126 Dielectrics and Inductive Capacity [CH. v 



lines of force from the point charge terminate on this plane, so that there 

 must be a total charge e spread over it. If the plane were actually a 

 conductor this would be simply an induced charge. If, however, the plane 

 is the boundary of a dielectric, the charge differs from an induced charge on 

 a conductor in that it cannot disappear if the original charge e is removed. 

 For this reason, Faraday described it as a " bound " charge. The charge has 

 of course come to the dielectric through the conducting flame. 



MOLECULAR ACTION IN A DIELECTRIC. 



142. From the observed influence of the structure of a dielectric upon 

 the electric phenomena occurring in a field in which it was placed, Faraday 

 was led to suppose that the particles of the dielectric themselves took part 

 in this electric action. After describing his researches on the electric 

 action "induction" to use his own term in a space occupied by dielectric 

 he says* : 



" Thus induction appears to be essentially an action of contiguous parti- 

 cles, through the intermediation of which the electric force, originating or 

 appearing at a certain place, is propagated to or sustained at a distance...." 



" Induction appears to consist in a certain polarised state of the particles, 

 into which they are thrown by the electrified body sustaining the action, the 

 particles assuming positive and negative points or parts...." 



"With respect to the term polarity..., I mean at present... a disposition 

 of force by which the same molecule acquires opposite powers on different 

 parts." 



And again, laterf, 



" I do not consider the powers when developed by the polarisation as 

 limited to two distinct points or spots on the surface of each particle to be 

 considered as the poles of an axis, but as resident on large portions of that 

 surface, as they are upon the surface of a conductor of sensible size when it 

 is thrown into a polar state." 



" In such solid bodies as glass, lac, sulphur, etc., the particles appear to 

 be able to become polarised in all directions, for a mass when experimented 

 upon so as to ascertain its inductive capacity in three or more directions, 

 gives no indication of a difference. Now, as the particles are fixed in the 

 mass, and as the direction of the induction through them must change with 

 its charge relative to the mass, the constant effect indicates that they can 

 be polarised electrically in any direction." 



* Experimental Researches, 1295, 1298, 1304. (Nov. 1837.) 

 f Experimental Researches, 1686, 1688, 1679. (June, 1838.) 



