181 183] DIELECTKICS AND MAGNETIZABLE BODIES. 359 



183. Apparent Charge. Since on passing from one medium 

 to another where the inductivity is different the force is dis- 

 continuous, the surface acts as a charged surface has previously 

 been found to act, 82. Also since the force is not solenoidal 

 in a heterogeneous medium, there appear to be bodily charges. 

 The magnitude of these apparent charges, whose densities are p, a, 

 are given by the usual equations 



and comparing these with the equations for the true densities 

 we find 



1 fiVfy Wbp dVdp 



or in a homogeneous medium 



(20') .',' ,' 



For the surface density 



The potential is then determined by 85 (18), as 



dS 



that is : 



A distribution of charges acting according to the Newtonian Law, 

 of densities p and cr', would produce everywhere exactly the same 

 field as that actually produced by the true charges p and a. The 

 Newtonian Law thus reappears, and may be used to calculate the 

 forces, only the true charges do not follow the law, but the apparent 

 charges, which are known as soon as the true charges and the 

 properties of the media are given. 



