HYSTERESIS:
Definition: The lagging behaviour of magnetic flux density (B) concerning the magnetic field intensity (H) is known as hysteresis.
When a ferromagnetic material is kept in a magnetic field then it becomes
magnetized non-linearly as shown in the figure. When the specimen is not in the magnetic
field then its magnetic flux density is also zero. It is represented by point ‘O’ in the figure. From this point, if magnetic field intensity (H) is increased then magnetic flux density (B) also increases up to the point ‘a’. If ‘H’ is decreased then ‘B’ also decreased in the direction of ‘ab’. Here ‘B’ lags ‘H’. If magnetic field intensity ‘H’ decreases then ‘B’ also decreases. At this point, ‘H’ becomes zero, but ‘B’ still has a value equal to ‘OB’. The magnetic flux density remaining in the material in the absence of a magnetic
field is known as residual magnetism. The power of retaining this magnetism is called
retentivity or remanence. If the magnetic intensity ‘H’ is again increased in the reverse direction, then magnetic flux density ‘B’ also decreased and becomes zero when H is equal to OC. This value of magnetic intensity is called coercive force or coercivity of the sample. Thus
coercivity (Hc) is a measure of the magnetic intensity required to destroy the residual
the magnetism of the sample. If ‘H’ is decreased beyond ‘OC’, again the sample is increasingly magnetised in the opposite direction and a point is reached. By taking ‘H’ back from its negative maximum value through ‘O’ to its original positive maximum value a similar curve ‘data’ is obtained. Their and ‘e’ where the sample is magnetised in the absence of an extan renal magnetic field exhibits permanent magnetism. If the curve is observed from ‘o’ to ‘e’, always magnetic intensity lags behind flux density.This lagging behaviour is known as hysteresis.