General description of the Earth‐current measuring system at the Watheroo Magnetic Observatory

The paper consists essentially of two parts, the first being an analysis setting forth the general requirements for the determination of the earth‐current density vector, and the second a description of the outstanding features of the system installed at the Watheroo Magnetic Observatory (in Western Australia) for measuring the potential gradient of earth currents. General requirements.—A complete description of earth currents requires a knowledge of the distribution of, (1) the earth resistivity, and (2) the earth‐current potential. A method suitable for determining the former is indicated. The major portion of the analysis, however, deals with the potential gradient. It is first indicated that a “potential survey” should be made in the vicinity of earth‐current lines in order to locate and measure any discontinuities that may exist in the potential field. General methods of approximately determining the potential gradient at a point are then given. The relations for resolving and combining earth‐current potential measurements are developed in such a manner as to include the less favorable case where the lines are not at right angles to each other. The earth‐current lines at Watheroo are so arranged that the earthed points determine a right angle, one limb of which extends due east and the other due north from the vertex. The point at the vertex is used as a common point of reference for the potentials of the other points. Two other points on each limb are situated, at present, 1.6 km. and 3.2 km. distant, respectively, from the common point. The potential difference between the common point and the nearer point on each branch can be alternately recorded by means of overhead and underground systems, and thus a close comparison of the relative virtue of these two types of line may be made. Only overhead lines connect with the farther point on each line. The overhead lines possess no features of special interest. The underground lines consist of leaded rubber‐covered copper conductors in bituminized fiber conduit, placed at a depth of 46 cm. below the earth's surface. For other details, reference may be made to the complete article.