﻿the a. particles expelled from Radium and Actinium. 359 



the mica screen, so that the electrostatic deflexion was 

 determined for the a particle from the unscreened wire. 



(2) The a particles passed through a mica screen equivalent 

 in stopping power to 3*5 cms. of air. The value of elm under 

 these conditions has been determined in the previous section. 



(3) The a particles passed through a screen of mica and 

 aluminium equivalent to about 6*5 cms. of air. 



The magnetic and electrostatic deflexion were separately 



mu 

 determined. The former gives the value of — and the 

 ,2 e 



latter 



The results of the measurements are collected 



in the following table, where D has the same meaning as 

 before. The value of L in all cases was 3*94 cms. 



Interposed absorbing 

 screen in terms of air. 



Volts. D. 



mu 

 e 



mu 2 

 e 



e 

 m 







3'5 cms. 



6-5 cms. 



340 

 340 



0-88 mm. 

 1-62 mm. 



4-06 X10 5 

 3-10 xlO 5 

 2-11 xlO 3 



9-4 XlO 14 

 4-87 XlO 14 

 2-11 xl0 u 



57 XlO 3 

 507 XlO 3 

 4-8 XlO 3 



The deflexion for the rays from the bare wire was small, but 

 could be measured with fair certainty. I think the value of ejm 

 obtained in this case, viz. 5*7 X 10 3 , is undoubtedly too high. On 

 removing the active wire after completion of the experiment. 

 it was noticed that its position was displaced somewhat to the 

 side of the opening of the parallel plates. This would tend 

 to make the observed width of the photographic trace too 

 small, and consequently to give too great a value of e/m, 

 when calculated from formula (1), which is based on the 

 assumption that the active source completely covers the 

 opening between the parallel plates. 



The impression on the photographic plate due to the u. 

 particles which have passed through an absorbing screen 

 equivalent to 6*5 cms. of air was weak but clearly defined, 

 and admitted of fairly accurate measurement. Allowing for 

 an error in the estimation of e/m for the a particles from the 

 bare wire, I think the agreement of the values of e/m obtained 

 under the different conditions is sufficiently close to prove 

 definitely that the value of e/m for the a. particle is unaltered 

 in its passage through matter. 



