OBSERVATIONS ON THE FICTIVE LAYER. IOI 



118. OBSERVATIONS ON THE FICTIVE LAYER. Although the 

 layer of density o- is a fictive layer, it must be noticed that if, 

 while the dielectric is under induction, its surface is brought by 

 any means to the neutral state, by moving along it a flame con- 

 nected with the earth for instance, and if the sources of induction 

 are removed, a real layer of density <r will be found on this surface. 



This observation enables us to explain the phenomena exhibited 

 by certain bodies ; for instance, uniaxial pyroelectrical crystals such 

 as tourmaline. We need only suppose that the normal state of these 

 bodies is analogous to that which dielectrics acquire under the 

 influence of electrical forces in other words, that they are naturally 

 polarized, and that their state of polarization is a function of the tem- 

 perature. 



A tourmaline which in appearance is neutral, is a tourmaline 

 which, in virtue of its polarization, would exert on the outside the 

 same forces as a layer of total mass zero, and density a-, distributed 

 on the surface, but which from any causes for instance, losses by 

 contact with the surrounding medium has become covered by a 

 real layer of density - o-, which for any external point neutralises 

 the effect of internal polarization. If the temperature of the 

 tourmaline alters, its internal condition may be changed without 

 modifying the layer developed on the surface; equilibrium is 

 broken, and could only be restored more or less slowly under the 

 action of causes which had brought about the previous neutralisa- 

 tion ; the effect observed under these conditions is the difference 

 between the actions of the fictive and of the real layer. 



119. CHARGES OF Two CORRESPONDING ELEMENTS. The 

 theorem of corresponding elements (36) also holds when the two 

 conductors are placed in different media. This will be clear if we 

 remember that we can always imagine the conductor separated from 

 the dielectric by an infinitely thin layer of air, between the surface 

 of the conductor itself and an infinitely near equipotential surface. 



Let A and B be the two conductors, /*, and ft 2 the inductive 

 capacities of the dielectrics with which they are respectively in 

 contact. If the bounding surface S of the two dielectrics has no 

 real electrical layer, the flow of induction is the same throughout the 

 whole extent of an orthogonal tube which cuts, on the conductors 

 and on this surface, the elements ^S a , ^/S 6 , and ^S. 



The force which in air, near the first conductor, would be F a , 



p 

 becomes F x = in the dielectric. 



The apparent density <r' a on the conductor that is to say, that 



