One method employed to detect surface flaws in metals, such as fatigue cracks, is to pass an alternating current through the specimen, and to probe the plane surface for irregularities in the field produced by the flaw. This paper compares the thin-skin surface field distributions obtained from spherical cap volume defects at the surface with the distributions typically found for normal cracks, in this case a crack of circular are form. When a uniform surface current is applied it is shown that for the former case no change in the E field arises on the plane surface, so that probe signals from the plane around the defect give no indication of its presence. This result is contrasted with the changes in the surface field produced by a circular are crack calculated from an unfolding theory given in earlier work. Experiments to measure these surface distributions are described and the results are in agreement with the theoretical predictions in both cases.