The earth testers designed for earth resistivity measurement have 4 terminals and 4 auxiliary earth electrodes.

According to the Wenner 4-pole method, apply AC current “I” between the “E” (earth electrode) and “H(C)” (current electrode) to find out the potential difference “V” between the two potential electrodes “S (P)” and “ES”. To obtain the earth resistance “Rg (Ω)”, divide the potential difference “V” by AC current “I”; where the distance between electrodes is “a” (m). Then use the formula: ρ = 2 π x a x Rg (Ωm).
Regarding the connections, stick all the 4 auxiliary Earth electrodes into the ground at the same inter distance a [m]. Note: The depth should be 5% or less of a.

With advanced earth resistivity testers, that use the above mentioned formula, the Earth Resistivity ρ measurement is automatically calculated and shown on the instrument display.