A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 573 



Electric Absorption. 



(85) When the dielectric of which the condenser is formed is not a perfect 

 insulator, the phenomena of conduction are combined with those of electric dis- 

 placement. The condenser, when left charged, gradually loses its charge, and in 

 some cases, after being discharged completely, it gradually acquires a new charge 

 of the same sign as the original charge, and this finally disappears. These 

 phenomena have been described by Professor Faraday (Experimental Researches, 

 Series XL) and by Mr F. Jenkin (Report of Committee of Board of Trade on 

 Submarine Cables), and may be classed under the name of " Electric Absorption." 



/ 



(86) We shall take the case of a condenser composed of any number of 



parallel layers of different materials. If a constant difference of potentials between 

 its extreme surfaces is kept up for a sufficient tune till a condition of perma- 

 nent steady flow of electricity is established, then each bounding surface will 

 have a charge of electricity depending on the nature of the substances on each 

 side of it. If the extreme surfaces be now discharged, these internal charges 

 will gradually be dissipated, and a certain charge may reappear on the extreme 

 surfaces if they are insulated, or, if they are connected by a conductor, a certain 

 quantity of electricity may be urged through the conductor during the re- 

 establishment of equilibrium. 



Let the thickness of the several layers of the condenser be a a a., &c. 

 Let the values of k for these layers be respectively k u k a , k t , and let 



ajc t + ajc t + &c. =ak (50), 



where k is the "electric elasticity" of air, and a is the thickness of an equiva- 

 lent condenser of air. 



Let the resistances of the layers be respectively r u r i} &c., and let 

 r 1 + r t + &c. = r be the resistance of the whole condenser, to a steady current 

 through it per unit of surface. 



Let the electric displacement in each layer be f u f v &c. 

 Let the electric current in each layer be p u p v &c. 



Let the potential on the first surface be and the electricity per unit 

 of surface e,. 



