218 JAMES CLEHK MAXWEU, 



ether it contains. As, then, we pass from the outside 

 of this conductor to its interior there is a sudden 

 change in some mechanical quantity connected with 

 the ether, and this change shows itself as a force of 

 attraction between the two conductors. Maxwell 

 called the change in structure, or in property, which 

 occurs when a dielectric is thus stored with electro- 

 static energy, KlcH rlc Displacement ; if we denote 

 it by I), then the electric force K is equal to 47rlVK, 

 and hence the energy in a unit of volume is 'l7r\)~/l\, 

 where K. is a quantity depending on the insulator. 



Now, 1), the electric displacement, is a quantity 

 which has direction as well as magnitude. Its value, 

 therefore, at any point can bo represented by a straight 

 line iu the usual way; inside a conductor it is zero. 

 The total change in 1), which takes place all over 

 the surface of a conductor as we enter it from the 

 outside measures, according to Maxwell, the total 

 charge on the conductor, At points at which the 

 lines representing 1) enter the conductor the charge 

 is negative; at points at which they leave it the 

 charge is positive; along the lines of the displacement 

 there exists throughout the ether a tension measured 

 by 27rl)7K; at right angles to these lines there is 

 a pressure of the same amount. 



In addition to the above the components of the 

 displacement D must satisfy certain relations which 

 can only be expressed in mathematical form, the 

 physical meaning of which it, is ditlicult to state in 

 non-mathematical language. 



When these relations are so expressed the problem 

 of finding the value of the displacement at all points 



