92 ON DIELECTRICS. 



bases. This amounts to saying that the lateral surface experiences 



P 

 a pressure p^ = - , and the bases a tension equal to the difference 



/! -/, that is to say equal to - . 



105. TENSION AND REPULSION OF LINES OF FORCE. If we 

 consider a layer bounded by two electrified equipotential surfaces, on 

 which we assume there are electrical masses capable of replacing the 

 action of bodies external to the layer, the two surfaces will attract 

 each other with a force equal to the general resultant of the tensions. 

 Dividing this layer into two by an orthogonal surface, a repulsion 

 will come into play, between the two portions, equal to the resultant 

 of the lateral pressures. 



It is easy to extend these considerations to the case in which 

 the second equipotential surface does not envelope the first. 



We may then suppose that conductors are connected with each other 

 by electric threads stretched along the lines of force ', and which repel 

 each other. This material representation of the phenomena is a 

 useful guide in a great number of applications. 



106. The foregoing properties are the mathematical translation 

 of the idea which Faraday formed for himself of the state of dielec- 

 trics, and which he himself summed up in the two following para- 

 graphs of his Experimental Researches, Series XL, 1297-1298 : 



"The direct inductive force which may be conceived to be 

 exerted in lines between two limiting and charged conducting 

 surfaces, is accompanied by a lateral or transverse force equivalent 

 to a dilatation or repulsion of these representative lines, (1224); 

 or the attractive force which exists amongst the particles of the 

 dielectric in the direction of the induction is accompanied by a 

 repulsive or a diverging force in the transverse direction (1304). 



" Induction appears to consist in a certain polarized state of the 

 particles, into which they are thrown by the electrified body sus- 

 taining the action, the particles assuming positive and negative points 

 or parts which are symmetrically arranged with respect to each other 

 and the inducting surfaces or particles. The state must be a forced 

 one, for it is originated and sustained only by force, and sinks to the 

 normal or quiescent state when that force is removed. It can be 

 continued only in insulators by the same portion of electricity, be- 

 cause they only can retain this state of the particles." 



107. ENERGY OF THE DIELECTRIC MEDIUM. From this point 

 of view, the whole energy of the electrical system must reside in the 

 dielectrical medium, and it is easy to calculate its value at any point. 



