  We investigate a one-dimensional electron liquid with two point scatterers of
different strength. In the presence of electron interactions, the nonlinear
conductance is shown to depend on the current direction. The resulting
asymmetry of the transport characteristic gives rise to a ratchet effect, i.e.,
the rectification of a dc current for an applied ac voltage. In the case of
strong repulsive interactions, the ratchet current grows in a wide voltage
interval with decreasing ac voltage. In the regime of weak interaction the
current-voltage curve exhibits oscillatory behavior. Our results apply to
single-band quantum wires and to tunneling between quantum Hall edges.
