548 Prof. J. J. Thomson on the Masses of 



see rny way to do this ; but another case, where negative 

 electricity is carried by charged particles (i. e. when a nega- 

 tively electrified metal plate in a gas at low pressure is 

 illuminated by ultra-violet light), seemed more hopeful, as in 

 this case we can determine the value of e by the method I 

 previously employed to determine the value of the charge 

 carried by the ions produced by Rontgen-ray radiation (Phil. 

 Mag. Dec. 1898). The following paper contains an account 

 of measurements of m/e and e for the negative electrification 

 discharged by ultra-violet light, and also of m/e for the negative 

 electrification produced by an incandescent carbon filament in 

 an atmosphere of hydrogen. I maybe allowed to anticipate the 

 description of these experiments by saying that they lead to the 

 result that the value of m/e in the case of (he ultra-violet light, 

 and also in that of the carbon filament, is the same as for the 

 cathode rays ; and that in the case of the ultra-violet light, 

 e is the same in magnitude as the charge carried by the 

 hydrogen atom in the electrolysis of solutions. In this case, 

 therefore, we have clear proof that the ions have a very much 

 smaller mass than ordinary atoms ; so that in the convection of 

 negative electricity at low pressures we have something 

 smaller even than the atom, something which involves the 

 splitting up of the atom, inasmuch as we have taken from 

 it a part, though only a small one, of its mass. 



The method of determining the value of m/e for the ions 

 carrying the negative electrification produced by ultra-violet 

 light is as follows : — Elster and Geitel (Wied. Ann. xli. 

 p. 166) have shown that the rate of escape of the negative 

 electrification at low pressures is much diminished by mag- 

 netic force if the lines of magnetic force are at right angles 

 to the lines of electric force. Let us consider what effect 

 a magnetic force would have on the motion of a negatively 

 electrified particle. Let the electric force be uniform and 

 parallel to the axis of x, while the magnetic force is also 

 uniform and parallel to the axis of z. Let the pressure be so 

 low that the mean free path of the particles is long compared 

 with the distance they move while under observation, so that 

 we may leave out of account the effect of collisions on the 

 movements of the particles. 



If m is the mass of a particle, e its charge, X the electric 

 force, H the magnetic force, the equations of motion are : — 



d' 2 x ^ T TT dy 



m dW =Xe -^ e dt' 



d 2 y n dx 

 m de= Re dt' 



