Spatial variations of the local electric fields in current‐carrying thin gold films were studied with a scanning tunneling microscope on a nanometer scale. With a refined potentiometry technique we resolved potential steps at grain boundaries and investigated the potential gradients within each grain. These gradients are caused by nonlocal background scattering of the conduction electrons and are used to measure the local current density. We determine the reflectivity of an individual grain boundary without invoking an averaging procedure over the whole film. We find that the reflectivity of grain boundaries varies between R=0.7 and 0.9 and depends on their orientation.