ELEMENTARY LESSONS ON [CHAP. in. 



the direction from north to south, it will cause the 

 N.- seeking end of the needle to turn eastwards: if the 

 current flows from south to north in the wire the N.-seek- 

 ing end of the needle will be deflected westwards. If 

 the wire is, however, below the needle, the motions will 

 be reversed, and a current flowing from north to south 

 (will cause the N. -seeking pole to turn westwards. 

 ^J 186. Ampere's Rule. To keep these movements 

 in memory, Ampere suggested the following fanciful but 

 useful rule. Suppose a man swimming in the wire with 

 the current, and that he ttirns so as to face the needle, then 

 the N. -seeking pole qf the needle will be deflected towards 

 his left hand. In other words, the deflection of the 

 N. -seeking pole of a magnetic needle, as viewed from 

 the conductor, is towards the left of the current. 



For certain particular cases in which a fixed magnet pole acts 

 on a movable, circuit, the following converse to Amperes Rule 

 will be found convenient. Suppose a man swimming in the 

 wire with the current, and that he turns so as to look along the 

 direction of the lines of force of the pole (i.e. as the lines of 

 force run, from the pole if it be N. -seeking, towards the pole if it 

 be S. -seeking), then he and the conducting wire with him will be 

 urged toward his left, 



187. A little consideration will show that if a current 

 be carried below a needle in one direc- 

 tion, and then back in the opposite 

 direction above the needle, by bending 

 the wire round, as in Fig. 79, the 

 forces exerted on the needle by both 

 portions of the current will be in the 

 same direction. For let a be the 

 N.-seeking, and b the S. -seeking, pole 

 of the suspended needle, then the 

 Fig 79 . tendency of the current in the lower 



part of the wire will be to turn the 

 "needle so that a comes towards the observer, while b 



