94 ON DIELECTRICS. 



Hence the energy for unit volume is equal at every point to the 

 electrostatic pressure. 



108. SPECIFIC INDUCTIVE CAPACITY. If the dielectric does play 

 this essential part in phenomena, it is not likely that all media 

 behave in exactly the same manner. 



We know, in fact, since Franklin's experiments, that the nature 

 of the glass is of great importance in the construction of electrical 

 batteries. Cavendish had already made a great number of ex- 

 periments to determine directly the comparative effect of various 

 substances used as insulators in condensers, but his experiments 

 were unpublished and unknown at the time when Faraday published 

 his important researches. 



Faraday connected the coatings of two spherical Leyden jars 

 of the same dimensions, in one of which the insulating layer of air 

 had been replaced by a solid dielectric such as melted sulphur or 

 resin ; he thus found that when a definite charge of electricity was 

 imparted to this system of conductors it did not divide equally 

 between the two jars. That in which the dielectric was solid, took 

 the larger charge. 



This is a general phenomenon, and falls under a very simple law. 

 The charge, acquired by a closed condenser, with a solid or liquid 

 dielectric, is in a constant ratio with the charge which it would take, 

 for the same difference of potential, if the dielectric were replaced by 

 a layer of air. 



Experiment shows, in fact, that air and gases, even when moist, 

 behave in virtually the same manner, whatever be the pressure and 

 temperature. If the nature of the gas does exert an appreciable 

 influence, to which we shall subsequently refer, it may be neglected 

 in practice. 



The ratio thus determined, is what Faraday calls the specific 

 inductive capacity of the dielectric. It is, as we see, the number by 

 which the capacity of an air-condenser must be multiplied, to give 

 that of the same condenser, in which the layer of air has been 

 replaced by the dielectric in question. 



109. ELECTRICAL ABSORPTION. The determination of this 

 constant offers considerable difficulties for most substances, owing 

 to the occurrence of a phenomenon, to which Faraday gave the 

 name of electrical absorption, and which is due to the same cause as 

 the residual charge of condensers. The capacity of a condenser, in 

 which the dielectric is solid, appears as a function of the time ; it 

 increases and seems to tend towards a limit, in proportion as the 

 duration of the charge increases. Conversely, when the condenser 



