August 25, 1923] 



NA TURE 



295 



The Earth's Magnetic Field for 1922. 



Bv Dr. Louis A. Bauer. 



'T^HE precise constitution of the earth's magnetic 

 -*■ field at any one time, and the causes of the 

 constituent fields, are problems of fascinating interest, 

 the solution of which appears destined to reveal 

 hitherto unknown properties of matter. A most 

 intimate knowledge of the earth's magnetic and 

 electric phenomena, as well as a thorough acquaintance 

 with all the latest developments of theoretical physics, 

 seems requisite to success in the proper interpretation 

 of the mysteries presented. Whatever theory is 

 advanced, either for the earth's magnetic or its 

 electric field, a hypothesis must be introduced some- 

 where implying new properties of matter, or changes 

 in the classical laws of electro-dynamics, or physical 

 conditions below or above the earth's surface, of 

 which we have no knowledge at present. This being 

 so, it behoves us to keep an open mind with regard to 

 any new magnetic or electric phenomena which may 

 come to light. 



We fortunately have now three bodies, vastly 

 differing from one another in their physical con- 

 stitution, the magnetic and electric fields of which may 

 come within the scope of our investigations and help 

 us in our theoretical views, namely — the earth, the 

 atmosphere, and the sun. To anticipate, we now 

 know that the direction of the magnetic axis of each 

 one of these bodies is related in the same way for all 

 three to the direction of rotation of the body, and 

 that the magnetic -axis of each is inclined to the axis 

 of rotation, namely, at present, about 11-5° for the 

 earth, about 14° for the atmosphere, and about 6° for 

 the sun. If / be the physical factor, w the angular 

 velocity of rotation, r the radius, and D the density 

 of the body, then the strength of the magnetic fields 

 of these three bodies, at their magnetic poles for 

 example, may be expressed approximately by a 

 formula of the following type : 

 F=/.wy2D. 



The magnetic field expressed by (i) has thus far defied 

 laboratory detection, because of the size and speed 

 of rotation of bodies we may experiment with, but it 

 becomes readily appreciable when we are dealing with 

 a body of mass, size, and angular velocity of rotation 

 comparable with those of a member of our solar 

 system. If (i) holds universally, Jupiter, for 

 example, would be enveloped by a magnetic field of 

 about the same strength as that of the sun. Thus 

 we may have to look for assistance in making notable 

 advances concerning the structure and properties of 

 matter to experiments performed by Nature at large. 

 The chief questions pertaining to the earth's 

 magnetic field may be stated as follows : (a) Is any 

 appreciable portion of the magnetic force observed on 

 the earth's surface to be referred'' to a non-potential 

 system N ? (6) Is there, besides an internal magnetic 

 potential system, I, also an appreciable external mag- 

 netic potential system, E, existing in our atmosphere ? 



(c) If measurable N- and E-systems are disclosed, 

 may any portions arise from relativity effects, R ? 



(d) Is the integral of dii. over the earth's surface and 

 for all constituent systems equal to zero, where dii. 

 represents the elemental quantity of magnetism, or 

 any other corresponding physical quantity that may 

 evoke a magnetic field ? [e) What physical con- 

 ditions must the causes for the various systems fulfil 

 to account for the geographic variations, the secular 

 and other variations ? 



Any theory of the earth's magnetism and electricity 

 will have to give a complete and satisfying account 

 of these various questions before it can be accepted. 

 Doubtless for some time to come we shall have to be 

 content with trying out working hypotheses and must 



not permit ourselves to be bound to any one theory. 

 However, encouraging progress has been made, and 

 the object of the present communication is to tell of 

 answers more or less complete to some of our questions. 



While the magnetic survey of the globe has been in 

 progress by the Carnegie Institution of Washington 

 and other organisations during the past eighteen 

 years, the writer has published the results oi various 

 investigations preparatory to a rigorous and complete 

 analysis of the earth's magnetic field. It thus 

 became possible to decide in what regions of the 

 earth the field work should be intensified, and what 

 additional allied scientific data should be included 

 in the observational programme. Furthermore, to 

 satisfy practical demands for magnetic data, our 

 observational work was so arranged that sufficiently 

 accurate results for magnetic charts could be supplied 

 to leading hydrographic establishments within a few 

 months after the observations had actually been 

 made. Thus for the 1922 magnetic charts of the 

 British Admiralty, as constructed at the Greenwich 

 Observatory, Sir Frank Dyson states that all available 

 material was used, " the greatest source being the 

 observations made by the Carnegie and the land ob- 

 servations of the Carnegie Institution of Washington." 



Pending more accurate and complete reduction of 

 all observations to a common epoch by the Depart- 

 ment of Terrestrial Magnetism than was possible by 

 Greenwich Observatory, a preliminary analysis of 

 the earth's magnetic field for 1922, on the basis of 

 the latest charts and observations, was made by the 

 writer, with the assistance of various members of his 

 staff. The chief results were announced in a lecture 

 entitled " The Greater Problems of the Earth's 

 Magnetism and their Bearings on Astronomy, Geology, 

 and Physics," delivered at the Carnegie Institution 

 of Washington on November 21, 1922. Some later 

 results were also presented at the meetings of the 

 American Physical Society and the American 

 Astronomical Society at Boston on December 30 and 

 31, 1922.^ The analysis was made free as possible 

 from assumptions as to the systems composing the 

 entire magnetic field, and was restricted, for the time 

 being, to the region of the earth (86 per cent.) between 

 60° N. Lat. and 60° S. Lat. Treating the earth as a 

 spheroid of revolution, spherical harmonic series to 

 the sixth degree, and in some cases to the seventh, 

 were established separately for each of the rectangular 

 components : X, positive towards north ; Y, positive 

 towards east ; and Z, positive towards nadir. 



The magnetic data utilised apply in general to 

 longitude-intervals of 10° and latitude-intervals of 5°. 

 Before the polar caps may be safely included in the 

 analysis, the available magnetic data for these 

 regions will require careful examination, and it may 

 be found necessary to await additional data. Some 

 analyses were also made for the regions 30° N. Lat. 

 to 30° S. Lat., and 45° N. Lat. to 45° S. Lat. There 

 is evidence that the Gaussian coefficients definifig the 

 earth's magnetic field, are to be regarded at first as purely 

 empirical quantities and, hence, strictly applicable only 

 to the region of the earth from which they ivere derived. 

 It would seem, however, that inclusion of the polar 

 caps will not materially alter the main conclusions 

 given here. 



The chief conclusions from our analysis are as 

 follows : * 



I . For a satisfactory representation of the observed 

 data, it is necessary to recognise the existence of an 



• Phys. Rev., March 1923, pp. 370-371 and 388 ; also Pop. Astr., March 

 1923, p. 186. 



• For fuller details the interested reader may be referred to Terr. Mag. 

 and Aim. Elect, for March-June (pp. 1-28), and September 1923. 



NO. 2808, VOL. I 12] 



