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



themselves. The values for 171 and 255 volts are calculated 

 from formula (1), the natural width of the photographic 

 trace being 0*61 mm., and for the higher voltages from 

 formula (2). Some of the photographs from which the 

 measurements were made are reproduced in fig. 2, magnifi- 

 cation about 1*4. 



Two good photographs were obtained with a P.D. of 

 340 volts. In each case two active wires were used suc- 

 cessively to give a strong photographic impression. On 

 account of the greater distance, the photographic impression 

 was not so strongly marked for the distance 10 cms. 



mu 

 Giving a weight 1 to the measurement of ■ for 497 



volts, and a weight 2 for both 340 and 508*6 volts, the mean 

 value is given by 



9. 



= 4-87 x 10 14 electromagnetic units. ... (3) 



e 

 By measurement of the magnetic deflexion, the maximum 



771 U 



value of - — for the a rays from radium C was found to be 

 4*06 x 10 5 . The mica screen cut down the velocity of the 

 rays to '763 of the initial velocity, so that the value of — 

 for the rays which passed through the electric field is given 



^=3'10xl0 5 (4) 



By combining equations (3) and (4) 



u = l'57 x 10 9 cms. per second, 

 e/m= 5'07 x 10 3 electromagnetic units. 



I think that the values of u and e/m are certainly correct 

 within two per cent. 



The initial velocity of the a particles expelled from radium C 

 is consequently 2'06 X 10 9 cms. per second. 



Does the value of e/m for the a particle vary in its passage 

 through matter ? 



In order to test this point, the value of e/m for the a particle 

 was determined under the following conditions : — 



(1) The active wire was placed on top instead of under 



