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



for several days, for there is no necessity to continually watch 

 the state of the vacuum. 



On account of the small distance between the plates, there 

 is no necessity to correct for the disturbance of the electric 

 field near the ends of the plates. In addition, the parallel 

 plates acted as a slit in order to obtain a narrow pencil of 

 rays. In its passage through the electric field each a particle 

 describes a parabolic path, and after emergence travels in a 

 straight line to the photographic plate. By reversing the 

 electric field at intervals, the direction of deflexion of the 

 pencil of rays is reversed. v 



The general effect of the electric field in altering the ap- 

 pearance of the trace of the pencil of rays impinging on the 

 photographic plate is shown in fig. 2 (PL V.) A shows the 

 natural width of the line without an electric field, B for a P.D. 

 of 255 volts, for 340 volts, and D for 497 volts. These are 

 reproduced from the actual photographs (magnification about 

 1*4 times). When a small P.D. is applied, the natural width 

 of the photographic trace is broadened. Above a certain 

 voltage, the single band breaks into two. As the voltage is 

 further increased, the distance apart of these bands increases 

 while the width of each band steadily narrows. The outside 

 edge of each band is sharply defined, but it is difficult to fix 

 with certainty the inner boundary of the bands. 



Theory of the Experiment. 



The theory of the experimental arrangement where the 

 parallel plates act both as a slit and a means of applying the 

 electric field, is more complicated than the ordinary case 

 where a narrow pencil of a rays is made to pass between the 

 two parallel plates of the condenser without impinging on 

 the sides. 



A diagram of the experimental arrangement is shown in 

 fig. 3. AB and CD are the two charged parallel plates, and 

 C-B the radiant source which was of greater width than the 

 distance between the plates. It is required to find the width 

 of the trace on the photographic plate when a P.D. V is 

 applied between the plates. 

 Let m=mass of a particle, 



e = charge on a particle, 



u = velocity of a particle in passing between plates, 

 AB = Z 1; CD = Z a ,Bft = Z 8 , 

 d = distance between plates, 



D = distance between extreme edges of the photo- 

 graphic trace for reversal of the electric field. 



