ACTION OF TERRESTRIAL CURRENTS. IO3 



as the end of the needle N. Consequently, as 

 the N poles of magnets reciprocally repel each 

 other, the needle is swung around to the position 

 denoted by the dotted needle N s. 



In proportion as the end D is lowered to its 

 previous horizontal position A B, in an east and 

 west direction, the magnetic needle swings back, 

 and is attracted again by either end of the bar, 

 indifferently. 



This experiment shows not only the action of 

 the electric currents circulating about the earth, 

 but also the sensitiveness of all bodies and mole- 

 cules of matter to terrestrial electro-magnetic 

 excitation, even by slight changes of relative po- 

 sition in regard to one another, while serving as 

 electrodes. An iron fire-poker, if it happens to 

 remain in the inclined position of a dipping needle, 

 becomes magnetically excited by the terrestrial 

 currents. The large vertical steel-drills used in 

 machine-shops are rendered so powerfully mag- 

 netic, that iron chips cling to them after the drill 

 has been used. 



