ASTATIC NEEDLES. 745 



and also on the direction of the current itself. The 

 following rule is generally given to assist the memory 

 in determining the various deflections of the needle 

 under the influence of a current. If we imagine an 

 observer placed in the conducting wire in such a manner 

 that the current entering by his feet issues by his head, 

 and that his face is always turned towards the needle, 

 the north pole of the needle is always deflected towards 

 his left hand. 



The student should prepare a small cylinder of wood, which may 

 represent the needle, and after drawing round the surface several 

 circles with arrow-heads to indicate the direction of the Amperian 

 currents, and marking the two ends correctly N and S (see fig. 356) 

 to distinguish the poles, then the direction in which a needle is de- 

 flected in any given case may be found by simply holding the little 

 cylinder in such a manner that the Amperian currents drawn upon 

 it have the same direction as the current in the wire. The position 

 of a magnetic needle would in fact be exactly the same as that of 

 the small wooden cylinder, if the needle were not influenced by the 

 action of the magnetism of the earth, in addition to the action of the 

 current. In consequence of this the needle would, however, take 

 up a position intermediate between its natural north-south position 

 and that indicated by the wooden cylinder. 



A very weak current may produce a considerable 

 deflection if an ' astatic ' system of two needles, fig. 

 376, be used. Two mag- 

 netic needles, n $ and 

 n 2 s 2 , as nearly equal as pos- 

 sible, are rigidly connected 

 in such a manner that 

 their poles are turned 

 opposite ways. If the 

 magnetism of one needle 

 were exactly equal to that Fia 376 (rea! 



of the other, the earth would not exert any directive 



