654 MR. CLERK MAXWELL ON A COMPARISON OE TILE ELECTRIC UNITS 
in the polarized dielectric, we must also admit that within the dielectric there is a dis- 
placement 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 elec- 
tromotive force and the electric displacement, multiplied by the volume of that portion. 
It may also be shown 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 Eaeaday, 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 con- 
structed. 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 experi- 
ment. The experiment would certainly be a very delicate and difficult one. 
Let us now apply these four principles to the electromagnetic theory of light, consi- 
dered as a disturbance propagated in plane waves. 
Let the direction of propagation be taken as the axis of z, and let all the quantities 
be functions of z and of t the time ; that is, let every portion of any plane perpendicular 
to z be in the same condition at the same instant. 
Let us also suppose that the magnetic force is in the direction of the axis of y, and let 
/ 3 be the magnetic intensity in that direction at any point. 
Let the closed curve of Theorem A consist of a parallelogram in the plane y z, two of 
whose sides are b along the axis of y, and z along the axis of z. The integral of the 
magnetic intensity taken round this parallelogram is b((3 0 —{3), where /3 0 is the value of /3 
at the origin. 
Now let p be the quantity of electric current in the direction of x per unit of area 
taken at any point, then the whole current through the parallelogram will be 
§*bpdz, 
b(P 0 —(3)=4ir$*bpdz. 
and we have by (A), 
