398 Induced Magnetism [CH. xn 



same kind as in iron are called paramagnetic, while substances in which the 

 effect is of the opposite kind are called diamagnetic. 



The phenomenon of magnetic induction is much more marked in para- 

 magnetic, than in diamagnetic, substances. The most diamagnetic substance 

 known is bismuth, and its coefficient of susceptibility ( 461, below) is only 



about of that of the most paramagnetic samples of iron. 



Coefficients of Susceptibility and Permeability. 



461. When a body which possesses no permanent magnetism of its own 

 is placed in a magnetic field, each element of its volume will, for the time it 

 remains under the influence of the magnetic field, be a magnetic particle. 

 If the body is non-crystalline, the direction of the induced magnetisation at 

 any point will be that of the magnetic force at the point. Thus if H denote 

 the magnetic force at any point, we can suppose that the induced magnetism, 

 of an intensity /, has its direction the same as that of H. 



Thus if a, /3, 7 are the components of magnetic force, and A, B, C the 

 components of induced magnetisation, we shall have equations of the form 



A = KOL 



(390), 



the quantity K being the same in each equation because the directions of / 

 and H are the same. 



The quantity K is called the magnetic susceptibility. 



If the body has no permanent magnetisation, the whole components of 

 magnetisation are the quantities A, J5, G given by equations (390), and the 

 components of induction are given (cf. equations (359)) by 



a = a + 4>7rA = a (1 -i- 

 6 = ft + 47r = (1 + 

 c = 7 + 4-TrC = 7 (1 + 47T*). 

 If we put ^ = 1 + 4?: .............................. (391), 



we have a = //, 1 



& = /* 1 ....... ....................... (392), 



c = w } 

 and IJL is called the magnetic permeability. 



462. The quantities tc and p are by no means constant for a given 

 substance. Their value depends largely upon the physical conditions, 

 particularly the temperature, of the substance, upon the strength of the 

 magnetic field in which the substance is placed, and upon the previous 

 magnetic experiences of the substance in question. 



