THE SEMI-CENTENNIAL ANNIVERSARY 37 



AFTERNOON SESSION 



ADDRESS OF DR. GEORGE ELLERY HALE 



Mount Wilson Solar Observatory 



ON 



THE EARTH AND SUN AS MAGNETS 



In 1891, Professor Arthur Schuster, speaking before the Royal 

 Institution, 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 mathe- 

 matical investigation, substituted a large number of small mag- 

 nets, distributed in an irregular manner, for the single magnet 

 of Biot. Grover suggested that terrestrial magnetism may be 

 caused by electric currents, circulating around the earth and 

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

 stration in 1876 that a rotating electrically charged body pro- 

 duces 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 properties when rotated. 



We shall soon have occasion to examine the two hypotheses 

 advanced 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 mag- 

 netism. If we turn to experiment, we find that iron globes, spun 

 at great velocity in the laboratory, fail to exhibit magnetic prop- 

 erties. But this can be accounted for on either hypothesis. What 



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