Effects produced by the Motion of Electrified Bodies. 237 



The result we have just obtained (viz. that a moving body 

 charged with electricity produces the effect of an electric cur- 

 rent) shows that Prof. Rowland's experiments on electric con- 

 vection are in agreement with Maxwell's theory. 



§ 4. The fact that a moving body charged with electricity 

 produces a vector-potential in the field through which it is 

 moving, suggests a possible theory of the cause of the green 

 phosphorescence observed in vacuum-tubes at the places where 

 the molecular streams strike the glass, different from that put 

 forward by Mr. Crookes. It will be seen from the above work 

 that the moving particle produces a vector-potential whose 

 value depends on the velocity of the moving body. Now, 

 when a particle strikes the glass directly, its velocity is reversed 

 and the vector-potential changes sign ; thus during the short 

 time occupied by the collision the vector-potential must be 

 changing very rapidly. But any change in the vector-poten- 

 tial produces a corresponding electromotive force, and thus 

 the glass against which the molecules impinge is subjected to 

 a rapidly varying electromotive force. But this, if Maxwell's 

 electromagnetic theory of light be true, is exactly what it is 

 subjected to when a beam of light falls upon it, which we 

 know is the ordinary method of exciting phosphorescence. 

 Stokes's law, that the period of the vibrations exciting the 

 phosphorescence is smaller than the period of the emitted 

 light, compels us to assume that at some period of the collision 

 the velocity of the moving particle is changing at a greater 

 rate than the rate of vibration of green light : in our present 

 state of knowledge, however, there seems nothing impossible 

 in this. This, too, would explain the following difficulty: — 

 Since we have every reason for supposing the discharge in a 

 vacuum-tube to be discontinuous, the vector-potential due to 

 electricity moving through the tube will vary, producing a 

 varying electromotive force all over the tube ; another varying 

 electromotive force will be produced by the action of the 

 charge on the electrodes. Now it may be asked, why, if the 

 above theory be true, does not this variable electromotive force 

 make the whole tube phosphoresce, instead of the phospho- 

 rescence being confined to the places where the molecular 

 streams strike the glass. But Spottiswoode and Moulton have 

 proved (see Phil. Trans, for 1879, part 2) that the time occu- 

 pied by the negative discharge is greater than the time occupied 

 by the particles in going the length of the tube. Hence, even 

 if we made the extravagant assumption that these molecules 

 travel with a velocity as great as that of light, the time of dis- 

 charge, and consequently the period- of the electromotive force, 

 would be greater than the period of vibration of light whose 



