14 Prof. Maxwell on the Theory of Molecular Vortices 



not flow through them, electrical effects are propagated through 

 them, and the amount of these effects differs according to the 

 nature of the body; so that equally good insulators may act 

 differently as dielectrics*. 



Here then we have two independent qualities of bodies, one 

 by which they allow of the passage of electricity through them, 

 and the other by which they allow of electrical action being 

 transmitted through them without any electricity being allowed 

 to pass. A conducting body may be compared to a porous 

 membrane which opposes more or less resistance to the passage 

 of a fluid, while a dielectric is like au elastic membrane which 

 may be impervious to the fluid, but transmits the pressure of the 

 fluid on one side to that on the other. 



As long as electromotive force acts on a conductor, it produces 

 a current which, as it meets with resistance, occasions a continual 

 transformation of electrical energy into heat, which is incapable 

 of being restored again as electrical energy by any reversion of 

 the process. 



Electromotive force acting on a dielectric produces a state of 

 polarization of its parts similar in distribution to the polarity of 

 the particles of iron under the influence of a magnetf, and, like 

 the magnetic polarization, capable of being described as a state 

 in which every particle has its poles in opposite conditions. 



In a dielectric under induction, we may conceive that the 

 electricity in each molecule is so displaced that one side is ren- 

 dered positively, and the other negatively electrical, but that the 

 electricity remains entirely connected with the molecule, and 

 does not pass from one molecule to another. 



The effect of this action on the whole dielectric mass is to 

 produce a general displacement of the electricity in a certain 

 direction. This displacement does not amount to a current, 

 because when it has attained a certain value it remains constant, 

 but it is the commencement of a current, and its variations con- 

 stitute currents in the positive or negative direction, according 

 as the displacement is increasing or diminishing. The amount 

 of the displacement depends on the nature of the body, and on 

 the electromotive force ; so that if h is the displacement, R the 

 electromotive force, and E a coefficient depending on the nature 

 of the dielectric, R _ _ ^^ h . 



and if r is the value of the electric current due to displacement, 



dh 

 r=-r* 

 dt 



* Faraday, ' Experimental Researches,' Series XI. 

 t See Prof. Mossotti, "Discussione Analitica," Memorie delta Soc. It a- 

 liana (Modena), vol xxiv. part 2. p. 49. 



