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In a large class of materials, there exists an approximately linear
relationship between and . If the material
is isotropic then

(866) 
where is called the magnetic susceptibility. If is
positive then the material is called paramagnetic, and the magnetic field
is strengthened by the presence of the material. On the other hand, if is
negative then the material is diamagnetic, and the magnetic field
is weakened in the presence of the material. The magnetic
susceptibilities of paramagnetic and diamagnetic materials are
generally extremely small. A few sample values are given in Table. 2.^{1}
Table 2:
Material 

Aluminium 

Copper 

Diamond 

Tungsten 

Hydrogen (1 atm) 

Oxygen (1 atm) 

Nitrogen (1 atm) 


A linear relationship between and also
implies a linear relationship between and .
In fact, we can write

(867) 
where

(868) 
is termed the magnetic permeability of the material
in question. (Likewise,
is termed the permeability of free space.) Note that
has the same units as .
It is clear from
Table 2 that the permeabilities of common diamagnetic and paramagnetic
materials do not differ substantially from the permeability of free space. In fact,
to all intents and purposes, the magnetic properties of such materials
can be safely neglected (i.e., ).
Next: Ferromagnetism
Up: Dielectric and magnetic media
Previous: Magnetization
Richard Fitzpatrick
20060202