Magnetic Flux Density:
In simple matter, the magnetic flux density 
related to the magnetic field intensity 
as 
where 
called the permeability. In particular when we consider the free space 
where 
H/m is the permeability of the free space. Magnetic flux density is measured in terms of Wb/m 2 .
The magnetic flux density through a surface is given by:
Wb ......................................(4.18)
In the case of electrostatic field, we have seen that if the surface is a closed surface, the net flux passing through the surface is equal to the charge enclosed by the surface. In case of magnetic field isolated magnetic charge (i. e. pole) does not exist. Magnetic poles always occur in pair (as N-S). For example, if we desire to have an isolated magnetic pole by dividing the magnetic bar successively into two, we end up with pieces each having north (N) and south (S) pole as shown in Fig. 4.7 (a). This process could be continued until the magnets are of atomic dimensions; still we will have N-S pair occurring together. This means that the magnetic poles cannot be isolated.