What is magnetising field?
When a magnetic material is placed in a magnetic field, a magnetism is induced in it. Thus the magnetising field can be defined as:
Magnetising field: The magnetic field that exists in vaccum and induces magnetism in any magnetic material is called magnetising field. [latexpage]
Magnetising field explanation
Lets consider a toroidal solenoid that carrying a current $I$ and that’s placed in vaccum. If the solenoid has $n$ turns per unit length, then the magnetic field setup in the solenoid is given by: $$B_0=\mu_0 nI$$ So the magnetic field that arise in the toroidal solenoid due to the free current called magnetising field.
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