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



that these difficulties- would disappear if a homogeneous 

 pencil of a rays was employed. I showed that such a homo- 

 geneous pencil could be obtained by using as a source of rays 

 a small wire which had been made very active by exposure 

 to the radium emanation. Fifteen minutes after removal 

 from the emanation, radium A has been transformed, and the 

 a particles are then emitted only from radium C. 



An examination of the deflexion of the rays in a magnetic 

 field showed that such an active wire fulfilled the conditions 

 necessary for a homogeneous source of rays. The a particles 

 all escaped from the thin film of radioactive matter at the 

 same speed, and all suifered the same reduction of velocity 

 in passing through an absorbing screen. On account of the 

 rapid decay of the activity of the active deposit, it is necessary 

 to employ an intensely active wire to obtain a strong photo- 

 graphic effect. In most of the experiments described later,. 

 the active deposit was concentrated on the wire by making 

 it the only negatively charged surface in a vessel containing 

 a large quantity of the radium emanation. In this way, very 

 active wires were obtained which served as suitable sources 

 of homogeneous a rays. 



Electric Deflexion of the ol Rays. 



The determination of e/m and of the velocity of the ol 

 particle was made in the usual way by measuring the de- 

 flexion of a pencil of rays in passing through both a magnetic 

 and electric field of known strength. The method employed 

 for measuring the magnetic deflexion has already been de- 

 scribed in a previous paper. After some preliminary experi- 

 ments,, the following arrangement was adopted to determine 

 the deflexion of the a rays in passing through an electric 

 field. The rays from the active wire W (fig. 1), after tra- 

 versing a thin mica plate in the base of the brass vessel M r 

 passed between two parallel insulated plates A and B about 

 4 cms. high and 0*21 mm. apart. The distance between the 

 plates was fixed by thin strips of mica placed at the four 

 corners, and the plates were rigidly held together by rubber 

 bands. The terminals of a storage-battery were connected 

 with A and B so that a strong electric field could be produced 

 between the two plates. The pencil of rays, after emerging 

 from the plates, fell on a photographic plate P. The latter 

 was rigidly fixed to a ground-brass plate which fitted accu- 

 rately on the top surface of the vessel. The ground surfaces 

 were air-tight, an'd the photographic plate could thus easily 

 be placed in position or removed without disturbing the rest. 



