258 CARNEGIE INSTITUTION OF WASHINGTON. 



three electric instruments and the provisional International Magnetic 

 Standard of the Carnegie Institution of Washington. Table 2 summarizes 

 the results. 



General description of the earth-current measuring system at the Watheroo Magnetic 

 Observatory. O. H. Gish. Terr. Mag., vol. 28, 89-108 (September 1923). 



This 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 determin- 

 ing the potential gradient at a point are then given. The relations for resolv- 

 ing and combining earth-current potential measurements are developed in 

 such a manner as to include the less favorable cases 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 over- 

 head and underground hnes, and thus a close comparison of the relative 

 virtue of these two types may be made. Only overhead lines connect with 

 the farther point on each branch. 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. The recorder is a modified Leeds and Northrup 

 12-point curve-printing potentiometer. For other details, reference may 

 be made to the complete article, 



A simple quantitative determination of the contact resistance of individual earthed elec- 

 trodes. O. H. Gish. 



Although obviously capable of wider application, the method here outlined 

 was developed in connection with earth-current investigations and is dis- 

 cussed from that point of view. The electric-circuit resistance in a single arm 

 of an earth-current measuring system consists of (1) the resistance of the 

 metallic line, (2) that of the measuring instruments, (3) the contact resistance 

 of the electrodes, and (4) the earth resistance. 



The contact resistance of the electrodes (3) is usually of a higher order of 

 magnitude than that of the earth (4) and is to some extent capable of control. 

 It depends principally upon the effective contact surface of the electrode and 

 on the resistivity of the soil lying within a radius of 3 to 6 meters about the 

 electrode; it is not simply related to the actual surface of the electrode, but 

 depends on the physico-chemical condition of the electrode surface, as well as 

 on that of the soil, and even more upon the extension of the electrode in the 

 earth. 



When installing earth-current lines it may be desired to increase the exten- 

 sion of an electrode in order to decrease its contact resistance (and thus the 



