In order to be able to measure inclined cracks that are
expected to create a non-symmetric electrical field over
the surface, a third measuring electrode M is placed in
the middle of the specimen at 1.5 mm distance from the
free boundary of the opposite U-notch as shown in Fig. 5
For the execution of measurements a constant current
IAB passes through electrodes A and B creating a distribution
of electrical potential over the surface of the gauge
area of the specimen. The potential drop between points
L–R (VLR), L–M (VLM) and R–M (VRM) which changes as
the crack propagates is measured constantly. The governing
differential equations of the electrical field problem
are