THE SUN AND THE EARTH'S MAGNETIC FIELD 



By J. A. Fleming 

 Director, Department of Terrestrial Magnetism, Carnegie Instiiution of 



Washington 



[With 19 plates] 

 INTRODUCTION 



The Smithsonian Listitution has been interested since its founda- 

 tion in 1846 in researches involving the sun. Its first Secretary, 

 Joseph Henry, was also interested in the earth's magnetism and in 

 1830-31 made a series of observations at Albany, N. Y., to determine 

 its intensity, and observed and reported upon a magnetic disturbance 

 in connection with the appearance of an aurora. He nurtured the 

 idea that solar investigations might be advanced by the application 

 of laboratory technique. By the use of thermoelectric apparatus in 

 connection with the solar disk projected by a telescope on a screen 

 in a dark room, he concluded that a sunspot emitted less heat than the 

 surrounding parts of the luminous disk. As a member of the faculty 

 of Yale College, he accompanied his friend A. D. Bache — American 

 pioneer in the study of the earth's magnetic field — who went to Europe 

 in 1837-38 to purchase instruments for the first magnetic observatory 

 (Girard College, Philadelphia) and survey in the United States. In 

 one of his memoirs he says, "It must now be admitted that magnetism 

 is not confined to our earth, but is common to other and probably all 

 bodies of our system." Through these researches, coupled with others 

 pertaining to various aspects of solar radiation, Henry foreshadowed 

 the productive researches in solar physics so ably conducted and en- 

 couraged for many years by the present Secretary. 



We are all acquainted with certain familiar aspects of the sun. We 

 all know that it gives us light and warmth. We have all seen it set 

 in the evening and — since the introduction of war time — many of us 

 have seen it rise in the morning. Biologists have interpreted the 

 physiological effects of the sun's radiation on plants and animals and 

 have shown how the chlorophyll and other substances in vegetable 



* The eleventh Ai'thur lecture given under the auspices of the Smithsonian Institution, 

 February 26, 1942. 



173 



