MAGNETIC SHIELDING BY A CUBIC SHELL

Magnetic field in and around a magnetically permeable shell of cubic shape placed in a uniform external field is studied. The Laplace equation satisfied by magnetic potential is solved by use of relaxation technique in three-dimensional space. In contrast to the case of a spherical shell, rarefaction of magnetic equi-potential lines is not quite remarkable indicating considerable weakening of shielding effect in the shell interior. This is probably caused by leakage of magnetic flux from the shell wall near the sharp comers. It is therefore apparent that a shell of which the shape is close to a sphere should be used in order to have an efficient shielding. Magnetic shielding of a cubic shell having a defect on one of the shell wall is also studied. It turns out that the extent of shielding power is weakened only slightly even for a large defect. In one of the Appendixes, shielding by a flat square shell is studied reaching a conclusion that the shielding power of the shell is fairly small. It is not practicable to use such a shell for protecting a floppy disk from a magnetic disturbance arising from outside.