308 Prof. J. J. Thomson on Cathode Rays. 



m v 



«2 



If, instead of the electric intensity, the rays are acted on 

 by a magnetic force H at right angles to the rays, and 

 extending across the distance /, the velocity at right angles 

 to the original path of the rays is 



Rev J 



m v' 



so that <£, the angle through which the rays are deflected 

 when they leave the magnetic field, is given by the equation 



6-— l 



m v 



From these equations we get 



<4F 

 V =6K 

 and 



m _ we . I 



e ~ F<f> 2 ' 



In the actual experiments H was adjusted so that <j> = 0; 

 in this case the equations become 



F 



m_Wl 

 e ~F0' 



The apparatus used to measure v and m/'e by this means is 

 that represented in fig. 2. The electric field was produced 

 by connecting the two aluminium plates to the terminals of a 

 battery of storage-cells. The phosphorescent patch at the end 

 of the tube was deflected, and the deflexion measured by a 

 scale pasted, to the end of the tube. As it was necessary to 

 darken the room to see the phosphorescent patch, a needle 

 coated with luminous paint was placed so that by a screw it 

 could be moved up and down the scale ; this needle could be 

 seen when the room was darkened, and it was moved until it 

 coincided with the phosphorescent patch. Thus, when light 

 was admitted, the deflexion of the phosphorescent patch 

 could be measured. 



The magnetic field was produced by placing outside the 

 tube two coils whose diameter was equal to the length of the 

 plates; the coils were placed so that they covered the space 



