Measures for Electric and Magnetic Quantities. 439 



willing to drop the electrostatic system, constructed according 

 to Gauss' s principles, hitherto employed. The only thing he 

 says on this subject is contained in § 1 of his memoir. After 

 mentioning that the forces exerted upon each other by closed 

 electric currents may be regarded as indubitably known, he 

 continues: — "As, further, the small electric currents which 

 according to Ampere are to be assumed as existing in the 

 interior of a magnet are likewise closed, we have in magnetism 

 to do with forces of the same kind." Thereupon follows an 

 analysis, according to which the forces of two magnetic quanta 

 are to be regarded as dynamic. 



This reason, however, would be decisive only if it were cer- 

 tainly proved that Ampere's hypothesis corresponds to the 

 reality, while up to the present it can hardly be said that it 

 has been clearly and consistently worked out for all sorts of 

 diamagnetic and ferromagnetic bodies. In particular, this 

 hypothesis would also require that the hypothetic molecular 

 currents of magnetized bodies should show the changes which 

 are to be generated by electrodynamic induction, as they are 

 in fact logically assumed to do by W. Weber in his well- 

 known explanation of diamagnetic polarization. How this 

 can be reconciled with the properties of ferromagnetic bodies 

 I leave the adherents of this theory to explain. Meanwhile, 

 however interesting this theory may be, we may look upon it 

 as neither verified nor even completely worked out. 



Among the electrodynamic theories which assume direct 

 action at a distance, its quantity and direction depending on 

 the absolute or relative motions of two electric quanta, stands 

 that of Faraday and Maxwell. It has at least this superiority 

 to those, that it does not violate either the principle of the 

 finiteness and constancy of energy or that of the equality of 

 action and reaction ; and moreover it bases a theory of light, 

 free from many difficulties of the hitherto received undulation 

 theory, upon the identical hypotheses which form the ground- 

 work of electrodynamics. In order to discover the essential 

 character of the forces of electricity and magnetism, Maxwell 

 excludes those processes in which, according to the sort of 

 friction, heat is generated and electric or magnetic energy 

 lost, and founds his theory upon conservative processes only. 

 In particular, he excludes the conduction of electricity in con- 

 ductors, and the coercive force in magnets. Then, "however, 

 his fundamental equations present the most complete analogy, 

 not between magnets and moved electricity, but between 

 resting dielectric and resting magnetic polarization. It is 

 precisely to this analogy that Gauss's electrostatic system of 

 measures perfectly accommodates itself. 



