Magnetic Shielding

Magnetic Materials · Maharashtra HSC

Theory

When a soft ferromagnetic material is kept in a uniform magnetic field, a large number of magnetic lines of force crowd up inside the material, leaving only a few outside it. This is a direct consequence of its very large permeability.
Now take a closed structure of such a material — for example a spherical shell of iron — and place it in a magnetic field. Very few lines of force pass through the enclosed space; most of them are crowded into the iron shell itself.
The enclosed region is therefore almost free of magnetic field. This effect is known as magnetic shielding.

Draw a set of parallel horizontal field lines coming in from the left. Draw a thick circular annulus (the iron shell) in their path. Show the lines bending into the walls of the shell as they approach it, running round through the material, and emerging on the far side, with the cavity inside left almost empty of lines.

An instrument that must be protected from a magnetic field is completely surrounded by a case made of a soft ferromagnetic substance.
This technique is used in spaceships.
Some scientific experiments must be protected from the magnetic fields present in the laboratory; equally, the high field of a laboratory magnet may need to be confined. In both cases a soft ferromagnetic case provides the shield.
Board point: the intensity of the earth's magnetic field at a point inside a hollow iron box is less than that outside — reduced, but not exactly zero.

Electrical shielding: an electrical insulator acts as a barrier or shield, and comes in a wide range of materials; ordinary household wiring is shielded in this way.
Acoustic shielding: in audio recording, stray sound would interfere with the sound being recorded, so recording studios are sound-insulated with acoustic material.

Magnetic shielding uses a closed shell of soft ferromagnetic material with very large $\mu_r$ ; the lines of force are drawn into the shell walls and the enclosed space is left nearly field-free.
The field inside a hollow iron box is less than that outside.