292 HERMANN VON HELMHOLTZ 



1 Experiments on the Electromotive Forces induced in Open 

 Circuits by Motion/ he describes the experiments made with 

 this object on the electricity that accumulates at the surface 

 of a conductor rotating in the magnetic field. By the ordinary 

 laws of induction, electromotive force must be induced in any 

 conductor that is thrown into rotation round the axis of 

 a magnet, which does not follow from the law of potential 

 alone; and Helmholtz set himself to test the discrepancy 

 between the two theories experimentally. On the assumption 

 that the universalized law of potential of Neumann (con- 

 sidering only the motions of electricity occurring relatively 

 to the conductor in the conductors proper) gave a complete 

 statement of the law of electrodynamic action, the ex- 

 perimental results did not agree with Neumann's law of 

 induction. This discrepancy only disappears when the po- 

 tential law is combined with Faraday's view, that the dielectric 

 polarization which occurs in the insulators between two con- 

 ductors in process of being charged is an electric motion, 

 equivalent in intensity and in electrodynamic effect to the 

 current with which either portion of the conductor is charged. 

 All other theories, in which forces at a distance (with intensities 

 depending upon the distances, velocities, and accelerations) 

 have to be assumed, correspond fully with the phenomena 

 of closed currents, but they are contradictory to the universal 

 axioms of dynamics, when they are applied to open currents. 

 Weber's hypothesis results in unstable electrical equilibrium 

 in every conductor of moderate tri-dimensional extension, so 

 that no practicable laws can be derived from it for the motion of 

 electricity in materially extended conductors. The same applies 

 to Riemann's law, which, moreover, contradicts the axiom of 

 equality of action and reaction, while Clausius's hypothesis, 

 which is free from these errors, has to resort to a space-filling 

 medium, between which and the electricities the forces he 

 postulates come into play. 



Thus Helmholtz was led to recognize Faraday's hypothesis 

 as the only one that agreed with the observed facts, and was 

 in no way contradictory to the universal laws of dynamics. 

 While Clerk Maxwell had actually worked out the theory for 

 the case of closed circuits only, Helmholtz found that it also 

 agreed with the few facts that were then known for open 



