  We report the experimental observation of the alpha-effect. It consists in
the generation of a current parallel to a magnetic field B0 applied to a
turbulent swirling flow of liquid sodium. At low magnetic Reynolds number, Rm,
we show that the magnitude of the alpha-effect increases like Rm2 and that its
sign is determined by the flow helicity. It saturates and then decreases at
large Rm, primarily because of the expulsion of the applied field B0 from the
bulk of the flow. We show how this expulsion is affected by the flow geometry
by varying the relative amplitudes of the azimuthal and axial flows.
