All materials possess magnetic properties to a greater or lesser degree and these are determined by the facts that (1) a magnetic field exert forces and torques on the bodies, (2) a body place in a magnetic field distorts the field. The magnetic properties of the materials are characterised by their relative permeabilities. In accordance with the value of relative permeability the material may be classified in the following three ways:
1. Ferromagnetic materials. The relative permeabilities of these materials are much greater than unity and are dependent on the field strengths. They attract the lines of force strongly [Fig. 3 (a)]. The principal ferromagnetic elements are iron, cobalt, nickel. Gadolinium however also comes under
this classification. These have high susceptibility.
Fig. 3 (a) Ferromagnetic
2. Paramagnetic materials. These have relative permeability slightly greater than unity and are magnetised slightly. They attract the lines of forces weakly [Fig. 3 (b )]. Aluminium, platinum and oxygen belong to this category.
Fig. 3 (b) Paramentic
3. Diamagnetic materials. The relative permeability of these materials is slightly less than unity. They repel the lines of force slightly [Fig. 3 (c)]. The examples are Bismuth, silver, copper and hydrogen.
Fig. 3 (c) Diamagnetic