DEPARTMENT OF TERRESTRIAL MAGNETISM. 271 



obtained by them independently. Schuster's results were derived from line- 

 integrals of the magnetic force in Great Britain as based on the latest mag- 

 netic survey by G. W. Walker, and Dyson's results depend on the world 

 magnetic charts for 1922, based on the latest observations, chiefly those of 

 the Carnegie and the Department's land expeditions. It is hoped that before 

 long some experimental investigations may be undertaken for further testing 

 this important question. The combined results would indicate that, on the 

 average, about 2 per cent of the Earth's magnetic field is involved in the 

 production of vertical electric currents. (For further information, see pp. 

 296-297.) 



(2) Results of recent earth-current observations. — Preparatory to the initia- 

 tion of observational work on earth-currents at the Watheroo Observatory, 

 the Director, assisted by members of his staff, undertook a discussion of the 

 most extensive available series of observations, namely the eleven-year series, 

 1910-1920, obtained at the Observatorio del Ebro, Tortosa, Spain. One of 

 the chief conclusions reached was that the resultant electric current flowing 

 in the Earth's crust, as observed at the Ebro Observatory, was approximately 

 in the direction from the Magnetic North Pole towards south-southeast, hence 

 towards the equator (see pp. 290-291). It will be a matter of no little interest 

 to know whether at a station in the Southern Hemisphere — at the Watheroo 

 Observatory, for example — the earth-current will be reversed in direction to 

 that in Spain, so as to flow also towards the equator. If so, then we shall 

 have another clue bearing on the question raised under (1). It is rather 

 interesting in this connection that such computations as we are able to make 

 on the basis of the existing data would indicate that the strength of the elec- 

 tric currents found circulating in the Earth's crust is of about the same order 

 of magnitude as those apparently disclosed by the results reported upon under 

 (1). Concurrently with the Director's discussion of existing earth-current 

 data, an experimental study was assigned to Mr. Gish, in consultation with 

 Dr. Mauchly, regarding the best methods and instruments for earth-current 

 measurements. 



The results of the investigations by the Director and Mr. Gish indicated 

 that it would be well worth while trying out at the Watheroo Observatory 

 whether satisfactory data may not be obtainable from specially constructed 

 earth-current Hues, not exceeding 1 to 2 miles in length. (See pp. 300-301.) 

 For the proper interpretation of the results obtained from the potential- 

 gradient observations, it appears also desirable to make provision for obser- 

 vations regarding the electric conductivity of the soil. Investigations on 

 earth-currents, such as here outlined, are of interest not alone to students 

 of the Earth's magnetism and electricity, but also to those who are investi- 

 gating the causes of disturbances in telegraphic transmission, the causes of 

 corrosion of water and gas mains, and electrical methods of locating ore bodies. 



(3) Relations between the Earth's magnetic and electric phenomena and solar 

 activity. — The Director's investigations, based on the published eleven-year 

 series of observations at the Ebro Observatory, have furnished some further 

 confirmation of results given in previous reports. There would hardly seem 

 any question now that the normal electric condition of the Earth and of the 

 atmosphere is dependent upon the precise state of activity of the Sun, and 

 hence varies during the sun-spot cycle, just as magnetic phenomena have 



