494 PEOFESSOE CLEEK MAXWELL ON THE ELECTEOMAGNETIC FIELD. 
Specific Capacity of Electric Induction (D). 
(84) If the dielectric of the condenser be air, then its capacity in electrostatic mea- 
S 
sure is ^ (neglecting corrections arising from the conditions to be fulfilled at the 
edges). If the dielectric have a capacity whose ratio to that of air is D, then the capa- 
DS 
city of the condenser will be — • 
Atta 
Hence D=^a, ..... ■ (49) 
where k 0 is the value of k in air, which is taken for unity. 
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 displacement. 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 XI.) and by Mr. F. Jenkin (Report of Com- 
mittee 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 time till a condition of permanent 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 con- 
nected by a conductor, a certain quantity of electricity may be urged through the con- 
ductor during the reestablishment of equilibrium. 
Let the thickness of the several layers of the condenser be a x , a 2 , &c. 
Let the values of k for these layers be respectively k 2 , k 3 , and let 
ajc 2 -\-a 2 k 2 -f &c. =ak, (50) 
where k is the “ electric elasticity” of air, and a is the thickness of an equivalent con- 
denser of air. 
Let the resistances of the layers be respectively r„ r 2 , &c., and let r x -\-r 2 - & 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 befi,f 2 , &c. 
Let the electric current in each layer be p x ,p 2 , &c. 
Let the potential on the first surface be 'P,, and the electricity per unit of surface e t . 
Let the corresponding quantities at the boundary of the first and second surface be 
’Pa and e 2 , and so on. Then by equations (G) and (H), 
