The apparatus for determining the electric deflexion of the 

 a rays was the same as that used in the radium experiments. 

 The copper plate which served as a source of a rays was 

 kept active by an arrangement similar to that used for the 

 magnetic deflexion. The photographic plate was exposed 

 for six days. Through an accident in the connexions, the 

 electric field was not acting between the plates for the first 

 three days. The photographic plate consequently showed 

 the undeflected trace of the rays and the deflected trace on 

 one side of it. The distance between the centre of the un- 

 deflected trace and the outside edge of the deflected trace 



gives the value of — . The value of Z 3 was 10*00 cms. ; the 



voltage 340, and D was 3*174 cms. Consequently the value 



of — - for the 



e 



rays 



after 



passing 



throu 



was 















mu 2 

 e 



= 3-10 



x 10 14 , 



We have previously 



shown that 









mu 



= 2-56) 



<10 5 , 



Therefore 















e 

 m 



= 4*7 x 



10 3 , 



and 



u = 1*21 x 10 9 cms. per second. 



We may thus conclude that the a particle from actinium 

 has the same mass as that from radium. 



Connexion of the a. particle with the helium atom. 

 We have seen that, within the limit of experimental error, 

 the mass of the a particle expelled from radium A, radium C, 

 radium F, or actinium B is the same. In a later paper, in 

 conjunction with Dr. Hahn, it will be shown that the mass of 

 the a particle expelled from thorium C is also identical with 

 that expelled from the radium products. We have also shown 

 in a previous paper that the amount of the magnetic deflexion 



