  We present a scattering theory of the electromotive force and internal
resistance of an electron pump. The characterization of the device performance
in terms of only two parameters requires the assumption of incoherent multiple
scattering within the circuit and complete thermalization among electrons
moving in a given direction. The electromotive force is shown to be of the
order of the driving frequency in natural units. In an open setup, the
electromotive force adds to the voltage difference between reservoirs to drive
the current, both facing a contact resistance which is absent in the case of a
closed circuit of uniform width.
