62 BEPORT— 1891. 



Force of repulsion 



— — . _^1^^ (The law of the mverse square.) 

 From this and the foregoing results it is evident that 



— is -^ — i.e., the reciprocal of the specific inductive capacity 

 47r K 



of the medium. 



It is worth noticing that it is only when the radius of each 



sphere is small compared to the distance apart of the two 



spheres that the law of the inverse square holds. 



6. It is not, perhaps, at once evident what corresponds in 

 this " elastic medium " theory to the " force at any point due 

 to the attraction of an electrical system," and in particular 

 to the law^ that just outside a conductor the attractive force 



:= 47rp, or, more strictly, -~. 



But it must be remembered that these " forces " are only 

 mathematical quantities, and have no real existence. The 

 " force at any point due to the attraction of an electrical sys- 

 tem " is the attraction on a unit of electricity sujjposing the 

 jjresence of that unit not to disturb the ^^Te-existinrj distrihution 

 of electricity — an impossible condition. 



Instead of the law that just outside a conductor the force 



= -T?^, we have the simple and obvious fact that, if x be the 



distance the dielectric is pushed back, there is a force of resti- 

 tution per unit volume equal to Ex. 



And the "force at any point" corresponds to the actual 

 force of restitution per unit volume due to the strain existing 

 at that point. 



Or, more simply, force == E x displacement. 



7. The conception of the action of a condenser, and the 

 mathematical theory of it, become very simple. 



A and B are two con- 

 centric spherical conductors, 

 enclosing a shell of dielectric 

 between them. B is in com- 

 munication with the earth. 



A quantity Q of rether 

 being pumped into A, the 

 dielectric round it is pushed 

 back and encroaches on B, 

 also to an amount Q. But 

 the excess of aether in B 

 runs away; so the dielectric 

 outside B is not stra,ined, and the whole of the strain 



