CHAP, in.] ELECTRICITY AND MAGNETISM. 157 



84, then the motions would be just in the reverse sense. 

 It would seem from this as if a N.- seeking pole of a 

 magnet ought to revolve continuously round and round a 

 current ; but as we cannot obtain a magnet with one 

 pole only, and as the S. - seeking pole is urged in an 

 opposite direction, all that occurs is that the needle sets 

 itself as a tangent to a circular curve surrounding the 

 conductor. This is what Oerstedt meant when he 

 described the electric current as acting " in a revolving 

 manner," upon the magnetic needle. The field of force 

 with its circular lines sunx .ling 

 a current flowing in a straight 

 conductor, can be examined ex- 

 perimentally with iron filings in 

 the following way : A card is 

 placed horizontally and a stout 

 copper wire is passed vertically 

 through a hole in it (Fig. 85). 

 Iron filings are sifted over the 

 card (as described in Art. 108), 

 and a strong current from three 



or four large cells is passed through the wire. On 

 tapping the card gently the filings near the wire set 

 themselves in concentric circles round it. 



192. Equivalent Magnetic Shell: Clerk-Max- 

 well's Theorem. For many purposes the following 

 way of regarding the magnetic action of electric currents 

 is more convenient than the preceding. Suppose we take 

 a battery and connect its terminals by a circuit of wire, 

 and that a portion of the circuit be twisted, as in Fig. 86, 

 into a looped curve, it will be * th I the entire 



space enclosed by the loop pos=>^_ x tic properties. 



In our figure the current is supposed to be flowing round 

 the loop, as viewed from above, in the same direction as 

 the hands of a clock move round ; an imaginary man 

 swimming round the circuit and always facing towards 

 the centre would have his left side down. By Ampere's 



