69 



On the earth-currents. 



After the great discoveries of Oersted and Ampere, 

 concerning the effect of the electric currents on the mo- 

 veable magnetic needle, the investigators began to search 

 in electrical currents for the causes of the earth-magnetism. 

 It was soon brought forward that the earth could be re- 

 garded as a solenoid or a coil and thereby its magnetism 

 be explained. The next step was to investigate if currents, 

 constituting the solenoid, were really to be found in the 

 earth, flowing around its magnetic axis. Besides some scat- 

 tered researches of Walker and others, the first more sy- 

 stematic investigation was made by Lamont at Munnich and 

 was taken up anew by Airy in Greenwich (1868) with im- 

 proved apparatus. As the method, introduced by Lamont, 

 is still employed at present in its capital features, we will 

 describe it briefly. 



In two rectangular directions, magnetic S — N and E — W, 

 metallic plates (zinc or platina) are sunk in the earth a 

 few feet beneath the surface. These plates are united by 

 means of insulated wires (copper or iron) with a sensible 

 galvanometer, one for every direction. 





Fis. 6. 



In the fig. 6 p and p, represent the plates and g the 

 galvanometer only for the one direction f. ex. E— W. 



