542 JAMES CLERK MAXWELL. 



rendered positively and the other negatively electrical, but that 

 the electricity remains entirely connected with the molecule, and 

 does not pass from one molecule to another. 



The effect of this action on the whole dielectric is to pro- 

 duce a general displacement of the electricity in a certain direc- 

 tion. This displacement does not amount to a current, because 

 when it has attained a certain value it remains constant, but it 

 is the commencement of a current, and its variations constitute 

 currents in the positive or negative direction, according as the 

 displacement is increasing or diminishing. . . . When we find 

 electromotive force producing displacement in a dielectric, and 

 when we find the dielectric recovering from its state of electric 

 displacement with an equal electromotive force, we cannot help 

 regarding the phenomena as those of an elastic body, yielding to 

 a pressure and recovering its form when the pressure is removed. 



Suppose we have a body positively electrified. This 

 means that a displacement of the electricity in the medium 

 takes place in all directions around the body and away from 

 its surface. The cells are thus exposed to a shearing strain, 

 diminishing as the distance increases, because the surface 

 over which the displacement takes place being increased the 

 linear displacement of the electricity is proportionately 

 diminished, the particles of electricity behaving like a per- 

 fectly incompressible fluid. The medium being isotropic the 

 lines of electric displacement coincide with those of electric 

 stress, which stress is everywhere proportional to the dis- 

 placement. The distortion which the cells experience by 

 the pressure of the electric particles induces an elastic pres- 

 sure in all directions, at right angles to the direction of dis- 

 placement, so that there is a pressure in the medium at right 

 angles to the lines of force. 



Now suppose that we have two positively charged 

 bodies in the field, which we may suppose to possess equal 

 charges. Each produces a displacement of the medium 

 outwards from itself, but the electric particles behaving like 

 an incompressible fluid, it is clear that there can be no lines 

 of displacement from the one to the other, but that between 

 the bodies the lines of displacement will be curved so as to 

 avoid one another in the same way as the stream lines 



