Looking at a typical hysteresis loop for a ferromagnetic material after the initial magnetising force is applied and then removed, it is well nigh impossible to end the test with a zero flux density. Even if a negative coercive force is applied, it will only keep the flux density at zero, as long as it continues to be applied.
Figure 1 shows that the key to demagnetisation is that a reversing and reducing magnetising force must be applied, so that the hysteresis loop reduces until all the parameters achieve zero.
There are a number of ways to achieve this, and some are listed below:
Aperture coil, removal
Aperture coil, reducing AC
Aperture coil, reversing DC
Electromagnet, reversing DC
Electromagnet, AC yoke
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