360 ELECTROSTATICS AND MAGNETISM. [PT. II. CH. IX. 



184. Capacity. Reluctance. In the electric case we have 

 for the capacity of a condenser whose dielectric is homogeneous, so 

 that AF=0, 



ff 3F 

 ^ fji^-dS 



^= = 



that is, the capacity equal to that found in Chapter VI multiplied 

 by the inductivity of the medium. This is Faraday's capital dis- 

 covery,* leading to the development of the theory according to 

 which the energy resides in the medium, so that electrical actions 

 are transmitted by means of the medium, and not by action at a 

 distance. Faraday experimented with condensers in the form of 

 concentric spheres, the intervening space being filled by the 

 dielectric in question. The material of the dielectric outside the 

 larger sphere was accordingly immaterial. Instead of capacity the 

 term permittance has been proposed by Heaviside. 



In the magnetic case, the value of the quantity analogous to 

 the capacity has been called the permeance or inductance, while its 

 reciprocal, corresponding to the resistance in the case of electric 

 flow, was called magnetic resistance by Bosanquet, a name which 

 has given way to that of reluctance. 



185. Induced Charge. The apparent charges defined above 

 minus the true charges are called the induced charges due to the 

 action of the forces of the field. If we examine the amount of the 

 induced charge in a body r surrounded by a homogeneous medium, 

 we shall obtain an important result. Let the constant inductivity 

 of the external medium be fa, and let us denote the normal 

 toward the interior of the body r by n{ and the normal toward the 

 outside by n e . Then if we use the formula (21) for the apparent 

 charge on the surface we find 



and transforming the second integral on the right by Green's 

 theorem this becomes 



* Exp. Res. 1252 seq. 



