Abstract: In an apparatus for testing cable (comprising a metallic core jacketed by insulation) to detect localized defects on the insulation caused by its carbonization, the cable is moved in an axial path through an array of five ring electrodes radially spaced from that path and spaced from each other along the path by axial gaps. The first and fifth electrodes from left to right are grounded while high voltage circuits supply current to the middle three electrodes at DC voltages which causes (a) the second and fourth electrode, and (b) the third electrode to be positive and negative, respectively, relative to ground, and which voltages are high enough to produce flow of charge through the atmosphere adjacent the electrodes, and between the electrodes and the moving cable.

Abstract: A probe holder mounts a pair of spaced current probes, a pair of spaced primary voltage probes inward of such current probes and a pair of spaced secondary voltage probes outward of such current probes. Each of the secondary voltage probes and the nearer thereto of the primary voltage probes is on the same equipotential contour calculated by assuming that the current probes contact and pass current through a planar homogeneously resistive sheet with infinite boundaries and of infinitesimal thickness. Electroconductive bodies are evaluated for resistive anomalies therein by contacting the body of all the probes and passing current through the body between the current probes. The voltages V.sub.a, V.sub.b and V'.sub.a, V'.sub.b responsively sensed on the surface of such body by, respectively, such two primary probes and such two secondary probes are used to derive first and second signals as functions of the voltage differentials V.sub.a -V.sub.b and V'.sub.a -V'.sub.b respectively.