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



stopped by the plate AB, but other a particles previously 

 stopped by the plate CD are able to emerge. 



Suppose that the « particle in passing through the electric 

 field is deflected normally through a distance S represented 

 by FB. All the a particles which before the application of 

 the electric field passed through the point F now just emerge 

 at B at grazing incidence. The a particle which forms the 

 extreme edge of the photographic trace at / must obviously 

 be projected initially in the direction CF, and after emergence 

 will travel along the line B/. 



Since the normal acceleration of the a particle in passing 



Ye 

 through the electric field is 1 —. and the time occupied in 

 ° dm j 



passing between the charged plates is - , the distance 



£ S5 = S= fry- .—» , 



2dm u 2 ' 



s = \L J , where X= 7 — .-—. . 

 2 m du z 



At the moment of leaving the electric field, the tangent of 

 the angle 6, which the direction of motion makes with the 

 initial direction of projection CF, is given by tan0 = 2XZi. 



If the angle DCF = ^ 1 , the emerging ray makes an angle 

 + 6 X with the direction of the plates B b. The distance 

 bf=l d tan (0 + #i). Since the angles 6 and l are small, 



b/=k(e+d 1 ) 



(**+*?> 



In a similar way when the electric field is reversed, the 

 corresponding distance 



In this case, the a. particle which is most deflected enters 

 the electric field at grazing incidence at the point A. 



The distance D between the extreme edges //' of the 

 photographic impression is consequently given by 



~D=bf+ab+af 



/ 1 1 N 



= 4\l 1 l r rk{d-s)[j +j) + d- 



Substituting the value s=\l{ 2 , 



