Page 154 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Residual magnetism

Residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external
magnetic field is removed. It is also the measure of that magnetization. Residual Magnetism is needed for instant
voltage build up for DC generator. It is a material’s ability to retain a certain amount of residual magnetic field
when the magnetizing force is removed after achieving saturation.

Retentivity
It is the property of retaining magnetism by a magnetic substance after the magnetization has been reduced to
zero.

If two pieces, one of steel and other of soft iron of same dimensions are aimed at same magnetizing force, it
will be observed that when the magnetizing force is removed, the two pieces does not retain the same value of
magnetization

The soft iron has lesser magnetism than the piece of steel, hence their power of retaining magnetism, or say
retentively is different.
Magnetic susceptibility

It is the property of an un magnetized body of acquiring induced polarity when under the influence of a magnetic
field. The susceptibility of iron is much greater than that of hard steel. Magnetic susceptibility is the degree of
magnetization of a material in response to an applied magnetic field. If magnetic susceptibility is positive, then the
material can be paramagnetic or ferromagnetic.
Comparison between electrical and magnetic circuit

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CITS : Power - Electrician & Wireman - Lesson 26-29