Chap. ix. j AMPERE'S LAW. 95 



angle. The side to which the needle is deflected de- 

 pends on the direction of the current, and whether 

 passed above or below the needle. The laws of 

 direction were worked out by Ampere ; and he has 

 given an easily remembered rule for determining the 

 directions. Suppose an observer placed parallel to 

 and facing the wires, and let the current be directed 

 as if passing from his feet to his head, then the north 



Figs. 48_aud 49. Ampfcre's Law. 



pole will be deflected to his left, and the south pole in 

 the opposite direction. This rule is illustrated in 

 Figs. 48 and 49. 



It is seen that a current flowing above the needle, 

 which deflects the needle to the left, will, if it flow in 

 the same direction below the needle, deflect it in the 

 opposite direction. In the figures, AB is the magnetic 

 needle, A the north pole and B the south, and XY 

 is the wire along which the current passes, the 

 arrow indicating the direction of the current. 

 Arrows and dotted lines indicate the deflection 

 of the needle. Thus the north pole A is de- 

 flected in the direction of the arrow to A, and the 

 south pole B to B'. Again, a needle deflected to the 

 left by a current flowing in one direction above it, 

 will be deflected still farther to the same side by 

 a current below the needle in the opposite direction. 

 Thus a current carried right round the needle will 

 travel above the needle in one direction, and below 



