Hysteris loop-Hysteresis is defined as the legging of the magnetic induction B behind the corresponding magnetic field H.
When magnetic field H and the magnetic induction B are varried from zero to maximum in one direction and then back through zero to a maximum in the opposite direction and finally back again through zero to the first maximum, a cycle of magnetisation is said to be completed.
Using the value of H and B obtained above, we plot a graph by taking.H along x-axis and B along y-axis. When the magnetic field H is increased from zero to the maximum value Og.the induction B follows the curve Oa. However, when the magnetic field is brought to zero, the induction does not follows back the curve Oa but a different curve ab. The save pattern of variations is seen for the rest of the curve.The B-H curve for decreasing H does not coincide with the B-H curve for increasing H.
As is clear from figure, even when the magnetising fields H is zero, some magnetic induction (Ob or Oe) is still present in the specimen. It is denoted by Br and is called remanence or retentivity or residual magnetism.
The value of the intensity of magnetisation of a material when the magnetising filed is reduced to zero is called retentivity.
After the specimen has been magnetised to saturation (oh or oj) a reverses magnetising field (equal to OC or of) is required to reduce the magnetic induction to zero. This is called the coercivity or coercive force He
The value of the magnetising fild required to reduce residual magnetisim to zero is called corevicity of the material.
