CHAPTER I. 



EARTH-CURRENTS AND THEIR RELATION TO CERTAIN TERRESTRIAL 



MAGNETIC PHENOMENA. 



INTRODUCTION. 



140. As soon as the discovery of OERSTED, in 1819, of the effect produced by galvanic currents 

 on magnets was made known to the world, attempts were made to explain the earth's magnetism and 

 its variations by means of currents circulating in the earth. 



As early as 1821 DAVY(') suggested that the variation in declination might possibly be due to 

 such currents, and some years after the same view was taken up and carried further by CHRISTIE ( 2 ) 

 and P. BARLOW( S ). 



These ideas seem to have met with general acceptance, and soon became the current explanation 

 for the pulpit. 



The theory of magnetism as caused by earth-currents was merely founded on speculation, and years 

 had passed before the question was put to an actual experimental test. 



The first attempts at measuring currents in the earth's crust were made in mines in Cornwall ( 4 ). 

 It seems, however, hardly possible to decide whether the currents measured were real earth-currents 

 or not. 



Experiments of a similar kind were made by BECQUEREL ( 5 ) in the salt-mines of Dieuze. He observed 

 the currents called into play when various layers of the earth were connected by conducting wire. 



W. H. BARLOW () seems to have been the first to show that currents were almost always circu- 

 lating in the earth's crust. He used four telegraph lines starting in different directions from the same 

 central station at Derby. About simultaneously, earth-currents were observed by BAUMGARTNER on the 

 line between Vienna and Gratz. 



It was found that earth-currents ordinarily circulating in the earth were very variable in strength. 

 The first result of the actual test of earth-currents was that the view put forward by P. Barlow, that 

 the earth's magnetism was directly caused by currents circulating in the earth, was not confirmed by 

 experiment. This conclusion, as far as I know, was first positively stated by AIRY. 



But there still remained for investigation the question as to whether, or to what extent earth- 

 currents produce the magnetic variations. As the result of comparison of currents with the variation 

 of magnetic elements, Barlow finds that simultaneous observations showed no marked similarity in the 

 path described by the magnetic needle and the galvanometer, LLOYD, however, from the same obser- 



(!) Sir H. DAVY: Phil. Trans. 1821 p. 7. 



('-) C. H. CHRISTIE: Phil, Trans. 1827 p. 308. 



( 3 ) P. BARLOW: Phil. Trans. 1831 p. 99. 



( 4 ) R. W. Fox: Phil. Trans. 1830. R. W. Fox, HUNT, PHILLIPS: Annual Report of the Roy. Polytechnic Institution of 

 Cornwall, 1836, 1841, 1842. 



( f> ) BECQUEREI. : Comptes Rendus XIX, p. 1052. 

 I 11 ) Phil. Trans. 1849, p. 61. 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. \\-2 



