MOVEMENTS OF DIPPING NEEDLES. 



101- 



tion, the coil is turned around so as to bring the cur- 

 rent on the upper side in the same direction, and 

 the ends or poles of the spiral coil are reversed. 



The mathematical precision with which the 

 movements of electrodes conform to the rule 

 of " reciprocal at- 

 traction between 

 similarly direct- 

 ed currents," is 

 seen in the pecu- 

 liar movements of s* x 

 dipping needles 

 in arranging their 

 positions in rela- 

 tion to the circu- 

 lation of the ter- Fig. 24 . 

 restrial electric currents, as exhibited in Fig. 24. 



The direction of the currents from east to west 

 about the earth is represented by the arrows, and 

 also the direction of the currents about the compass 

 needles ABC; to which spherical outlines are 

 added, to show the directions of the electro-mag- 

 netic currents about each of them when transferred 

 from the earth's equator, at A, toward the pole, at 

 B and at c. The inclination of the compass 

 needle, at B, shows the actual movement of a dip- 

 ping needle ; and the currents about B, and about 

 the earth in that latitude, as will be manifest on 

 inspection, are brought to move in similar direc- 

 tions by the " dip " of the needle. The needle, at c, 



