174 MAGNETISM 



Horseshoe magnets. For purposes in which the two poles 

 are required close together, steel magnets of the horseshoe form 

 are often convenient. Bars of this shape may have their length 

 greater in proportion to breadth and thickness than straight bars, 

 and they may be conveniently magnetised either by the electro- 

 magnetic method, a coil being placed on each limb, or by the 

 method of double touch after connecting the two ends by a soft 

 iron cross-piece. 



Compound magnets. Very powerful " compound " magnets 

 are often formed by building together a number of single bars or 

 horseshoes magnetised separately. This method is not so much 

 used now as formerly, when it was difficult to get thick bars of 

 homogeneous steel. 



Distribution of magnetisation. If a straight bar magnet 

 is very carefully magnetised the magnetic action appears to radiate 

 from very near the ends, and at some distance from the bar the 

 resultant action is very nearly the same as if the poles were actual 

 points within the bar and near the ends. The thinner the bar in 

 comparison to its length, the nearer are the poles to the ends. If 

 the bar has dimensions, say, 10 x 1 X J, then the poles may perhaps 

 be considered as an eighth or a tenth of the length from the ends. 

 This is, however, only an approximation. If we imagine an ideal 

 magnet as consisting of a very fine wire or fibre, with its poles 

 actually at the two ends, then any real n induct may be regarded as a 

 bundle of such fibres of unequal lengths. If the magnet is well 

 made the fibres will have their middle points at the middle of the 

 bar so that the magnetisation is symmetrically distributed, and the 

 greater number will be of nearly the same length as the bar. 



Ball -ended magnets. Balls of steel or soft iron are bored 

 and are put one on each end of a steel rod which is magnetised. 

 The force is directed very much more nearly to a definite pole at 

 the centre of the ball than in other magnets.* 



Consequent poles. If there is any irregularity in the 

 stroking of a bar in the process of magnetisation as, for example, 

 magnetising it first in one direction and then in the other 

 consequent poles may be developed, i.e. poles of the same kind may 

 be formed together within the bar. We may easily obtain a 

 knitting needle magnetised as in Fig. 119, each end being North- 



s s 

 FIG. 119. 



seeking, two consequent South-seeking poles occurring somewhere 

 between them. 



Magnetisation chiefly near the surface. The per- 



* Searle, Camb. Phil. Soc. Proc., 12, p. 27, 1903. Ball-ended magnets were 

 devised by Kobison in the eighteenth century, but had been entirely forgotten 

 till they were re-invented by Searle. 



