  To describe the dynamics of a single peak of the Rosensweig instability a
model is proposed which approximates the peak by a half-ellipsoid atop a layer
of magnetic fluid. The resulting nonlinear equation for the height of the peak
leads to the correct subcritical character of the bifurcation for static
induction. For a time-dependent induction the effects of inertia and damping
are incorporated. The results of the model show qualitative agreement with the
experimental findings, as in the appearance of period doubling, trebling, and
higher multiples of the driving period. Furthermore a quantitative agreement is
also found for the parameter ranges of frequency and induction in which these
phenomena occur.
