To add to hypress description the center lead (Potential lead) should be placed at a distance that is 52% of the distance of the furthest lead (Current lead), record the value and move the center rod to 62%, record, then 72%, record. These values should be plotted to show a relativly flat line to verify your readings. If the plot is not flat your readings are being affected by the sphere of infulence of the ground rod under test and the current lead and the current lead should be moved further away from the rod under test and the entire procedure repeated until your 3 point plot is relativly flat.
Another method that is becoming more accepted is the clamp on method, simple and fast, but the test equipment is more expensive.
MV/HV Testing Specialist, "BKRMAN"