300 CARNEGIE INSTITUTION OF WASHINGTON. 



The report was concluded with a brief account of the development of aerial 

 instruments, particularly the design of magnetic instruments for direction and 

 position purposes at the United States Bureau of Standards. The desira- 

 bility of concentrating attention on the design of suitable apparatus for pre- 

 cise determinations of the magnetic elements on airships was emphasized as 

 of first importance in undertaking investigations of variations of the mag- 

 netic elements with height. 



Earth-currents : Methods and equipment, with special reference to the Watheroo Magnetic 

 Observatory, Western Australia. O. H. Gish. 



Beginning in March 1922, most of the writer's time was devoted to the vari- 

 ous matters pertaining to the design and installation of a system for the 

 measurement of earth-currents. Although this system was designed speci- 

 fically for the Watheroo Magnetic Observatory, the investigations and much 

 of the design will be equally applicable to such other systems as the Depart- 

 ment may install in the future. In this connection the literature of the sub- 

 ject was reviewed with special reference to methods and such analyses were 

 made as seemed required in outlining plans for the observational equipment 

 and program. Then followed correspondence, conferences, and other in- 

 vestigations in order to determine upon recommendations for design and 

 installation details. Some laboratory experiments with nonpolarizing elec- 

 trodes indicated that this type is open to serious criticism and, consequently, 

 further investigation of electrodes is urgently needed. The more important 

 features of the general plans that have been developed and of the equipment 

 designed are outlined in the paragraphs that follow. 



A complete plan for the study of earth-currents must provide not only for 

 the observation of earth-currents but also for that of phenomena that may be 

 corelated. The latter have been already largely provided for at the Watheroo 

 Observatory, but there remain two elements that should receive attention 

 because of their likely bearing on earth-current records, namely, soil tempera- 

 tures and soil moisture. An experimental well, equipped with a water-level 

 recorder, might suffice to show fluctuations in the last-named element. 



In the measurement of earth-currents direct methods can not be used, but 

 the magnitude and direction of these currents must instead be determined 

 from measurements of earth-current potentials and earth-resistivity. The 

 method of obtaining the latter has been outlined, but further development 

 must be deferred until next year. For the present year, then, the chief concern 

 is with methods and equipment for the measurement of potentials. Such is 

 the chief purpose of the earth-current system or lines. 



The system most frequently used consists of three earthed points, pref- 

 erably so placed that they may be the vertices of a right-angled triangle. 

 The point at the vertex of the right angle is used as a common reference or 

 zero-point in measuring the potentials of the other two. Lines connect each 

 of the other points to the common point through appropriate measuring in- 

 struments. Such a system is the simplest that has yet been devised for deter- 

 mining the magnitude and direction of earth-currents, but in using it, one is 

 compelled to make the highly restrictive assumption that the equipotential 

 surfaces are parallel planes perpendicular to the horizon plane. To detect 

 lack of parallelism or curvature in these surfaces a more elaborate surface 

 distribution of potentials than can be obtained from three points is needed; 

 while to detect a departure from perpendicularity a distribution with depth 

 must be obtained. In the design of this first installation, however, the 

 simpler system was rather closely adhered to, although provisions are made 

 so that, in case it seems advisable later to provide for obtaining a better dis- 

 tribution of potentials, nothing of the present system need be sacrificed. 



