Consider a completely demagnetized ferromagnetic material. [Completely Demagnetised material is the one in which B=H =0].It will be subjected to increasing values of magnetic field strength H and the corresponding flux density B measured.

The result is shown in the below figure by the curve o-a-b.At the point b, if the field intensity (H) is increased further the flux density(B) will not increase any more.This is calledsaturation( the material is said to be saturated). In this figure b-y is called assaturation flux density.

Now if the field intensity (H) is decreased, the flux density (B) will follow the curve b-c.When field intensity (H) is reduced to zero, flux remains in the iron.This is called asremanent flux density or remanence.It is shown in the figure as o-c.

Now if the H is increased in the opposite direction, theflux density decreases as will.

It will decrease until the point d. Here the flux density (B)is zero.

The magnetic field strength (points between 0 and d) required to remove the residual magnetism, i.e. reduce B to zero. It is called thecoercive force.

Now if the H is increased further in the reverse direction causes the flux density to increase in the reverse direction till the saturation point ( here point-e).

If H is varied backwards from 0x to 0y, the flux density(B) follows the curve e-f-g-b, similar to curve b-c-d-e.

From the figure the clear thatthe flux density changes lag behind the changes in the magnetic field strength.

This effect is calledHysteresis.

The closed figure b-c-d-e-f-g-b is called theHysteresis Loop (or the B/H Loop).