I08 ON DIELECTRICS. 



the original state, so that by interposing the dielectric the difference 

 of potential of two surfaces has become 



which gives the same result as the preceding. 



Looked at in this way, the coefficient /* acquires a physical 



signification; it is the ratio - - of the sum of the thicknesses 



a 



of two successive laminae to the thickness of that one which is 

 odd. Now, experiment shows that many solid dielectrics have a 

 specific inductive capacity of about 2, from which it follows that 



a 

 the ratio of the thicknesses of two successive laminae would be 



sensibly equal to unity. 



125. Experiment shows also, and the experiments of Gaugain 

 on this subject are particularly interesting, that the specific inductive 

 capacity varies with the time. It has first a minimum value at the 

 moment of charge ; it then increases rapidly and afterwards more 

 slowly, tending then towards a limit. In other words, the potential 

 of the inner coating of a condenser first diminishes rapidly after the 

 charge, and then more slowly. 



The force being zero in each of the ft, /?'..., we see in fact that 

 the positive layers are all impelled outwards and the others inwards, 

 and that in consequence of this mutual action the layers which 

 bound the laminae a tend to come nearer, which more and more 

 increases the inductive capacity. 



Generalising this reasoning, we are led to attribute to conductors 

 an infinitely great specific inductive capacity. 



126. MAXWELL'S THEORY OF DISPLACEMENT. In order to 

 explain the properties of dielectrics and to account for the phe- 

 nomena by the intervention of the medium only, Maxwell supposed 

 that when a dielectric is submitted to induction a phenomenon is 

 produced equivalent to a displacement or gliding of electricity in 

 the direction of the induction. For instance, in a Ley den jar 

 whose inner coating is charged positively, and outer negatively, the 

 displacement takes place in the substance of the glass from within 

 outwards. 



Any increase of the charge increases the displacement, and 

 corresponds to a current of positive electricity from the inside 

 towards the outside; any diminution, to a current going from the 



