MAGNETIC EFFECT OF ELECTRIC CURRENT. 



95 



move around the wire in a counter-clockwise direction in Figs. 

 59 and 60. The direction of a current in a wire may be deter- 

 mined by means of the compass, as follows : Bring the compass 







K / 



F X / 



N 



Fig. 59. 



near the wire, and, knowing that the forces which act on the two 

 poles of the compass needle are at right angles to lines drawn from 

 the poles to the wire, infer, from the observed movements of the 

 needle, the direction in which the north-pointing pole of the nee- 

 dle tends to move around the wire. The direction of the current 

 in the wire * is the direction in which a right-handed screw with its 

 axis parallel to the wire would travel if the screw is turned in the 

 direction in which the north-pointing pole tends to move around the 

 wire. 



51. The composite magnetic field which is produced when a 

 straight electric wire is stretched across a region which, but for the 



* See Art. 2. 



Fig. 61. 



