18.104.22.168 Four-Terminal Method. In the four-terminal method developed by the U.S. Bureau of Standards (2-8),
four electrodes are inserted into the soil in a straight line with equal spacings. A known current is injected into
the soil through the end electrodes and the voltage drop between the two inside electrodes is measured.
Consider four deeply buried spheres placed in a straight line, separated by a distance, a, as shown in Figure 2-5.
Connection is made to the spheres by insulated conductors. Assume that a current, I, is introduced into one of
the outermost spheres (No. 1) and flows out of the earth through the other (No. 4) outermost sphere. The
voltage from the left hand (No. 2) to the right hand (No. 3) inner sphere can be viewed as resulting from a
current flowing to infinity and another returning from infinity. The two resulting components of the voltage
= output current.
the total potential V is
v = v1 + v2