An applied outer magnetic field polarizes a body made from magnetic material. The effective field becomes the sum of the applied field and the inherent field due to the magnetic polarization of the material. The permeability of a material is a measure of the amount of polarization that is generated in a body. This Demonstration shows the direction and magnitude (color of the arrows with red being strongest and blue being weakest) of the magnetic field in and around a spherical shell. A plane cross section through the center of the spherical shell is shown. The applied magnetic field is directed from right to left.