MAGNETIC METHODS 



89 



change of dip with latitude is more rapid near the equator than at the poles. 

 From Gilbert's work, the basic concepts of geomagnetics were evolved. 



Building on the discoveries of Gilbert and others, C. F. Gauss in 1832 

 put forward a general theory of earth magnetism. In it he presented a 

 fairly accurate and quantitative idea of the nature and distribution of the 

 magnetic field of the earth. Gauss developed a potential formula in terms 

 of spherical harmonics to represent the facts of the earth's magnetism as 

 then known. Based on the work of Gauss, the science of terrestrial mag- 

 netism has progressed steadily. It was, in fact, one of the first fields of 

 international cooperation for the study of a world-embracing natural 

 phenomenon. 



The earth acts as a magnetized sphere surrounded and permeated by 

 a magnetic field of strength of about 0.6 gauss. As a first approximation 

 this field is so distributed at the surface and is of such an amount that it 

 could arise from a short powerful bar magnet at the center of the earth. 

 The moment of such a hypothetical magnet has been calculated and was 

 found in 1922 to be some 8 • 10-^ c.g.s. units. 



Toto/ /nffnjfty^ T 

 C^Z.'^Horizontal Coniponmnf, // 

 Verfical Cofnpon^nt, 2 

 Dtp or /n<//naf^n, i 



Maanffic equator 



Fig. 23 — Schematic diagram illustrating magnetic field produced by an 

 imaginary bar magnet located at the earth's center. 



The case under discussion is pictured in Figure 23 which shows an ideal 

 earth and the magnetic field developed in it from a short bar magnet at its 

 center. 



Calculations indicate that the earth's field is most closely duplicated 

 when the poles of the imaginary magnet, shown in Figure 23, are practically 

 coincident. The magnetic poles, therefore, of the spherical magnetic earth 



