Intensity of magnetising field (H)
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When a substance is placed in an external magnetic field, the substance experiences a torque due to the field and aligns in the same direction as the field. The magnetisation so produced in the substance is called Induced magnetisation and strength of external field is called intensity of magnetising field (H).
H is measured in Ampere/meter or Joule/Tesla-m
3.
Intensity of magnetisation (I)
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This gives us the measure of the extent to which substance has been magnetised under the influence of H-field and depends upon the nature of the substance.
Mathematically
where
is the intensity of magnetisation,
net magnetic moment, v the volume of the material
Magnetic induction
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The iron bar gets magnetised with north pole at B and south pole at A. The field inside the specimen gets modified. The magnetic induction at a point inside the magnetised specimen is the total number of magnetic field lines crossing a unit area around that point, the area being held perpendicular to the field lines.
The flux density produced by the magnetising field vacuum is proportional to the intensity of field 
.
Magnetic susceptibility
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This indicates how easily the material can be magnetised. It is represented as Xm.
i.e., the ratio of intensity of magnetisation induced in the material to magnetising field (H).
Therefore X
m is a number and has no units.
Relation between mr and Xm
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mr = 1 + xm
e.g., electromagnet
