Assignment:
The existence of the surface of a piece of magnetizable material profoundly alters the internal magnetic field, Hint.
Mostly this is a nuisance as in the demagnetization correction required to determine the true, (i.e. internal) susceptibility of a sample.
Demagnetization can however be put to good use. One example is the magnetic shielding of instruments and experimental chambers.
Following the method used to derive the field of a uniformly magnetized sphere, derive the field in the cavity of a hollow sphere.
