MAGNETISM. 



17 



net strictly confined to particular points 

 in the magnet : for although much con- 

 centrated at the two ends, they exist 

 with less intensity in other parts of the 

 magnet. 



(69.) Let us, however, suppose, for 

 the sake of simplification, that the mag- 

 netic forces emanate solely from the 

 two poles at the extremities of the mag- 

 net M,Jlg. 26, with its axis placed hori- 



Fig. 26. 



zontally, while a smaller magnet B sus- 

 pended on a point, or in other words, the 

 needle of a mariner's compass, and 

 which we shall therefore designate as 

 the needle, is presented to it in the vi- 

 cinity of its north pole, N, and with its 

 centre in a line with the axis of the mag- 

 net. The north pole of the magnet at- 

 tracts the south pole of the needle, and 

 tends to turn it in the direction indicated 

 by the arrow at s. It also repels the 

 north pole of the needle, turning it in the 

 direction indicated by the arrow at n. 

 These two actions, it will be seen, both 

 conspire to give the needle a rotatory 

 motion in the same direction with regard 

 to its centre, and to bring it into the po- 

 sition represented in the next figure, (27,) 



in which the south pole of the needle is 

 turned directly towards the north pole of 

 the magnet. 



The influence of the south pole, S, of 

 the magnet operates in a manner exactly 

 contrary to that of its north pole ; but 

 Leing at a greater distance, its intensity 

 is less ; and all that it can effect is to 

 subtra. ^ somewhat from the forces \vith 

 which ,the needle would have been im- 

 pellt. .f the north pole of the magnet 

 had acted alone. The general result as 

 to the rotatory motion, is therefore de- 

 termined by the predominance of the 

 actions of the north pole of the magnet, 

 and remains as before stated. 



(70.) The tendency in one magnet to 

 assume a particular position with rela- 

 tion to another magnet, is termed its 



directive force *. It results, as we have 

 seen, from the conjoined influence of two 

 forces, the one acting on the north, and 

 the other on the south pole, and is there- 

 fore equal to the sum of these forces. 



(71.) If we now consider what ten- 

 dency the needle has to approach to, or 

 recede from the magnet, we shall find 

 the same forces, which in the former in- 

 stance conspired together, now op- 

 posing each other. It is, in the first 

 place, evident that while the needle is in 

 the position shewn iny?g\ 26, that is, at 

 right angles to the magnet, the attrac- 

 tion of the adjoining north pole of the 

 magnet for the south pole of the needle, 

 is balanced by its repulsion for its north 

 pole ; and the needle, although strongly 

 urged by these forces to turn round its 

 centre, has no tendency, on the whole, to 

 recede from, or approach the magnet. 

 When, however, it arrives at the posi- 

 tion shewn in/g-. 27, its south pole s 

 being nearer to N than its north pole n, 

 the attractive action is more powerful 

 than the repulsion, and the needle is, 

 consequently, now impelled towards the 

 magnet. But the force which thus im- 

 pels it results from the difference only of 

 two contrary forces, the one attractive, 

 the other repulsive. 



(72.) Hence we may conclude that the 

 directive force, which consists of the 

 sum of two forces, is in all cases con- 

 siderably greater than the attractive force 

 exerted upon the whole needle ; this 

 latter force being only equal to the diffe- 

 rence between the same forces. The 

 ratio between the directive and attractive 

 forces will be increased, either by di- 

 minishing the length of the needle, or 

 increasing that of the magnet. Hence 

 the polarity of a small needle may be 

 considerable, while its attraction is quite 

 insensible. 



(73.) Let us next transfer the needle 

 to the situation shown in Jigs. 28 and 29, 

 in which its centre is in a line drawn 

 from the centre of the magnet, and at 

 right angles to its axis ; the needle being 

 supposed, as in the former case, to be so 

 balanced as to turn freely in a plans 

 which passes through the centre of the 

 needle, and both poles of the magnet. 

 Let it be placed in the position indicated 

 in fig. 28, with one of its poles di- 

 rected towards the middle of the mag- 

 net. The directive force is here com- 

 pounded of four forces: the attrac- 

 tions of N for s and of S for n ; and 



* Dr. Gilbert expressed it by the term verticity, 



c 



