AND ON THE ELECTROMAGNETIC THEORY OF LIGHT. 139 



THEOREM C. When a dielectric is acted on by electromotive force it ex- 

 periences what we may call electric polarization. If the direction of the elec- 

 tromotive force is called positive, and if we suppose the dielectric bounded by 

 two conductors, A on the negative, and B on the positive side, then the 

 surface of the conductor A is positively electrified, and that of B negatively. 



If we admit that the energy of the system so electrified resides in the 

 polarized dielectric, we must also admit that within the dielectric there is a 

 displacement of electricity in the direction of the electromotive force, the amount 

 of this displacement being proportional to the electromotive force at each point, 

 and depending also on the nature of the dielectric. 



The energy stored up in any portion of the dielectric is half the product 

 of the electromotive force and the electric displacement, multiplied by the volume 

 of that portion. 



It may also be shewn that at every point of the dielectric there is a 

 mechanical tension along the lines of electric force, combined with an equal 

 pressure in all directions at right angles to these lines, the amount of this 

 tension on unit of area being equal to the amount of energy in unit of volume. 



I think that these statements are an accurate rendering of the ideas of 

 Faraday, as developed in various parts of his "Experimental Researches." 



THEOREM D. When the electric displacement increases or diminishes, the 

 effect is equivalent to that of an electric current in the positive or negative 

 direction. 



Thus, if the two conductors in the last case are now joined by a wire, 

 there will be a current in the wire from A to B. 



At the same time, since the electric displacement in the dielectric is 

 diminishing, there will be an action electromagnetically equivalent to that of an 

 electric current from B to A through the dielectric. 



According to this view, the current produced in discharging a condenser 

 is a complete circuit, and might be traced within the dielectric itself by a 

 galvanometer properly constructed. I am not aware that this has been done, 

 so that this part of the theory, though apparently a natural consequence of 

 the former, has not been verified by direct experiment. The experiment would 

 certainly be a very delicate and difficult one. 



Let us now apply these four principles to the electromagnetic theory of 

 light, considered as a disturbance propagated in plane waves. 



182 



