Magnetic Shielding

A closed shell of soft ferromagnetic material, placed in a region of magnetic field, behaves very differently from an open piece of the same material: because the shell offers the field lines an easy, high-permeability path through its own thickness, a very large number of field lines crowd into the shell's material itself, leaving only a small number of lines to pass through the hollow space enclosed by the shell (Fig. 11.12). In other words, a spherical (or other closed) shell of soft iron placed in a magnetic field strongly reduces the field found inside the enclosed cavity compared with the field just outside the shell -- an effect called magnetic shielding.

This is a genuinely practical technique: any instrument that needs to be protected from an ambient or stray magnetic field can be completely enclosed within such a soft ferromagnetic case, and the technique is used aboard spacecraft to protect sensitive equipment. Conversely, some scientific experiments must instead be protected from the HIGH magnetic fields of nearby laboratory magnets; in that case too, the same principle applies -- a case made of soft ferromagnetic material shields the experiment by drawing the strong field into the case's own material rather than letting it reach the enclosed experimental region. …