Prototype Martin-Clark resistivity built by John Martin and Antony Clark, UK, 1956. The meter is small handheld instrument with a grey rectangular metal casing, on the top side of which are two dials to control the voltage of the meter and to measure electrical resistance in ohms. Connected to the meter from one of its sides are five separate wires, which would connect to the steel probes. Prototype Martin-Clark resistivity meter by John Martin and Antony Clark 1956