AEMC 6422 User Manual page 23

If readings were taken by moving the auxiliary potential electrode S towards
either E or H, the reading differentials would be great and we could not obtain a
reading within a reasonable band of tolerance. The sensitive areas overlap and
act constantly to increase resistance as S is moved away from E.
Now consider the illustration below, where the E and H electrodes are sufficiently
spaced so that the areas of effective resistance do not overlap. If we plot the
resistance, measured we find that the measurements level off when S is placed
at 62% of the distance from E to H, and that the readings on either side of the
initial Y setting are most likely to be within the established tolerance band. This
tolerance band is defined by the user and expressed as a percent of the initial
reading: ±2%, ±5%, ±10%, etc.
23
Ground Tester Models 6422/6424