ON PHYSICAL LINES OF FORCE. 491 



while a dielectric is like an 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 electri- 

 cal 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 magnet*, 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 rendered 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 displace- 

 ment 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 constitute 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 = - 



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



_dh 

 ~dt' 



These relations are independent of any theory about the internal mechanism 

 of dielectrics ; but when we find electromotive force producing electric displace- 

 ment in a dielectric, and when we find the dielectric recovering from its state 

 of electric displacement with an equal electromotive force, we cannot help 



* See Prof. Mossotti, " Discussione Analitica," Memorie della Soc. Italiana (Modena), Vol. xxiv. 

 Part 2, p. 49. 



622 



