The Schlumberger array requires that four steel electrodes are arranged and pinned collinearly into the earth with the current electrodes spacing much greater than the potential electrodes and ensuring that AB/2 ≥ 5MN/2 where “AB” is current electrodes separation and “MN” is potential electrodes separation (Fig.2).
This implies that for MN = a, AB = na+a from which their values on table 1 were generated and approximated for convenience. The
electrodes were connected appropriately to their respective terminals on the terrameter through cables and
hammered to make good contact with the earth.