from Faraday tcr J. J . Thomson. 405 



Thomson's lecture drew from Fitz Gerald* the suggestion 

 that " we are dealing with free electrons in these cathode rays " 

 a remark the point of which will become more evident when 

 we come to consider the direction in which the Maxwellian 

 theory was being developed at this time. 



Shortly afterwards Thomson himself published an accountf of 

 experiments in which the only outstanding objections to the 

 charged-particle theory were removed. The chief of these was 

 Hertz' failure to deflect the cathode rays by an electrostatic 

 field. Hertz had caused the rays to travel between parallel 

 plates of metal maintained at different potentials ; but Thomson 

 now showed that in these circumstances the rays generate 

 ions in the rarefied gas, which settle on the plates, and annul 

 the electric force in the intervening space. By carrying the 

 exhaustion to a much higher degree, he removed this source of 

 confusion, and obtained the expected deflexion of the rays. 



The electrostatic and magnetic deflexions taken together 

 suffice to determine the ratio of the mass of a cathode particle 

 to the charge which it carries. For the equation of motion of 

 the particle is 



rar = eE .+ e[v. H], 



where r denotes the vector from the origin to the position of 

 the particle ; E and H denote the electric and magnetic forces ; 

 e the charge, m the mass, and v the velocity of the particle. 

 By observing the circumstances in which the force #E, due to 

 the electric field, exactly balances the force e [v . H], due to the 

 magnetic field, it is possible to determine v ; and it is readily 

 seen from the above equation that a measurement of the 

 deflexion in the magnetic field supplies a relation between v 

 and m/e ; so both v and m/e may be determined. Thomson 

 found the value of m/e to be independent of the nature of the 

 rarefied gas : its amount was 10~ 7 (grammes/electromagnetic units 

 of charge), which is only about the thousandth part of the value 

 of m/e for the hydrogen atom in electrolysis. If the charge 



* Electrician, May 21, 1897. t Phil. Mag. xliv (1897), p. 298. 



