8.11 Soft Iron as Magnetic Shield
Soft iron has high magnetic permeability. The permeability is the ability of a material to allow the magnetic flux or lines of force through it when the material is placed inside a magnetic field. When a piece of soft iron is put into a magnetic field, it generates a magnetic field due to magnetisation.
If a sensitive magnetic device is enclosed in a casing of soft iron, the magnetic flux gets established in the soft iron rather than the device. Thus, the device is shielded from external magnetic fields.
A soft iron casing (shell) is placed inside a magnetic field produced by opposite poles of two bar magnets. Since the magnetic permeability of the iron shell is higher than that of air, the magnetic flux is established in the soft iron. As a result, the device is protected from the magnetic field. Usually, the casing is made with rounded corners to facilitate the magnetic field lines lining up easily.
Soft iron is generally used in the cores of transformers and electromagnets because of its high permeability. In the case of an electromagnet, the core of soft iron can be easily magnetised when current is passed around it and quickly loses magnetism when the current is stopped. That is why electromagnets are widely used in electric bells, loudspeakers, picking and releasing iron scraps by cranes, and in many more appliances. The sensitivity of a moving-coil galvanometer is also increased by placing a soft iron core inside the coil.