572 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



PART V. 



THEORY OF CONDENSERS. 



Capacity of a Condenser. 



(83) The simplest form of condenser consists of a uniform layer of insulating 

 matter bounded by two conducting surfaces, and its capacity is measured by the 

 quantity of electricity on either surface when the difference of potentials is unity. 



Let S be the area of either surface, a the thickness of the dielectric, and 

 k its coefficient of electric elasticity; then on one side of the condenser the 

 potential is and on the other side t +l, and within its substance 



<> 



Since -j and therefore f is zero outside the condenser, the quantity of electricity 

 on its first surface = Sf, and on the second +Sf. The capacity of the con- 

 denser is therefore Sf=-r in electromagnetic measure. 



Specific Capacity of Electric Induction (D). 

 (84) If the dielectric of the condenser be air, then its capacity in electro- 



o 



static measure is (neglecting corrections arising from the conditions to be 



fulfilled at the edges). If the dielectric have a capacity whose ratio to that of 



DS 



air is D, then the capacity of the condenser will be - - . 



4 Trot 



Hence D-i (49), 



where k t is the value of k in air, which is taken for unity. 



