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



electrode. It is suggested that radium A, which is respon- 

 sible for this constant activity, is capable of forming a par- 

 tition between the metal surfaces and the space in the 

 apparatus, at ordinary temperatures. 



In conclusion I wish to express my best thanks to Pro- 

 fessor Sir Ernest Rutherford for his kind interest in the 

 work. 



The Physical Laboratories, 



The University, Manchester, 



June 1914. 



L VII. The Mass and Velocities of the a Particles from Radio- 

 active Substances. By Sir E. Rutherford, F.R.S., and 

 H. Robinson, University of Manchester* '. 



[Plate XII.] 



IT was shown by Rutherfordf in 1903 that the a. rays emitted 

 from a radium preparation consisted of a stream of 

 positively electrifiod particles moving with a high velocity, 

 which were deflected in their passage through a magnetic or 

 electric field. The rays employed were heterogeneous ; but 

 by comparing with an electric method the deflexions in mag- 

 netic and electric fields of known intensities an approximate 

 estimate was made of the average velocity of the a particle 

 and of the value E/m, the ratio of the charge to the mass of 

 the a particle. The velocity was found to be 2*5 x 10 9 cm. 

 per second, and E/??i = 6000 e.m. units. These results were 

 soon afterwards confirmed by Des Coudres % using the photo- 

 graphic method. He found a value E/??? = 6400 for the 

 a. particles from radium. Later experiments were made by 

 Mackenzie § and Huff || using polonium as a homogeneous 

 source of a rays. The former found a value of E/??i = 4699, 

 and the latter E/m = 4300. 



In 1906 Rutherford^] repeated his experiments, using a 

 photographic method and employing several distinct sources 

 of homogeneous a rays. The electric deflexion obtained 

 under the experimental conditions was small and difficult to 

 measure with certainty. The value E/m= 5070 was obtained. 



* Communicated by the Authors. This paper was communicated to 

 the Kaiserlich. Akad, der Wissensch. in Wien, in October 1913, and read 

 at the Meeting of Dec. 4th, 1913. 



t Butherford, Phys. Zeit. iv. p. 235 (1903). 



X Des Coudres, Phys. Zeit. iv. p. 483 (1903). 



§ Mackenzie, Phil. Mag. x. p. 538 (1905). 



II Huff, Proc. Rov. Soc. A. Ixxviii. p. 77 (1906). 



IT Rutherford, Phil. Mag. xii. p. 348 (1906). 



