DEPARTMENT OF TERRESTRIAL MAGNETISM.^ 



Louis A. Bauer, Director. 



John A. Fleming, Assistant Director. 



GENERAL SUMMARY. 



COMPOSITION OF THE EARTH'S MAGNETIC FIELD. 



The present annual report, the twentieth since the estabhshment of the 

 Department in 1904, is largely devoted to a narration of results obtained 

 from a discussion of accumulated observational and experimental data. 

 As our studies progress, the wide significance and broad bearings of the 

 phenomena of the Earth's magnetism and its electricity are becoming more 

 and more manifest. It would seem not unreasonable to expect that these 

 subjects may contribute facts of interest and importance not alone to the 

 magnetician, but to the physicist, astronomer, and geologist as well. 



Preparatory to a lecture, entitled "The greater problems of the Earth's 

 magnetism and their bearings on astronomy, geology, and physics," de- 

 livered by the Director at the Carnegie Institution of Washington on Novem- 

 ber 21, 1922, a preliminary analysis of the Earth's magnetic field for 1922 

 was undertaken. Owing to the extent and accuracy of the accumulated 

 data, it has become possible to give definite answers to some of the greater 

 questions outstanding. For example, there can now no longer be any question 

 that the Earth's intensity of magnetization has been steadily diminishing 

 during the past 80 years at a rather large average annual rate, namely, about 

 1/1,500 of itself. Even though no statement at present may be made as 

 to how much longer the diminishing will continue, or as to whether the Earth's 

 magnetism has been steadily running down for centuries past, what has 

 already been found is of profound bearing upon theories as to the origin of 

 the magnetic field enveloping our planet. 



About 94 per cent of the Earth's magnetic field is to be ascribed to internal 

 magnetic and electric systems (/), about 3 per cent to external systems (E), 

 or systems in the atmosphere, and the remainder (A^), about 3 per cent, 

 to a magnetic system the evoking agencies of which, as, for example, vertical 

 electric currents, may pass from the atmosphere into the Earth and out 

 again. As the result of the A'' system, the path followed in going around 

 the Earth, let us say eastward, always at right angles to the direction of 

 the compass needle, is generally not a closed curve, i. e., the supposed traveler 

 would at the end of his journey be about 30 miles to the north or south of 

 his starting-place, according to the region of the Earth traversed. Though 

 the E and A'' systems contribute, as stated, only about 6 per cent to the mag- 

 netic force as observed on the Earth's surface, their further study and elucida- 

 tion may reveal some entirely new facts pertaining to matter, which facts 

 may be also of significance in studies concerning the origin of the Earth's 

 electric field, another great outstanding problem. 



The I system — the 94 per cent of the Earth's total magnetic field — exhibits 

 certain characteristics that are of peculiar interest, both to the geophysicist 

 and to the geologist. It is by no means so simple a magnetic system as 



^Address: Thirty-Sixth Street and Broad Branch Road, Washington, D. C. 



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