When a ferromagnetic sample is placed in a magnetising field H varies,the magnetic induction B does not vary linearly with H,i.e the permeability µ=B/H is not constant but varies with H.In fact,it also depends on the past history of the sample. The fig shows the variation of magnetic induction B with magnetising field intensity H.Point O represents the initial unmagnetised state of a ferromagnetic sample.AS the magnetising field intensity H increases,the magnetic induction B first gradually increase and then attains a constant value.In other words,the magnetic induction B saturates at a certain value +Hmax Now if the magnetising field intensity of H is gradually decreased to zero,B decreases but along a new path AB.It is found that the magnetic induction B does not become zero even when the magnetising field H is zero.,the sample is not demagnetised even when the magnetising field has been removed.The magnetic induction(=OB)left behind in the sample after the magnetising field has been removed is called residual magnetism or retentivity  To reduce the magnetism to  zero ,the field H is gradually increased in the reverse direction,the induction B decreases and becomes zero at a value of H=OC.The value of reverse magnetising field intensity H required for the residual magnetism of a sample to become zero is called coercivity of the sample. On further increasing H in the reverse direction to a value-Hmax,we reach the saturation point D located symmetrically to point A.Now if H is decreased gradually,the point A is reached after going through the path DEFA.The closed curve ABCDEFA which represents a cycle of magnetisation of a ferromagnetic sample is called its hysteresis loop.throughout the cycle,the magnetic field B lags behind the magnetising field intensity H,the value of B when H is increasing.The phenomenon of the lagging of magnetic induction behind the magnetising field is called hysteresis Significance of the area of hysteresis loopThe product BH= $B(B/\mu)=B^2/\mu0\mur$ has the dimensions of energy per unit volume.Hence the area within the B-H loop represents the energy dissipated per unit volume in the material when it is carried through a cycle of magnetisation.The source is the source of emf used in magnetising the material and the sink is the hysteretic heat loss in the magnetic field. A study of hysteresis loop Provides  us information about retentivity.coercivity and hysteresis loss of a magnetic material.This helps in proper selection of materials for designing cores of transformation and electromagnets and in making permanent magnets.