575 



Tables 723-738 



TERRESTRIAL MAGNETISM 



TABLE 723. — Magnetic Constants of the Earth 



(Prepared by J. A. Fleming, Department of Terrestrial Magnetism, Carnegie Institution 



of Washington.) 



If V be the magnetic potential of the earth, then 



V/R = 2cm"Pm n sin cos (m\ + a m n ) 

 where R — earth's mean radius (6.37 X io 8 cm), = latitude, X — east longitude, n varies 

 from 1 to co, and m from to n. The field-components of total intensity F designated, 

 X positive towards geographic north, Y positive towards geographic east, and Z positive 

 towards nadir, are 



X=-(l/R) (^A=-^Cm n (dPm n /d<t>) COS OX + a,„«) 



Y=— (i/R cos 0) OF/3\) = (i/cos <p)2mcm n Pm n sin (m\ + a m n ) 

 Z = — 2(» + l)cm n Pm n cos (mX + a m n ) 



L. A. Bauer (Terr. Mag., 28, 1-28, 1923) made an analysis based on the latest values of 

 the magnetic elements, epoch 1922, between the parallels 60° N. and 6o° S. He found the 

 following for the uniform portion of the earth's X, Y, and Z magnetic systems : 



where M is the earth's moment, and Mp and Me are its axial and equatorial components. 



For the same date Bauer deduced the following values in magnetic units : 



M = 8.04 X io 25 c.g.s. M P = 7.88 X io 25 c.g.s. Me = 1.60 X io 35 c.g.s. 



The magnetic field of the earth approximates that of a uniformly magnetized sphere, 

 its magnetic axis inclined to that of geographical rotation. The equivalent axis intercepts 

 the northern hemisphere in latitude 78 2> 2 ' N. and longitude 69 08' W. 



The intensity of the earth's magnetic field above the surface may be expressed as a first 

 approximation (according to Schmidt) by F(i — sh/R) where h is the elevation and R 

 the earth's radius ; that is, for each 2 km the field diminishes by approximately 0.1 per cent 

 while the direction is practically unchanged. 



If the earth's magnetism were distributed uniformly throughout its volume, the average 

 intensity of magnetization would be 0.074 c.g.s. The equivalent intensity of magnetization 

 has been steadily diminishing during the past 80 years at the average annual rate of 

 about 1/1,500 part. 



A. Nippoldt (Veroff. Preus. Meteor. Inst., Berlin, no. 372, 137-143, 1930) gives the 

 following positions based on observations : 



TABLE 724. — North Magnetic Pole 



The magnetic poles are not diametrically opposite, each being approximately 2300 

 kilometers (Gutenberg, Lehrb. Geophys., 400, 1929) from the antipodes of the other. These 

 poles are defined as the points at which magnetic lines of force are normal to the earth's 

 surface, and are to be distinguished from the extremities of the magnetic axis derived 

 from analysis. 



Smithsonian Tables 



