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Open circuit magnetization: magnetizing a workpiece that needs to be magnetized in a coil or magnetizing a large workpiece around a cable, often referred to as a coil method. When the coil method magnetizes the workpiece, a magnetic field is generated at both ends of the workpiece, and a demagnetizing field is generated.

Demagnetization field definition
The magnetization state of a material depends not only on its magnetic susceptibility but also on the shape of the sample. When a finite-sized sample is magnetized by an external magnetic field, the free magnetic pole appearing at both ends of it will produce a magnetic field opposite to the direction of magnetization, which is called a demagnetizing field. When the ferromagnetic material is magnetized, the magnetic field generated by the magnetic pole in the material is called the demagnetizing field, which weakens the applied magnetic field, and symbolizes ΔH.

The demagnetizing field is proportional to the magnetization of the material △H= N * M
△H---Demagnetizing field, M---Magnetization strength, N---Demagnetization factor

Demagnetization factor: It is only related to the shape of the material. For example, for an elongated sample magnetized along the long axis, N is close to zero, and for a thick and short sample, N is large. For a magnet of a general shape, it is difficult to find the size of N. The sample shape that can strictly calculate its demagnetization factor is only an ellipsoid.

Effective magnetic field
When the ferromagnetic material is magnetized, as long as the Zen magnetic pole is on the workpiece, a demagnetizing field is generated, which weakens the applied magnetic field, so the effective magnetic field on the workpiece is represented by H, which is equal to the applied magnetic field minus the demagnetizing field. The expression is: H=H0 -△H
H---effective magnetic field (A/m), H0---external magnetic field (A/m), △H---demagnetizing field (A/m)