In a ferromagnetic material, the flux density B increases when external applied magnetic field H to it is increased. When the saturation arrives, the increase in flux ceases even though H may be increased. This has be shown by OS in the fig. If the external field is gradually reduced, the original curve OS is not retraced but follows curve SR. The external field H is reduced to zero but B does not reduce to zero i.e. the material remains magnetised. The value of R flux density is called remanent flux density or residual magnetism. In order to demagnetise the material completely, external magnetic field must be reversed and when it reaches the value OC in reverse direction, it is seen that B is zero. This applied reverse magnetising force, which causes B to become zero is called coercive force. Further increase of H in reverse direction will now increase in B in reverse direction and again at the point S saturation occurs. The residual magnetism in reverse direction is represented by OR and to neutralise it H must be increased in positive direction to the value OC. Further increase in H will again magnetise the material and saturation will occur at S. The above property is characteristic of magnetic behaviour of the ferromagnetic material. When the material is taken through one complete cycle of magnetisation, it traces a loop called hysteresis loop. When a material is subjected to cyclic changes of magnetisation, the domains change the direction of their orientation in accordance with H. Work is done in changing the direction of domains, which leads to production of heat within the material. The energy required to take the material through one complete cycle of magnetisation is proportional to the area enclosed by the loop.