and the Absolute Scale of Electric Potential. 15 



trie current possesses momentum*. He expressed himself as 

 of opinion that, were any such action to be discovered, we 

 should be able to regard " one of the so-called kinds of elec- 

 tricity" as a "real substance," and "we should be able to 

 describe the electric current as a true motion of this substance 

 in a particular direction." The experiment which Maxwell 

 chose wherewith to test this question consisted in starting a 

 current suddenly in a horizontal conducting ring or coil, sus- 

 pended by a thin wire in a space in which the horizontal com- 

 ponent of magnetic force was absent. He then expected to 

 find that, "if electricity were a fluid like water," then at the 

 moment of starting the current, and while its velocity was in- 

 creasing, there would be a mechanical reaction upon the con- 

 ducting coil or ring which would set the coil in rotation in the 

 opposite sense to that of the flow of the current, a cessation 

 of the current being likewise accompanied by a rotational 

 reaction in the same direction as the flow. No such pheno- 

 menon was, however, observed. It is rather difficult to un- 

 derstand how a mechanical reaction could have been expected. 

 To start a current in the coil requires the application of elec- 

 tromotive force, not of ponderomotive force. The suggestion 

 appears, indeed, to have arisen out of a dilemma which is stated 

 in art. 568 of Maxwell's treatise, as to whether the energy of 

 an electric current is " of that form which consists in the 

 actual motion of matter (kinetic energy), or of that which 

 consists in the capacity for being set in motion" (potential 

 energy). The solution to the dilemma is, that energy is not 

 necessarily of either of these kinds, but may also be either 

 electrokinetic or electropotential if of that form which consists 

 in the actual motion of electricity or of that which consists in 

 the capacity of electricity for being set in motion. We must, 

 of course, accept the view that a system containing an electric 

 current " is a seat of energy of some kind "—and that this 

 energy must be of a kinetic order " since we can form no 

 conception of an electric current except as a kinetic pheno- 

 menon " (Maxwell, art. 552). Yet its energy is certainly 

 not ponderokinectic, but electrokinetic, being the energy 

 which moving electricity has in virtue of its motion. 



One other consideration advanced by Maxwell, as against 

 the supposition that the electricity in the conductor may be 

 regarded as the moving body in which we are to find the 

 energy, is that whereas the (electrokinetic) energy of the cur- 

 rent depends upon the external medium and its conditions, as 

 well as upon the current itself, the (ponderokinetic) energy 

 of a moving mass " does not depend on anything external to 

 * ' Electricity and Magnetism/ art. 574. 



