PROFESSOR IN BERLIN 291 



goes a step farther in the development of the general theory. 

 Till now he had only dealt with the action of electrical currents 

 upon one another and upon conductors on the assumption that 

 all conductors were at rest, so that it was only the alterations 

 of current strength that had to be taken into consideration, 

 the potential (as extended by him) of two superposed current- 

 elements due to one another being definable as the work-value 

 of the electrical currents present in them. He now went on 

 to derive the equations of motion of electricity in moving 

 ponderable conductors from the same principles, and en- 

 deavours to show that his generalization of Neumann's law 

 of potential contradicts none of the known results which 

 referred almost exclusively to closed circuits, while at the 

 same time it agrees with the law of the conservation of energy. 

 On the other hand he did not extend the inquiry to the case 

 in which, besides the moving conductors, the dielectric polariz- 

 able media are also in motion, so that the electric motions 

 occurring in these are also electrodynamically active. 



In his three published papers on Electrodynamics Helmholtz 

 had thus in the first place expressed F. E. Neumann's law of 

 potential (which derived the strength of the induced currents 

 not from the action of one point on another, but from that 

 of one longitudinal element of the conductor on another), and 

 was able in this way to represent, all the phenomena of 

 closed circuits in quantitative agreement with the facts, more 

 simply than by Ampere's original law. For the usually much 

 weaker electrodynamic action of open currents, in which 

 electricity tends to accumulate at single points of the con- 

 ductor, he was able to show that the application of the potential 

 law never contradicted the universal 'axioms of mechanics, 

 wherein lay the great superiority of Neumann's law to all 

 other hypotheses of electrical action at a distance. It differed, 

 however, in one important particular from Faraday's assump- 

 tion, since electrodynamic action was only ascribed to the 

 passage of currents in the conductors, while the dielectric 

 charges generated in the insulators lying between the con- 

 ductors were not thought to be electrodynamically active. It 

 thus remained for Helmholtz to plan experiments which should 

 enable him to decide for one or other of the two hypotheses. 



In the memoir laid before the Berlin Academy in June, 1875, 



u 2 



