AEMC 6422 User Manual page 24

2.3.1.3 Auxiliary Electrode Spacing
No definite distance between X and Z can be given, since this distance is relative
to the diameter of the electrode tested, its length, the homogeneity of the soil
tested, and particularly, the effective resistance areas. However, an approximate
distance may be determined from the following chart which is given for a
homogeneous soil and an electrode of 1" in diameter. (For a diameter of 1/2",
reduce the distance by 10%; for a diameter of 2" increase the distance by 10%.)
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Ground Tester Models 6422/6424