MIL-HDBK-419A

2.4.2.2 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

are (2-8)

where

input current,

and

(2-10)

**where**

**= output current.**

**But since**

**the total potential V is**

**(2-11)**

**v = v1 + v2**

**2-13**