Field Measurement
Potentials, current, and resistance
Potential measurements require a stable reference electrode in contact with the electrolyte and a high-impedance meter. Current is measured through shunts, current spans, or clamp-on meters; a clamp-on device only works when it encircles a single isolated conductor, so bonded cables must be separated temporarily. Resistance and continuity checks must account for lead resistance - use a four-terminal method where accuracy matters.
Soil resistivity
Use the Wenner four-pin method: drive four equally spaced pins in a straight line at spacing a, pass current through the outer pair, measure voltage across the inner pair, then compute resistivity. With spacing in centimeters, rho = 2 x pi x a x R; with spacing in feet, rho (ohm-cm) = 191.5 x a x R. Record spacing, depth, and moisture conditions, because resistivity changes seasonally.
Traps: using a two-lead resistance check and including test-lead resistance; unequal pin spacing; testing resistivity after a rain and comparing it to dry-season data; clamping a bonded cable and reporting a fraction of the true current.
Worked example: spacing a = 5 ft, measured R = 8 ohm. Then rho = 191.5 x 5 x 8 = 7,660 ohm-cm, or about 77 ohm-m.
Rule of thumb: measure, do not infer - and always record the geometry that goes with a resistivity number.