Electroluminescence 261 



the aurora and the magnetic needle 



Edmund Halley called attention to the relation between auroral 

 streamers and the magnetic field of the earth in 1716. The influ- 

 ence of the aurora in causing irregular movement of the magnetic 

 needle during displays appears to have been first observed in 1741 

 by Olav Peter Hjorter ^^ (1696-1750) , an assistant of Anders Celsius 

 (1701-1744) , professor of astronomy at Upsala. The observations 

 were continued by Celsius and additions made in 1750 by Peter 

 Wilhelm Wargetin ^* (1717-1783), an astronomer and secretary of 

 the Swedish Academy of Sciences. 



In 1759 Canton confirmed the Swedish observations and remarked 

 that Wargetin was " silent as to the cause." He then proceeded to 

 give his own explanation. Canton believed that the well-known 

 diurnal variation of the needle is connected with the daily tem- 

 perature change on the earth and applied the same explanation to 

 the aurora. Since the aurora is "electricity of the heated air above 

 (the earth) ," it would also affect the needle and would " appear 

 chiefly in the northern regions as the alterations of the heat of the 

 air in those parts will be greatest." 



Actually the magnetic variations on the earth are primarily re- 

 lated to the appearance of magnetic storms on the sun, visible as 

 the sunspots, described by Johann Fabricius (1611) , and by Galilei. 

 Cassini, in his work on the zodiacal light, had related this phenomen 

 to sunspots and said that after the year 1688, when the zodiacal 

 light began to decline, the sunspots also disappeared. 



In later years the relation of aurorae to magnetic fields on the 

 earth was particularly studied by John Dalton (1766-1844) , the dis- 

 tinguished English chemist and author of Meteorological Observa- 

 tions and Essays (London, 1793: 153-194) , who paid much attention 

 to the disturbance of the magnetic needle during a display and also 

 emphasized that the luminous beams were always parallel to the 

 dipping needle and the arches bore definite relations to the magnetic 

 meridian. The effect on the compass was also studied by Dominique 

 Francois Jean Arago (1786-1853), the famous French astronomer, 

 during the years 1818 to 1835. 



MISCELLANEOUS THEORIES OF THE AURORA 



After the publication of Different Kinds of Air by Joseph Priestley 

 (1774), Richard Kirwan (1733-1812), Irish chemist and meteor- 

 ologist, published An Essay on the Variations of the Barometer 



^'' Vetensk. Acad. HandL, 1747. 



"P. W, Wargetin, Phil. Trans. 47: 126-131, 1752, in Latin. 



