Electricity in Equilibrium, 56 



that the dissimulating power of a Leyden phial depends on the 

 nature of the glass of which it is made, as well as on its thick- 

 ness, have been by some attributed to a slight degree of con- 

 ducting power, or of penetrability, possessed by solid insulators. 

 This explanation, however, seems to be very insufficient ; and 

 besides, Faraday has estimated the nature of the effects of imper- 

 fect insulation, by independent experiments, and has established, 

 in what seems to be a very satisfactory manner, the existence of 

 a peculiar action in the interior of solid insulators when subjected 

 to electrical influence. As far as can be gathered from the 

 experiments which have yet been made, it seems probable that a 

 dielectric, subjected to electrical influence, becomes excited in 

 such a manner that every portion of it, however small, possesses 

 polarity exactly analogous to the magnetic polarity induced in 

 the substance of a piece of soft iron under the influence of a 

 magnet. By means of a certain hypothesis regarding the nature 

 of magnetic action*, Poisson has investigated the mathematical" 

 laws of the distribution of magnetism and of magnetic attrac- 

 tions and repulsions. These laws seem to represent in the most 

 general manner the state of a body polarized by influence, and 

 therefore, without adopting any particular mechanical hypothesis, 

 we may make use of them to form a mathematical theory of 

 electrical influence in dielectrics, the truth of which can only be 

 established by a rigorous comparison of its results with experi- 

 ment. 



Let us therefore consider what would be the effect, according 

 to this theory, which would be produced by the presence of a 

 solid dielectric, C, placed in the space between A and B, the rest 

 of which is occupied by air. The action of C, when excited by 

 the influence of the electricities on A and B, may (as Poisson has 

 shown for magnetism) be represented, whether on points within 

 or without C, by a certain distribution of positive electricity on 

 one portion of the surface of C, and of an equal quantity of 

 negative electricity on the remainder. The condition necessary 



* Faraday adopts the corresponding hypothesis to explain the action of 

 a sohd dielectric, which he states thus : — " If the space round a charged 

 globe were filled with a mixture of an insulating dielectric, as oil of tur- 

 pentine or air, and small globular conductors, as shot, the latter being at a 

 little distance from .each other, so as to be insulated, then these in their 

 condition and action exactly resemble what I consider to be the condition * 

 and action of the particles of the insulating dielectric itself. If the globe 

 were charged, these little conductors would all be polar ; if the globe were 

 discharged, they would all return to their normal state, to be polarized 

 again upon the recharging of the globe." — (Experimental Researches, 

 § 1679.) The results of the mathematical analysis of such an action are 

 given in the text. It may be added that the value of the coefficient k will 

 diflfer sensibly from unity if the volume occupied by the small conducting 

 balls bear a finite ratio to that occupied by the insulating medium. 



