Periodically forced ferrofluid pendulum: effect of polydispersity

We investigate a torsional pendulum containing a ferrofluid that is forced periodically to undergo small-amplitude oscillations. A homogeneous magnetic field is applied perpendicular to the pendulum axis. We give an analytical formula for the ferrofluid-induced ``selfenergy'' in the pendulum's dynamic response function for monodisperse as well as for polydisperse ferrofluids.