146 PHYSICAL SCIENCE 



affected by the presence of the magnetic field. If, 

 however, the electrometer plate be moved away 

 from the zuic plate till it lies beyond the path of 

 the ions, it will receive none of them, and the 

 establishment of the magnetic force should stop 

 completely the supply of negative electricity to the 

 electrometer. If AT be the electric force and H 

 the magnetic force, theory shows that no ions 

 should cross the space between the plates if 

 the distance between them exceeds 2XmjeH'^, 

 while below that distance the addition of the 

 magnetic force H should produce no effect on 

 the rate of gain of negative charge by the 

 electrometer. 



The experiments which Thomson carried out 

 by this method showed that no such sudden 

 change could be produced. As the distance was 

 diminished, or the magnetic field increased, at first 

 the effect of putting on or taking off the magnetic 

 force was small. Then a stage was reached at 

 which a considerable effect was produced ; while 

 finally, in a third stage, the magnetic force cut off 

 almost all the ions from the electrometer plate. 

 This somewhat gradual change is explained if we 

 suppose that the negative ions are not all formed 

 at the surface of the zinc plate, but that, as the 

 primary ions there produced move forward under 

 the action of the electric force, they produce new 

 ions by their collisions with the molecules of the 

 gas. The ions are thus formed, not exclusively 

 at the surface of the plate, but throughout a thin 

 layer of gas near the plate. This secondary pro- 

 duction of ions by primary ions moving with high 

 velocities occurs in many other cases, and has 

 been studied systematically by Townsend. It 



