  We show that for a thin superconducting strip placed in a transverse dc
magnetic field - the typical geometry of experiments with high-Tc
superconductors - the application of a weak ac magnetic field perpendicular to
the dc field generates a dc voltage in the strip. This voltage leads to the
decay of the critical currents circulating in the strip, and eventually the
equilibrium state of the superconductor is established. This relaxation is not
due to thermally activated flux creep but to the "walking" motion of vortices
in the two-dimensional critical state of the strip with in-plane ac field. Our
theory explains the shaking effect that was used for detecting phase
transitions of the vortex lattice in superconductors with pinning. Some recent
experiments on this subject are discussed.
