102 



POSITIONS OF DIPPING NEEDLES. 



Fig. 25. 



is turned entirely around. The several relative 



positions of the dipping 

 needle are designated in 

 Fig. 25 by the arrows, ar- 

 ranged on different parts 

 of the earth's surface. 



That the phenomenon 

 of magnetism is excited 

 by terrestrial electric 

 currents, is verified by 

 the following experi- 

 ments. If a bar of iron be held in a horizontal 

 east and west position, in the line of movement 

 of the terrestrial currents, as represented at B A 

 in Fig. 26, and a magnetic needle be brought near 



either end of 

 the bar, an 

 equal recipro- 

 cal attraction 

 takes place be- 

 tween them. 

 On lifting the 

 end A to a 

 sloping posi- 

 tion, as repre- 

 sented by c D, in a line directed toward the north 

 pole of the earth, the terrestrial currents will in- 

 duce the circulation of currents about the bar of 

 iron, c D, and convert it into an electro-magnet ; 

 with the lower end, c, having the same polarity 



Fig. 26. 



