564 Prof. J. J. Thomson on 



through electric and magnetic fields the spot was not simply 

 deflected to another place, but was drawn out into bands or 

 patches, sometimes covering a considerable area. To deter- 

 mine the velocity o£ the rays and the value of e/m, it was 

 necessary to have a record o£ the shape o£ these patches. 

 This might have been done by substituting a photographic 

 plate for the Willemite screen. This, however, was not the 

 method adopted, as, in addition to other inconveniences, it 

 involves opening the tube and repumping for each obser- 

 vation, a procedure which would have involved a great 

 expenditure of time. The method actually adopted was as 

 follows : — The tube was placed in a dark room from which 

 all light was carefully excluded, the tube itself being painted 

 over so that no light escaped from it. Under these circum- 

 stances the phosphorescence on the screen appeared bright 

 and its boundaries well defined. The observer traced in 

 Indian ink on the outside of the thin flat screen the outline 

 of the phosphorescence. When this had been satisfactorily 

 accomplished the discharge was stopped, the light admitted 

 into the room, and the pattern on the screen transferred to 

 tracing-paper ; the deviations were then measured on these 

 tracings. 



Calculation of the Magnetic and Electric Deviation of 

 the Rays. 



If we assume the electric field to be uniform between the 

 plates and zero outside them, then we can easily show that ,c, 

 the horizontal deflexion of a ray whose charge is e, mass m, 

 and velocity v, is given by the equation 



where X is the force between the plates, I the length of path 

 of the rays between the plates, and d the distance of the 

 screen from the nearer end of the parallel plates. 



To find the deflexion due to the magnetic field, we have, 

 if p is the radius of curvature of the path at a point where 

 the magnetic force is H, 



mv 2 



= tier, 



P 

 or 



P mv 



