AND MODERN PHYSICS. 183 



Several writers in recent years adopt these equations 

 as the fundamental relations of the field, establishing 

 them by the argument that they lead to consequences 

 which are found to bo in accordance with experiment 



Wo have endeavoured to give some account of 

 Maxwell's historical method, according to which the 

 equations are deduced from the laws of electric 

 currents and of electro-magnetic induction derived 

 directly from experiment 



While the manner in which Maxwell obtained 

 his equations is all his own, ho was not alone in 

 stating and discussing general equations of tho electro- 

 magnetic field. Tho next steps which wo are about 

 to consider are, however, in a special manner duo 

 to him. An electrical or magnetic system is tho 

 scat of energy ; this energy is partly electrical, partly 

 magnetic, and various expressions can be found for 

 it. In Maxwell's theory it is a fundamental assump- 

 tion that energy has position. "The electric and mag- 

 netic energies of any electro-magnetic system," says 

 Professor Poynting, "reside, therefore, somewhere in 

 tho field." It follows from this that they are present 

 wherever electric and magnetic force can be shown to 

 exist Maxwell showed that all tho electric energy is 

 accounted for by supposing that in tho neighbourhood 

 of a point at which the electric force is II there is 

 an amount of energy per unit of volume equal to 

 KH'/S^, K being tho inductive capacity of tho 

 medium, while in tho neighbourhood of a point at 

 which the magnetic force is H, the magnetic energy 

 per unit of volume is /iH 2 /87r, p being the per- 

 meability. Ho supposes, then, that at each point of 



