184 186] DIELECTRICS AND MAGNETIZABLE BODIES. 361 



/ aF 



r te 



~ dr - 



But by the definitions of true and apparent volume density this is 



(26) 



so that, transposing, 



(27) JJo-US + JJJVdr = I [JjVdS + ffjp dr^ , 



or the total apparent charge of a body surrounded by a homo- 

 geneous medium is equal to the true charge of the body divided 

 by the inductivity of the surrounding medium. In particular a 

 body which has no true charge has a total apparent charge equal 

 to zero, and since this remains true however the body may be 

 subdivided, the body is polarized. In the magnetic case, the body 

 is always found to be polarized, consequently we must conclude 

 that the true magnetic charge of all bodies is zero, or in other 

 words, true magnetism exists only as polarization. This is a second 

 apparent difference between electricity and magnetism, but if we 

 remember that whenever electrification is produced equal quan- 

 tities of both signs appear, the difference disappears. 



186. Polarizations. Since experiments on electrification 

 and magnetization are almost always made on bodies surrounded by 

 a homogeneous medium, namely air, it has become customary to 

 regard their apparent charges as due to the polarizations of the 

 bodies themselves, although it is evident by 120 that the surface 

 charges are due only to differences of polarization on the two sides 

 of the surface. The surrounding medium might be uniformly 

 polarized to any degree without producing any effect, consequently 

 its absolute polarization cannot be determined, and is of no import- 

 ance whatever. The apparent polarization of the body must 

 produce the surface density, by 120 (2), 



(28) </ = - /cos (/<) 



