Q.Intensity of magnetic field of the earth at the point inside a hollow iron box is (A) less than that outside (B) more than that outside (C) same as that outside (D) zero

A closed shell of soft ferromagnetic material (for example, a hollow iron sphere) placed in a magnetic field offers the field lines a much easier, high-permeability path through its own thickness; consequently, a large number of field lines crowd into the shell's material, leaving only a small number to pass through the hollow space it encloses. This effect, called magnetic shielding, strongly reduces the magnetic field found inside the enclosed cavity compared with the field just outside the shell.

Instruments needing protection from an ambient or stray magnetic field can be completely enclosed within such a soft-ferromagnetic case -- a technique used aboard spacecraft -- and, conversely, sensitive laboratory experiments that must instead be protected from the strong field of a nearby magnet can be shielded the same way. Magnetic shielding is one member of a broader family of shielding techniques for different physical quantities: electrical shielding uses an insulating barrier (as in ordinary household electrical wiring), and acoustic shielding uses sound-absorbing material (as in a sound-insulated recording studio) to keep out unwanted interference of their respective kind.