A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 531 



(11) But when electromotive force acts on a dielectric it produces a state 

 of polarization of its parts similar in distribution to the polarity of the parts 

 of a mass 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 opposite poles in opposite conditions'*. 



In a dielectric under the action of electromotive force, 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 electricity in a certain direction. This displacement 

 does not amount to a current, because when it has attained to a certain value 

 it remains constant, but it is the commencement of a current, and its varia- 

 tions constitute currents in the positive or the negative direction ac2ording as 

 the displacement is increasing or decreasing. In the interior of the dielectric 

 there is no indication of electrification, because the electrification of the surface 

 of any molecule is neutralized by the opposite electrification of the surface of 

 the molecules in contact with it ; but at the bounding surface of the dielectric, 

 where the electrification is not neutralized, we find the phenomena which indicate 

 positive or negative electrification. 



The relation between the electromotive force and the amount of electric 

 displacement it produces depends on the nature of the dielectric, the same 

 electromotive force producing generally a greater electric displacement in solid 

 dielectrics, such as glass or sulphur, than in air. 



(12) Here, then, we perceive another effect of electromotive force, namely, 

 electric displacement, which according to our theory is a kind of elastic yielding 

 to the action of the force, similar to that which takes place in structures and 

 machines owing to the want of perfect rigidity of the connexions. 



(13) The practical investigation of the inductive capacity of dielectrics is 

 rendered difficult on account of two disturbing phenomena. The first is the 

 conductivity of the dielectric, which, though in many cases exceedingly small, 

 is not altogether insensible. The second is the phenomenon called electric absorp- 



* Faraday, Experimental ResearcJias, Series XI. ; Mossotti, Mem. delta Soc. Italiana (Modena), 

 Vol. xxiv. Part 2, p. 49. 



672 



