A geophysical instrument used to measure the electrical resistivity of the earth to identify buried features and structures. Since the resistivity of the soil changes with humidity, humus content, etc., the machine can detect pits, ditches, roads, floors, etc. This is generally done through an array of four electrodes, pushed into the ground surface. Despite their name, resistivity meters do not actually measure resistivity, but ground resistance. Resistivity is this resistance, standardized for the distance between the electrodes in the ground. The instrument consists of a source of electricity (a handle-operated dynamo in the megger earth tester, batteries in the tellohm, a transistor oscillator in the Martin-Clark meter) and a meter to record the results. All systems employ four steel probes connected by cable to the meter, two to carry the activating current, two to pick up the current passing through the ground. Also, the resistance between two roving probes is now compared with that between two distant static ones. Different spacing between the probes is employed in different conditions; where the probes are spaced equally, as in the Wenner configuration, features up to a depth equal to the probe-separation can be detected. Anomalous readings may indicate the presence of archaeological material.