Magnetization curve or
saturation curve is basically the graphical relationship between the magnetic
flux density B and magnetic filed intensity H for a given magnetic material. As
we know that,

B = μ

_{0}μ_{r}H
where μ

_{0 }= Permeability of free space = 4πx10^{-7}Tm/A
μ

_{r }= Relative Permeability of material
Thus by using the above
equation, we can find the relative permeability of the material. Unfortunately
relative permeability do not remain constant for a given material rather it
varies with the value of magnetic flux density B and magnetic field intensity
H. This variation in the value of relative permeability μ

_{r}is due to saturation of the material which can be better understood by domain theory. However we will concentrate here on the magnetization curve.
Figure below shows a typical
B-H curve for a ferromagnetic material. As can be seen from the figure below,
it consists of three zones i.e. OA, AC and beyond point C.

Initial zone OA is non
linear while the zone AC is linear and beyond point C, it is again non linear.
This non linear zone beyond point C is called Saturation region. It shall be
noted here that tough OA is also non linear but in this zone the rate of change
of B with respect to increase in H is increasing which means that increasing
the value of H increases the magnetic flux density B rapidly (How am I saying
this? Just draw tangent at different points between OA. You will notice that
slope of tangent is increasing). But thing is not so in Saturation Region. In
saturation region, the rate of increase of B with respect to increase in H is
decreasing (Just draw tangent at different point beyond C. You will notice that
slope of tangent is decreasing) which simply means that flux density B in the
Saturation Region increases less rapidly with H as compared to its change in
linear zone AC.

It shall also be noticed
that in saturation region,

Relative permeability μ

_{r}= B/ μ_{0}H = (1/ μ_{0})xSlope
Therefore relative
permeability decreases in saturation region as the slope of curve is
decreasing. Thus during magnetic circuit calculations, the value of μ

_{r }and H should correspond to the flux density under consideration. For free space or non magnetic material as μ_{0 }is constant, hence magnetization curve shall be a straight line (B = μ_{0}H).
You may like to read Basics of Magnetic Circuit

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