CH. V.] 



OF CATHODE RAYS 



49 



method to assist in measuring the velocity and 

 other constants of the cathode ray particles. 



By noting the shift of the luminous spot, the 

 deflexion of the narrow beam which has travelled 

 through a length I of either an electric field of 

 strength E, or a magnetic field of strength H, can 

 be directly measured. 



If u is the original velocity of the ray particles, 

 travelling at right angles to one of the deflecting 

 fields, either of them will have a time l/u in which 



FIG. 4. Thomson's apparatus for observing and measuring the 

 electrostatic deflexion of cathode rajs. 



to act ; and in that time an extra velocity w 

 will be caused in the direction of the electric force, or 

 perpendicular to the direction of the magnetic force, 

 such that the rate of change of momentum of each par- 

 ticle will be jj- = Ee in the one case, and = juXieu in 



ijU 



the other ; wherefore the deflexion, if small, will be 



/ w e EZ . ,, 



= = s- in the one case, 



u m u 



and O f = - in the other. 



m u 



L.E. 



