8 



ELECTRO-MAGNETISM. 



above stated. Ampere has hit upon an 

 ingenious device for imprinting this 

 rule more firmly in the memory, and 

 enabling us to apply it under a great di- 

 versity of circumstances. The electric 

 currents are not only characterized as 

 positive and negative, and as flowing in 

 one or other of two directions along the 

 wire that conducts them, but may be 

 actually personified and conceived as 

 endowed with a head and feet, with a 

 face and back, and with a right and a left 

 hand. In order to turn this idea to the 

 best account, and being at liberty to 

 choose, with respect to the various kinds 

 of conditions belonging to the subject, 

 one or other of two alternatives, we shall 

 select in each case those which seem 

 naturally entitled to the preference : and 

 it fortunately happens that, on combin- 

 ing these conditions so selected, they 

 accord exactly with nature, and are 

 therefore well calculated to answer the 

 purpose of a kind of artificial memory. 



(25.) First, it is more natural to fix 

 our attention on the current of positive, 

 than of negative electricity. Secondly, 

 in a vertical wire, a descending current 

 will occur to us more readily than an 

 ascending one : or, if we imagine our- 

 selves borne along by the current, it 

 would be more natural to conceive our- 

 selves moving with our feet foremost ; 

 but if, on the contrary, we suppose our- 

 selves to be at rest, we should conceive 

 the current to be passing from our head 

 to our feet. Our/ace would, of course, be 

 turned towards the magnetic pole to which 

 we are directing our attention ; we should 

 attend to the north pole in preference 

 to the south ; and the movement with 

 which we are most familiar, is that 

 which we perform with our right hand, 

 as in writing for instance, that is, from 

 left to right. Combining these condi- 

 tions, then, we may always recollect, 



Fig. 10. 



P 



that if we conceive ourselves lying in 

 the direction of the current, the stream 

 of positive electricity flowing through 

 our head towards our feet, with the 

 magnet before us, tJie north pole of that 

 magnet will be directed towards our 

 right hand. If any one of these condi- 

 tions be reversed, the result is reversed 

 likewise. 



(26.) The action of the conducting 

 wire on the pole of a magnet is necessa- 

 rily accompanied by a corresponding 

 and opposite action of the magnet on 

 the wire. When the wire impels the 

 pole from left to right, the pole impels 

 the wire from right to left, and vice 

 versa. Thus we have seen that a posi- 

 tive current, descending along a wire, 

 of which W, Jig. 9 A, represents the sec- 



Fig. 9. 



W W 



tion by a horizontal plane, urges the 

 north pole N to the right, in the direc- 

 tion N n, but the wire Itself is also urged 

 in the direction W w, to the left. The 

 contrary action takes place between the 

 south pole S, jig. 9 B, so that if either 

 pole of a magnet were fixed, and the 

 wire moveable, the motion of the latter 

 would, as in the case we have already 

 considered, be circular, and the force 

 which impels it tangential. This is 

 shown in Jigs. 10 and 11, where pn, pn, 

 &c. show the successive positions of the 

 wire urged to move in the direction 

 shown by the arrows, by the influence 



Fig.U. 

 I 



of the north or south poles N or S. direction to that exerted by the other. 

 The influence exerted on the same cur- When the currents are reversed, all the 

 rent by the one being in the opposite effects just described are again reversed. 



