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



Value of e/mfor the a particles from Radium A. 



In a previous paper (loc. cit.) I gave the results of the measure- 



mu 

 ments by the photographic method of for the a particle 



emitted from radium A. For the unscreened wire the value 



mil 

 of ■ — =3*67 x 10 5 , and for the wire covered with a mica 

 e 



plate of the same thickness as that over the opening in the 



base of the electrostatic apparatus, I found the value of 



— =2-19 xlO 5 . 



e 



I pointed out in that paper the difficulty of accurately 

 measuring the magnetic deflexion of the a. particles from 

 such a rapidly changing product as radium A, which is half 

 tran sformed in t hree minutes. The difficulty of obtainin g 

 a sufficiently marked photographic impression, in order to 

 measure the electric deflexions of the rays, was still greater. 

 It was found necessary to place twenty active wires succes- 

 sively in position under the base of the apparatus, in order to 

 obtain a measurable darkening of the photographic plate. 

 Each wire was exposed for two minutes as negative electrode 

 in a vessel containing a large quantity of the radium eman- 

 ation. Eor such a short exposure, the initial radiation from 

 the wire mainly comes from radium A. The wire was 

 rapidly removed from the emanation vessel and placed in 

 position and left for six minutes. In that time, the a ray 

 activity of radium A is reduced to one quarter of its initial 

 value. The a rays from the active wire passed through the 

 standard mica plate before entering the electric field. The 

 electric deflexion of the a. rays from radium A is considerably 

 greater than that for the swifter a particles from radium C ; 

 so that there is no danger of confusion between the two types 

 of rays, even though the photographic impression of the 

 rays from radium C present on the active wire is comparable 

 with that due to the rays from radium A. 



In the experiment the voltage was 255 ; / 3 = 3"94 cms. ; 

 -and 1) = 1'30 mm. This gave 



mu 



= 2-61x10 



14 



The value for the a rays from radium A after traversing 



the standard mica was 2*19 x 10 5 . This gives the values 

 u = 1*22 x 10 9 cms. per sec. 

 elm = 5-6 x 10 3 . 



