568 Sir E. Rutherford and Mr. H. Robinson on Mass and 



The values of E/;?i obtained are seen to be in close accord. 

 The measurements made on the a rays from radium C are 

 probably the most accurate, as the photographic bands in 

 that case were the most sharply defined. 



Our final results for the value of E/wi for the a particle 

 fall between the extremes 4-813 x 10 3 and 4*826 x 10 3 , 

 with a mean value 4*82 x 10 3 , E being measured in absolute 

 electromagnetic units. This agrees with the calculated 

 value 4826 well within the limit of probable error, since out- 

 value depends upon both the electrostatic and magnetic 

 deflexion, and in the latter case the square of the result is 

 involved. 



As would be expected, the photographic traces corre- 

 sponding to the a rays of radium A and the emanation are 

 not so sharply defined as the radium C lines. Nevertheless, 

 some of the later photographs were sufficiently clear to 

 permit of the measurement of these lines with considerable 

 accuracy, and we were able to calculate the values of E/??i 

 for the a. particles of radium A and the emanation. The 

 values found were very nearly equal to the value for the 

 a particles from radium C. Also in general we found an 



excellent accord between the values of -=- deduced directly 



from the deviation by the magnetic field, and the value ob- 

 tained by correcting the initial velocity for the stopping- 

 power of the a-ray tube, which was kindly determined for 

 us by Mr. Marsden by the scintillation method. 



Summary of Results. 



From the measurements of the deflexion of the a rays in 

 magnetic and electric fields, we have found that for the 

 unscreened a particle from radium C 



^r =3*985 xlO 5 , 



and E/m = 4823 electromagnetic units. 



Comparing these results, the initial velocity of expulsion 

 of the a particle from radium C is given by 



v= 1*922 x 10 9 cm. per second. 



This is a value about 7 per cent, lower than the previously 

 accepted value. 



Assuming this value of v the initial velocity of expulsion 

 of the a particles from the various a-ray products can be at 



