Magnetic shielding by a spherical shell having a hole is studied. Solving the differential equation satisfied by magnetic potential, distributions of the potential are obtained for various combinations of hole size, shell thickness and magnetic permeability. Relaxation technique is used for solving the problem numerically. The shell is placed in a uniform external field parallel to the direction connecting the hole center to that of the shell.
As expected, it turns out that the smaller the hole size, the higher the shielding power. It is interesting and important, however, that the shielding power decreases only by a few tens of percent when the hole aperture radius is 30° or smaller. A fairly large defect of the shell may be allowed without losing much shielding power.