The potential distribution near a surface emitting in a magnetic field has been calculated. It is shown that the characteristic transverse scale of the return current region is determined by the perpendicular ion viscosity (classical or anomalous). This scale is much larger than the emitting channel width. The longitudinal scale depends on the classical conductivity. A criterion for unipolar arc formation has been put forward for an arbitrary angle between the magnetic field and the wall. In the criterion the region of the return current is specified, in contrast to previous publications