A very long cylindrical iron rod of radius R is given a uniform longitudinal magnetization M, and then bent into a circle with a narrow gap of length w (this is essentially a toroid with a circular cross section from which a very small portion of.

Magnetisation M which will produce magnetic field at the center of the gap.

Explanation:

A bound surface current will flow around the square cross section in the direction perpendicular to the field.

This is like a torus and a square loop with a current flowing the opposite way.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia