S40 



LESSONS IN ELECTRICITY. 



the action of the two poles of the magnet 

 on n ? Your reply will of course be that the 

 pole S attracts n while the pole N repels it. 

 Let the magnitude and direction of the at- 

 traction be expressed by the line n m, and the 

 magnitude and direction of the repulsion by 

 the line n o. Now the particle n being equally 

 distant from S and N, the line no, expressing 

 the repulsion, will be equal to m n, which 

 expresses the attraction, and the particle n t 

 acted upon by two such force-s, must evi- 

 dently move in the direction^? n, exactly mid- 

 way between in n and n o. Hence you see 

 that, although there is no tendency of the 

 particle n to move toward the magnetic 

 equator, there is a tendency on its part to 

 move parallel to the magnet. If instead of a 

 particle of north magnetism we placed a par- 

 ticle of south magnetism opposite to the mag- 



FIG. 1. 



netic equator, it would evidently be urged 

 along the line n q ; and if instead of two sep- 

 arate particles of magnetism we place H little 

 magnetic needle, containing both north and 

 south magnetism, opposite the magnetic 

 equator, its south pole being urged along n q, 

 and its north along n p, the little needle will 

 be compelled to set itself parallel to the mag- 

 net S N. Make the experiment, and satisfy 

 yourselves that this is the case. 



Substitute for your magnetic needle a bit 

 of iron wire, devoid of permanent magnetism, 

 and it will set itself exactly as the needle 

 does. Acted upon by the magnet, the wire, 

 as you know, becomes a magnet and behaves 

 as such ; it will, of course, turn its north pole 

 to ward p, and south pole toward q, just like 

 the needle. 



But supposing you shift the position of 

 your particle of north magnetism, and bring 

 it nearer to one end of your magnet, than to 

 the other, the forces acting on the particle 

 are no longer equal ; the nearest pole of the 

 magnet will act more powerfully on the par- 



Fio. 2. 



tide than the more distant one. Let S N, 

 Fig. 2, be the magnet and n the particle of 

 north magnetism in its new position. Well, 

 it is repelled by N, and attracted by S. Let 

 the repulsion be represented in magnitude 

 and direction by the line n o, and the attrac- 

 tion by the shorter line n in. The resultant 

 of these two forces will bo found by complet- 

 ing the parallelogram m n o p, and drawing 

 its diagonal n p. Along np, Ihrn, a particle 

 of north magnetism would be urged by the 

 simultaneous action of S and N. Substitut- 

 ing a particle of south magnetism for n, the 

 same reasoning would lead to the conclusion 

 that the particle would be urged along n q, 

 and if we place at n a short magnetic needle, 

 its north pole will be urged along n p, its 

 south pole along n q, 9nd"the onlv ^osition 

 possible to the needle, thus acted on, is along 

 the line p q, which, as you see, is no longer 

 parallel to the magnet. Verify this by actual 

 experiment. 



In this way we might go round the entire 

 magnet, and considering its two poles as two 

 centres from which the force emanates, we 

 could, in accordance with ordinary mechani- 

 cal principles, assign a definite direction to 

 the magnetic needle at every particular place. 

 And substituting, as before, a bit of iron 

 wire for the magnetic needle, the positions of 

 both will be the same. 



Now, I think, without further preface, you 

 will be able to comprehend for yourselves, 

 and explain to others, one of the most in- 

 teresting effects in the whole domain of mag- 

 netism. Iron filings you know are particles 

 of iron, irregular in shape, being longer in 

 some directions than in others. For the pres- 

 ent experiment, moreover, instead of the iroo 

 filings, very small scraps of thin iron wire 

 might be employed. I place a sheet of paper 

 over the magnet ; it is all the better if the 

 paper be stretched on a wooden frame, as 

 this enables us to keep it quite level. I scat- 

 ter the filings, or the scraps of wire, from a 

 sieve upon the paper, and tap the latter gently, 

 so as to liberate the particles for a moment 

 from its friction. The magnet acts on the fil- 

 ings through the paper, and see how it 

 arranges them ! They embrace the magnet 

 in u series of beautiful curves, which are 

 technically called magnetic curves, or lines 

 of magnetic force. t)oes the meaning of 

 these lines yet flash upon you? Set your 

 magnetic needle or your suspended bit of 

 wire at any point of one of the curves, and 

 you will find the direction of the needle or of 

 the wire to be exactly that of the particle cf 

 iron, or of the magnetic curve at the point. 

 Go round and round the magnet ; the direc- 

 tion of your needle always coincides with the 

 direction of the curve on which it is placed. 

 These, then, are the lines along which a par- 

 ticle of south magnetism, if you could detach 

 it, would move to the north pole, and a bit 

 of north magnetism to the south pole ; they 

 are the lines along which the decomposition 

 of the neutral fluFd takes place, ancf in tho 

 case of the magnetic needle, one of its poles 

 being urged in one direction, and the other 



