  We investigate the behavior of the time-dependent voltage drop in a
periodically driven quantum conductor sensed by weakly coupled dynamical
voltages probes.
  We introduce the concepts of ac-dc local voltage and four point impedance in
an electronic system driven by ac fields. We discuss the properties of the
different components of these quantities in a simple model of a quantum pump,
where two ac voltages oscillating with a phase lag are applied at the walls of
a quantum dot.
