THE EARTH AND SUN AS MAGNETS.^ 



By Dr. Gbx)bge Ellery Hale,^ 

 Mount Wilson Solar Observatory. 



[With 8 plates.] 



In 1891 Prof. x\.rthiir Schuster, speaking before the Royal Insti- 

 tution, asked a question which has been widely debated in recent 

 years : " Is every large rotating body a magnet ? " Since the days of 

 Gilbert, who first recognized that the earth is a great magnet, many 

 theories have been advanced to account for its magnetic properties. 

 Biot, in 1805, ascribed them to a relatively short magnet near its 

 center. Gauss, after an extended mathematical investigation, sub- 

 stituted a large number of small magnets, distributed in an irregu- 

 lar manner, for the single magnet of Biot. Grover suggested that 

 terrestrial magnetism may be caused by electric currents, circulating 

 around the earth and generated bj^ the solar radiation. Soon after 

 Rowland's demonstration in 1876, that a rotating electrically charged 

 body produces a magnetic field, Ayrton and Perry attempted to 

 apply this principle to the case of the earth. Rowland at once 

 pointed out a mistake in their calculation, and showed that the high 

 potential electric charge demanded by their theory could not possibly 

 exist on the earth's surface. It remained for Schuster to suggest 

 that a body made up of molecules which are neutral in the ordinary 

 electrical or magnetic sense may nevertheless develop magnetic prop- 

 erties when rotated. 



We shall soon have occasion to examine the tAvo hypotheses ad- 

 vanced in support of this view. While both are promising, it can 

 not be said that either has been sufficiently developed to explain com- 

 pletely the principal phenomena of terrestrial magnetism. If we 

 turn to experiment, we find that iron globes spun at great velocity 

 in the laboratory fail to exhibit magnetic properties. But this can 

 be accounted for on either hypothesis. Wliat we need is a globe of 

 great size, which has been rotating for centuries at high velocity. 

 The sun, with a diameter 100 times that of the earth (fig. 1), may 



^Address delivered at the semicentennial of the National Academy of Sciences, at 

 Washington, D. C, May, 1913. 



- The author had expected, before reprinting this address, to subject it to a thorough 

 revision and to insert the results of recent observations, but he has been prevented by 

 illness from doing so. (Aug. 24, 1914.) 



44863°— SM 1913 10 145 



