The three divisions of Electromagnetism 3 



It has, however, been said that magnetostatic forces are found to be 

 produced by the motion of electric charges. Now if electric charges at rest 

 produce simple stresses in the ether, the motion of electric charges must be 

 accompanied by changes in the stresses in the ether. It is now possible to 

 identify magnetostatic force with change in the system of stresses in the 

 ether. This interpretation of magnetic force forms an essential part of 

 Maxwell's theory. If we compare the ether to an elastic material medium, 

 we may say that the electric forces must be interpreted as the statical 

 pressures and strains in the medium, which accompany the compression, 

 dilatation or displacement of the medium, while magnetic forces must be 

 interpreted as the pressures and strains in the medium caused by the motion 

 and momentum of the medium. Thus electrostatic energy must be regarded 

 as the potential energy of the medium, while magnetic energy is regarded as 

 kinetic energy. Maxwell has shewn that the whole series of electric and 

 magnetic phenomena may without inconsistency be interpreted as phenomena 

 produced by the motion of a medium, this motion being in conformity with 

 the laws of dynamics. More recently, Larmor has shewn how an imaginary 

 medium can actually be constructed, which shall produce all these phenomena 

 by its motion. 



The question now arises : If magnetostatic forces are interpreted as 

 motion of the medium, what properties are we to assign to the magnetic 

 bodies from which these magnetostatic forces originate ? An answer sug- 

 gested by Ampere and Weber needs but little modification to represent the 

 answer to which modern investigations have led. Recent experimental 

 researches shew that all matter must be supposed to consist, either partially 

 or entirely, of electric charges. This being so, the kinetic theory of matter 

 tells us that these charges will possess a certain amount of motion. Every- 

 thing leads us to suppose that all magnetic phenomena can be explained by 

 the motion of these charges. If the motion of the charges is governed by a 

 regularity of a certain kind, the body as a whole will shew magnetic pro- 

 perties. If this regularity does not obtain, the magnetic forces produced by 

 the motions of the individual charges will on the whole neutralise one 

 another, and the body will appear to be non-magnetic. Thus on this view 

 the electricity and magnetism which at first sight appeared to exist inde- 

 pendently in the universe, are resolved into electricity alone electricity 

 and magnetism become electricity at rest and electricity in motion. 



This discovery of the ultimate identity of electricity and magnetism is 

 by no means the last word of the science of Electromagnetism. As far back 

 as the time of Maxwell and Faraday, it was recognised that the forces at 

 work in chemical phenomena must be regarded largely if not entirely, as 

 electrical forces. Later, Maxwell shewed light to be an electromagnetic 



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