CHAPTER VIII 



THE DIELECTRIC. SPECIFIC INDUCTIVE 

 CAPACITY. RESIDUAL EFFECTS 



Specific inductive capacity Faraday's experiments Effect of specific 

 inductive capacity on the relations between electric quantities Condi- 

 tions to be satisfied where tubes of strain pass from one dielectric to 

 another Law of refraction Capacity of a condenser with a plate of 

 dielectric inserted The effect of placing a dielectric sphere in a pre- 

 viously uniform field Residual charge and discharge Mechanical model. 



FARADAY, as we have seen in Chapter IV, abandoned the old 

 method of regarding electric forces as due to direct action of the 

 charges at a distance, and sought to explain electric induction 

 by the action of contiguous particles on each other in the 

 dielectric, an action which he suppo>ed to be "the first step in 

 the process of elect rolysution." Taking this view of electric 

 induction, " there seemed reason to expect some particular relation 

 of it to the different kinds of matter through which it would be 

 exerted, or something, equivalent to a specific electric induction for 

 different bodies, which, if it existed, would unequivocally prove 

 the dependence of induction on the particles/* He was thus led 

 to the great discovery that the Quantity of electricity which a 

 condenser will receive when charged to a given potential that is, 



-ipacity depends on the nature of the dielectric. This implies 

 that the force which a given electric charge will exert depends on 

 the medium through and by which it acts. 



The nature of Faraday's discovery may be illustrated by 

 supposing that we have two exactly equal condensers of the same 

 dimensions, the plates in one being separated by air and those in 

 the other bv, say, ebonite. If each is charged to the same 

 potential difference, that with ebonite will have about two and a 

 li ilf times as great a charge as that with air as the dielectric, and 

 the ebonite is said to have two and a half times the specific induc- 

 tive capacity of air. 



A very simple experiment with a gold-leaf electroscope suffices 

 t<> -how the greater inductive capacity of ebonite. Let the 

 electroscope have a table on the top of the rod to which the gold 

 leaves are attached, and let there be a cover the size of the table 

 provided with an in-ulating handle. Put a thin plate of ebonite 

 * Exp. Re*. Ser. XI. i. p. 373 (November 1837). 



