The effects of a uniform magnetic field applied to one of the electrodes in a flow cell were investigated. It was found that the cell potential is either increased or decreased, depending on the relative orientations of the magnetic field, the electric field, and the flow. The change of the cell potential is directly proportional to the magnetic field strength and the flow velocity. The proportionality constant, however, depends on the relative orientation of the fields, contrary to expectation from magnetohydrodynamic theory on laminar flow. The orientation dependence is explained in terms of a turbulent boundary layer. The conductivity of this boundary layer is calculated.