Theoretical modeling studies associated with the a. c. field measurement technique have been a major feature of work at UCL for almost the last decade [1]. In that technique, the objective is to establish a spatially uniform current flow in the surface of a metal and to use this to interrogate a defect such as a surface-breaking crack by directing it broadside on to the crack and measuring the perturbations in the surface voltage distribution which it produces (Figure 1a). We have described the first major result of the theoretical studies as an unfolding theory. It shows that when the electrical skin depth δ is small compared with the defect size, the field problem posed by the interception of a uniform current flow by a surface-breaking crack is the plane potential problem shown in Figure 1c and d. The descriptive name was adopted because the problem domain is that formed by conceptually sectioning the material in the plane of the crack, as in figure 1b, and unfolding the crack plane about the surface edge BC to make it coplanar with the metal surface. This approach has been successfully used to solve the field problems associated with cracks of various forms, for example elliptical, circular arc, rectangular and triangular, and it has enabled us to incorporate the influence of crack aspect ratio on the readings of an a.c.f.m. instrument such as the Crack Microgauge [2], Earlier review papers [1,3] give numerous examples of the comparison between theory and experiment.