the magnetic intensity created at point b of the arc and the bottom scan line represents point c the path of the arc can be reconstructed by a correlation of high intensity parallel planar areas of exposure on the tape with the areas of less intense exposure the magnetic tape sleeve used in this invention is preferably similar in construction to the tapes used in the sound and video industry but is not limited to such construction any device that can measure the intensity of a magnetic field that is propagated to the surface of the sample will do the test sample is preferably a right cylinder in shape but it is not limited to such a shape with computerized analysis of magnetic field intensity at the surface of a sample it is possible to map the path of an arc through a sample of almost any shape the magnetic intensity of the arc flows out radially and approximately perpendicular to the longitudinal axis of the sample the intensity is approximately equal for a given distance from the arc path but it diminishes with distance away from the arc path the surface of the tape is magnetized with an intensity that is proportional to the magnetic flux level created by the arc for a circle that is formed by a plane taken through the right cylinder sample said plane being perpendicular to the longitudinal axis of the sample a line on the magnetic tape that corresponds to.