Faraday. 209 



degree, between atoms which belong to different molecules, 

 thus producing the phenomena of cohesion. When an electric 

 field is set up, a change takes place in the distribution of these 

 forces ; some are strengthened and some are weakened, the 

 effect being symmetrical about the direction of the applied 

 electric force. 



Such a polarized condition acquired by a dielectric when 

 placed in an electric field presents an evident analogy to the 

 condition of magnetic polarization which is acquired by a mass 

 of soft iron when placed in a magnetic field ; and it was there- 

 fore natural that in discussing the matter Faraday should 

 introduce lines of electric force, similar to the lines of magnetic 

 force which he had employed so successfully in his previous 

 researches. A line of electric force he defined to be a curve 

 whose tangent at every point has the same direction as the 

 electric intensity. 



The changes which take place in an electric field when the 

 dielectric is varied may be very simply described in terms of 

 lines of force. Thus if a mass of sulphur, or other substance of 

 high specific inductive capacity, is introduced into the field, 

 the effect is as if the lines of force tend to crowd into it : as 

 W. Thomson (Kelvin) showed later, they are altered in the 

 same way as the lines of flow of heat, in a case of steady con- 

 duction of heat, would be altered by introducing a body of 

 greater conducting power for heat. By studying the figures of 

 the lines of force in a great number of individual cases, Faraday 

 was led to notice that they always dispose themselves as if they 

 were subject to a mutual repulsion, or as if the tubes of force 

 had an inherent tendency to dilate.* 



It is interesting to interpret by aid of these conceptions the 

 law of Priestley and Coulomb regarding the attraction between 

 two oppositely-charged spheres. In Faraday's view, the medium 

 intervening between the spheres is the seat of a system of 

 stresses, which may be represented by an attraction or tension 

 along the lines of electric force at every point, together with a 



* Exp. Res., 1224, 1297 (1837). 

 P 



