  It is demonstrated that the now well-established `flip-flop' mechanism of
spin exchange between electrons and nuclei in the quantum Hall effect can be
reversed. We use a sample geometry which utilizes separately contacted edge
states to establish a local nuclear spin polarization --close to the maximum
value achievable-- by driving a current between electron states of different
spin orientation. When the externally applied current is switched off, the
sample exhibits an output voltage of up to a few tenths of a meV, which decays
with a time constant typical for the nuclear spin relaxation. The surprizing
fact that a sample with a local nuclear spin polarization can act as a source
of energy and that this energy is well above the nuclear Zeeman splitting is
explained by a simple model which takes into account the effect of a local
Overhauser shift on the edge state reconstruction.
