2 BAUER : OEIGIN OF THE EARTH's MAGNETIC FIELD 



raised the question as to whether the magnetism of the Earth and 

 celestial bodies, in general, may not be connected in some manner 

 with the fact of their rotation.* J. J. Thomson had also pointed 

 out in 1894* that if atoms possess a specific attraction for the two 

 electricities— attracting one kind slightly more than the other — 

 then a large rotating body ought to produce a magnetic field. 



Sutherland's hypothesis avoids the difficulties pointed out by 

 Rowland^ in 1879 as also some of those advanced in connection 

 with other theories. Unfortunately, however, it thus far admits 

 of no conclusive proof, chiefly for the reason that it leads to an 

 expression for the magnetic potential precisely the same, as far as 

 effects on or above the Earth's surface are concerned, whether 

 the magnetic field be due to an elemental magnet at the center of 

 the Earth, or to a uniformly magnetized sphere, or to an appro- 

 priate system of electric currents imbedded in the Earth, or to a 

 rotating electrically charged sphere such as Sutherland supposed, 

 or to a combination of these causes. Mathematically the mag- 

 netic fields ascribed to these various, distinct causes can not be 

 differentiated from one another. 



If the supposed electrical distribution really exists within the 

 Earth, then it may be that, because of gravitational force, or of 

 some other central force, the elemental charges tend to arrange 

 themselves so that the "center of gravity" of one kind of elec- 

 trical charge is, on the average, slightly nearer the Earth's center 

 than that of the other kind, or, still better, so that the mean 

 volume density of the positive body charge is slightly different 

 from that of the negative. This being assumed, it seems rational 

 to inquire whether, during the Earth's rotation there may occur 

 some alteration in the mean densities of the opposite charges or 

 in the radial separation, or in both, to be attributed perhaps to 

 some action of the Earth's centrifugal force. Though the ques- 



' The reader desiring to familiarize himself with the difficulties attaching to 

 the chief hypotheses advanced is advised to read the following suggestive papers: 

 Schuster, A., A critical examination of the possible causes of terrestrial magnet- 

 ism, Proc. Phys. Soc. London, 24: pt. Ill, p. 127; and Swann, W. F. G., The Earth's 

 magnetic field, Phil. Mag., 24: 80. 1912. 



* Thomson, J. J., On the electricity of drops, Phil. Mag., [5], 37: 358. 1904. 



" Rowland, Physical papers, p. 182, Johns Hopkins Press, Baltimore, 1902. 



