Prof. E. Edlund on Unipolar Induction, 305 



between a and a , nor of that between k and k ; they can only 

 be determined by experiment. There is nevertheless a phy- 

 sical phenomenon which can make them known ; and that is 

 the development of electricity by friction. In this phenome- 

 non the aether molecules pass from the rubber to the body 

 rubbed, or vice versa. This passage of the molecules from one 

 side to the other can hardly have its cause except in the fact 

 that the repulsion exerted upon them by the body in motion 

 has been modified in some way by the motion itself. Now 

 the action of the moving body upon an aether molecule present 

 in the motionless rubber is given by the sum of the three ex- 

 pressions above cited. If now we put -^ = '-$■ = °, that 



a h m 



sum will be equal to ^; or, in other words, we get the 



same value for the repulsion as before the body was put in 

 motion. Therefore, in order that friction may be capable of 



producing electricity, — and -f- cannot have precisely the same 



a rC 



value. 



What has just been said applies to the case in which the 

 moving body and the rubber are in contact — that is to say, 

 when r is infinitely little. If the two bodies are at a certain 

 distance from one another, it is impossible to produce a sen- 

 sible development of electricity by the movement of one of 



them ; consequently — and -j- must be sensibly equal to 



as soon as r has a finite value. If, then, we put a body 



in motion together with the aether, both free and condensed, 

 which it contains, its action upon a motionless exterior aether 

 molecule at a given distance will be nearly as great as if the 

 body were at rest at the same distance. It can easily be shown 

 that the case is still the same when the exterior aether mole- 

 cule is itself in motion, and consequently forms an element of 

 a true galvanic current. A cylindrical tube put in rotation 

 about its axis will therefore not exert any electrodynamic 

 action ; or if it does so, at least the action will be insignificant. 

 In fact, the action of the currents due to the aether being 

 carried in the direction of the rotation is neutralized by the 

 opposite electrodynamic action produced by the motion of the 

 proper molecules of the body in the same direction*. 



* When deducing the formulae expressing the reciprocal action of two 

 galvanic current-elements while the conductors in which the electric fluid 

 is moving are at rest, we have assumed (op. cit. p. 12) that a = a and 



Phil Mag. S. 5. Vol. 6. No. 37. Oct. 1878. X 



