A device that consists of a spring bow very analogous to a section gage.
On one arm of the bow is a rubber pad shaped to fit the curvature of the
hole. In this pad, slightly recessed, are three electrodes of about
diameter 1/2 in (1.3 cm) and located at 1-in (2.54-cm) intervals. These
three electrodes are used to record two resistivity curves. One curve is a
three-electrode type with a spacing of 1-1/2 in (3.8 cm), and the second
is a two-electrode type with a spacing of 2 in. Wyllie
Dictionary of Mining, Mineral, and Related Terms