Dynamical Theory of the Electromagnetic Field. 153 



The kind of motion attributed to the medium when transmitting 

 light is that called transverse vibration. 



An elastic medium capable of such motions must be also capable 

 of a vast variety of other motions, and its elasticity may be called 

 into play in other ways, some of which may be discoverable by their 

 effects. 



One phenomenon which seems to indicate the existence of other 

 motions than those of light in the medium, is that discovered by 

 Faraday, in which the plane of polarization of a ray of light is caused 

 to rotate by the action of magnetic force. Professor W. Thomson* 

 has shown that this phenomenon cannot be explained without ad- 

 mitting that there is motion of the luminiferous medium in the neigh- 

 bourhood of magnets and currents. 



The phenomena of electromotive force seem also to indicate the 

 elasticity or tenacity of the medium. When the state of the field is 

 being altered by the introduction or motion of currents or magnets, 

 every part of the field experiences a force, which, if the medium in 

 that part of the field is a conductor, produces a current. If the me- 

 dium is an electrolyte, and the electromotive force is strong enough, 

 the components of the electrolyte are separated in spite of their 

 chemical affinity, and carried in opposite directions. If the medium 

 is a dielectric, all its parts are put into a state of electric polari- 

 zation,^ a state in which the opposite sides of every such part are 

 oppositely electrified, and this to an extent proportioned to the inten- 

 sity of the electromotive force which causes the polarization. If 

 the intensity of this polarization is increased beyond a certain limit, 

 the electric tenacity of the medium gives way, and there is a spark 

 or " disruptive discharge." 



Thus the action of electromotive force on a dielectric produces an 

 electric displacement within it, and in this way stores up energy 

 which will reappear when the dielectric is relieved from this state of 

 constraint. 



A dynamical theory of the Electromagnetic Field must therefore 

 assume that, wherever magnetic effects occur, there is matter in motion, 

 and that, wherever electromotive force is exerted, there is a medium 

 in a state of constraint ; so that the medium must be regarded as the 

 recipient of two kinds of energy — -the actual energy of the magnetic 

 motion, and the potential energy of the electric displacement. Accord- 

 ing to this theory we look for the explanation of electric and mag- 

 netic phenomena to the mutual actions between the medium and the 

 electrified or magnetic bodies, and not to any direct action between 

 those bodies themselves. 



In the case of an electric current flowing in a circuit A, we know 

 that the magnetic action at every point of the field depends on its 

 position relative to A, and is proportional to the strength of the cur- 

 rent. If there is another circuit B in the field, the magnetic effects 

 due to B are simply added to those due to A, according to the well- 

 known law of composition of forces, velocities, &c. According to our 

 theory, the motion of every part of the medium depends partly on 

 the strength of the current in A, and partly on that in B, and 

 * Proceedings of the Royal Society June 1856 and June 1861. 



