178 JAMES CLEIIK MAXWELL 



and the polarisation of dielectrics, I wish merely to 

 direct tho mind of tho reader to mechanical pheno- 

 mena, which will assist him in understanding tho 

 electrical ones. All such phrases in tho present 

 paper are to bo considered as illustrative and not as 

 explanatory." He then continues : 



" In speaking of the energy of the field, however, I wish to 

 be understood literally. All energy is the same as mechanical 

 energy, whether it exists in the form of motion or in that of 

 elasticity, or in any other form. 



"The energy in electro magnetic phenomena is mechanical 

 energy. The only question is, Where does it reside ? 



"On the old theories it resides in the electrified bodies, con- 

 ducting circuits, and magnets, in the form of an unknown 

 quality called potential energy, or the power of producing 

 certain effects tit a distance. On our theory it resides in tho 

 electro-magnetic field, in the space surrounding the electrified 

 and magnetic tiodies, as well as in those Ixxlies themselves, 

 and is in two different forms, which may he described without 

 hy|K>thesis as magnetic polarisation and electric polarisation, 

 or, according to a very probable hypothesis, as the motion and 

 the strain of one and the same medium. 



"The conclusions arrived at in the present pu|er are inde- 

 pendent of this hypothesis, being deduced from experimental 

 facts of three kinds : 



"(1) The induction of electric currents by the increase or 

 diminution of neighbouring currents according to the changes 

 in the lines of force passing through tho circuit. 



"(2) The distribution of magnetic intensity according to 

 the variations of a magnetic potential. 



44 (3) The induction (or influence) of statical electricity 

 through dielectrics. 



" We may now proceed to demonstrate from these principles 

 the existence and laws of tho mechanical forces, which act 

 upon electric currents, magnets, and electrified bodies placed 

 in the electro-magnetic field.' 1 



