write the difference between self and forced demagnetization
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Answer:
Induction of magnetic fields in and around ferromagnetic materials and components which have the appropriate intensity and suitable directions for magnetic particle inspection involves techniques which demand skill and understanding both in design and actual use. After taking such trouble to magnetize material and components for magnetic particle inspection it is frequently, but not always necessary to reduce the remaining magnetic field as much as possible. This can prove to be a more difficult problem than that of magnetizing materials and components. The difficulty may arise from the magnetic characteristics of the material, or a component may be made up of several materials having quite different magnetic characteristics. A common example is of a large component made of steel of high coercivity and retentivity having one or more bolts in it which have much higher relative permeability. Another common source of difficulty in demagnetization is the equipment available. It is not uncommon to find operators working under great difficulty because they have only 50 or 60 Hz alternating current demagnetizing equipment and the magnetic field was induced by use of full-wave or half-wave rectified current.