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LXI. The Deflexion of a Rays from Radium and Polonium. 

 By A. Stanley Mackenzie, Munro Professor of Physics, 

 Dalhousie University, Halifax, JW.S.* 

 [Plate VIII.] 

 HE following investigation was begun with two aims in 



T 



view : first, to measure from the deviations of a beam 

 of a. rays in a magnetic and in an electrostatic field the 

 value of the velocity v, and of the ratio of the charge e on an 

 <x particle to its mass m, for the rays from radium and from 

 polonium ; and, secondly, to see if there w r ere any evidence 

 of a change in any of the quantities v, e, or m as the rays 

 travelled in an ordinarily good vacuum. 



The former problem for the case of radium was first treated 

 by Rutherford f, using an electroscope to determine the 

 deviations ; and later by Des Coudres J, who let the rays fall 

 directly upon a photographic plate. Both used radium in 

 radioactive equilibrium, and this method was followed in the 

 present case ; but it has the disadvantage that the beam of a 

 rays not being homogeneous, it gives only the mean velocity 

 of the rays. The method employed by Rutherford was 

 admittedly not well adapted to give accurate numerical 

 determinations ; and Des Coudres states that too much stress 

 must not be laid on the accuracy of his electrostatic experi- 

 ment. In a paper just published Rutherford § describes some 

 very suggestive experiments made on the magnetic deflexion 

 of the a rays from radium C, and promises to give later similar 

 results for the deflexion in an electric field. The great 

 advantage gained by the use of a thin coating of radium C 

 on a wire is that the rays all leave the wire with the same 

 velocity, and the beam is accordingly homogeneous. An 



e 

 accurate determination of the values of v and — is to be 



expected from his completed measurements. 



An effort was made to find a way of observing the position 

 of the beam of rays coming through two slits without having 

 to let them fall directly upon a photographic plate, since in the 

 case of a path longer than a few centimetres this necessitates 

 the use of a closed vessel and the production of a vacuum 

 each time an observation has to be made. After experiment- 

 ing with a thin layer of powdered zinc sulphide on glass, 

 and finding that the scintillations were still quite brilliant 

 when observed after transmission through the glass, I adopted 



* Communicated by Prof. J. J. Thomson. 

 t Phil. Mag-. Feb. 1903, p. 177. 

 % Physik. Zeitschr. iv. p. 483 (1903). 

 § Phil. Mag-. July 1905, p. 163. 



