If the probe depth, h, is less than the probe separation distance, a, the potential drop measured between the
inner electrodes divided by the current measured into (or out of) one of the outer electrodes is (2-8):
a = distance between four, equally spaced, in-line probes, and
h = depth of burial of probes (insulated leads to surface).
If h << a, Equation 2-12 simplifies to
Short rods provide an effective approximation to the buried sphere, particularly at distances large with respect
to the depth of insertion.
The typical earth resistance test set contains a hand powered generator which can generate an ac signal at
frequencies of 40 to 100 hertz or so. (Fifty or sixty hertz should not be used because errors may be produced
by stray power currents in the soil. Direct current is not usually used because of polarization effects.) By
adjusting the resistance of an internal double balanced bridge, the instrument provides a direct indication of the
R required in Equation 2-14.
Figure 2-5. Idealized Method of Determining Soil Resistivity