Getting Down to Earth 57APPENDIX IV Measurement of the Resistance of Large Earth-Electrode Systems: Intersecting Curves Method13The difficulties of measuring the resistance of large electrode systems involve the use of very long leads to connect the potential and current probes. An alternative method, in which such long leads are not necessary, has been devised. The basic principle is to obtain earth-resistance curves for several current-electrode spacings and, by assuming a number of successive positions for the electrical center of the system, to produce intersection curves which will give the earth resistance and the position of the electrical center.Some rather difficult problems are encountered when the resistance of an earth-electrode system, consisting of a number of rods, tapes, etc., all connected in parallel and spread over a large area, is to be measured. The usual method of measurement that worked very well has one disadvantage; namely, that it is generally necessary to place the auxiliary current probe at a considerable distance from the earth-electrode system. In some cases, this distance can be as much as 3000 ft, and this is not always convenient or possible.A method which does not require such long lengths of cable would obviously be better. Therefore, the following is suggested.Suppose that all measurements are made from an arbitrary starting point O, the distance C to the current probe and the variable distance P to the potential probe being measured from this point. Then a curve such as abc (Fig. 28), giving the measured resistance against the value of P, can be obtained. Now suppose the electrical center of the earth-electrode system is actually at D, distance X from O. Then the true distance from the center to the current probe is C + X, and the true resistance is obtained when the potential probe is at a distance 0.618 (C + X) from D. This means that the value of P, measured from O, is 0.618 (C + X) - X. If X is now given a number of values, the corresponding values of P can be calculated and 13 Reference 22 (page 77)
