The penetration of the magnetic field intensity into the interior of an infinite slab of ferromagnetic material having a nonlinearB — Hrelationship is governed by a nonlinear diffusion equation. For finite thickness of the slab, the problem is solved by an approximate method. A rigorous solution of the same problem when the thickness of the slab approaches infinity is given to justify the approximate method used. In the paper, one term only is used to represent an infinite series in order to give a readily calculable result in ``switching time.'' The result is then compared with that of the graphical solution of the same problem.