6 1815 



THE 



POPULAR SCIENCE 



MONTHLY. 



AUGUST, 1913 



THE EAETH AND SUN" AS MAGNETS 1 



By De. GEORGE ELLERY HALE 



MOUNT WILSON SOLAR OBSERVATORY 



IN 1891, Professor Arthur Schuster, speaking before the Eoyal 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, substituted a 

 large number of small magnets, distributed in an irregular manner, 

 for the single magnet of Biot. Grover suggested that terrestrial mag- 

 netism may be caused by electric currents, circulating around the earth 

 and generated by the solar radiation. Soon after Bowland's demon- 

 stration 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. Bowland 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 re- 

 mained for Schuster to suggest that a body made up of molecules which 

 are neutral in the ordinary electrical or magnetic sense may neverthe- 

 less develop magnetic properties when rotated. 



We shall soon have occasion to examine the two 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 completely 

 the principal phenomena of terrestrial magnetism. If we turn to ex- 

 periment, we find that iron globes, spun at great velocity in the lab- 

 oratory, fail to exhibit magnetic properties. But this can be accounted 

 for on either hypothesis. What we need is a globe of great size, which 



1 Address delivered upon the occasion of the semi-centennial anniversary of 

 the foundation of the National Academy of Sciences, April 22, 1913. 



vol. lxxxiii.— 8. 



