MAGNETISM. 



the action of B, reacts upon B> and, ex- 

 erting a new inductive influence, tends 

 to increase its magnetism in the same 

 manner as if it had been a permanent 

 magnet. Thus it is that in a series of 

 iron bars held together by induced mag- 

 netism, each piece tends to increase the 

 strength of the preceding piece, and 

 the whole coheres together -with greater 

 force than if no such reaction took 

 place. This circumstance affords a fur- 

 ther explanation of the strong cohesion 

 we observe among the particles of iron 

 filings, which hang in long threads from 

 the poles of a magnet. 



(34.) A closer attention to the conse- 

 quences which flow from magnetic in- 

 duction, will also enable us to explain 

 another remarkable fact, which the ad- 

 hesion of the strings of iron filings pre- 

 sents. It is that each separate filament, 

 although composed of parts that attract 

 each other in the direction of their 

 length, yet shew a tendency to keep dis- 

 tinct from the neighbouring filaments, 

 and even appear to repel one another. 

 In order to understand this, let us con- 

 sider the condition of several slender 

 iron bars placed side by side, and ad- 

 hering to the north pole of a magnet, as 

 shewn in fig. 14. The in- 14 

 ductive power of the mag- 

 net, as we have seen, will 

 render each of the ends in 

 contact with that pole, a 

 south pole, while all the re- 

 mote ends will be north 

 poles. Hence, the bars will 

 all have their similar poles 

 near each other, and this 

 will happen at both their 

 extremities, and they will 

 accordingly repel one ano- 

 ther. As long as they ad- 

 here to the magnets by one end, this 

 repulsion will be prevented by that ad- 

 hesion from shewing itself; but at the 

 other ends, which are at liberty to move, 

 it will be strongly manifested, and the 

 bars will be observed to separate or 

 diverge from one another. Now this is 

 very nearly the condition of the fila- 

 ments composed of particles of iron. 

 The polarities of those parts of each 

 which are in contact, are neutralized and 

 become scarcely sensible ; but those of 

 the extremities, being uncompensated, 

 exert their full power, and produce the 

 observed repulsion of the filaments. 



(35.) This effect of induction is exceed- 

 ingly well illustrated by the following 

 experiment of Cavallo: let two short 



pieces of iron wire, fig. 15, be each fas- 

 tened to a thread, the threads being 

 joined at their other ends and formed 

 into a loop, by which they are to be sus- 

 pended from a hook or pin, so as to have 

 full liberty to move. On bringing the 

 pole of a magnet, the south pole for 

 instance, at a certain distance below the 

 wires, it will occasion them to recede 

 from each other, as shewn in fig. 16, in- 



Fig.15. Fig.]6. Fig. 17. 



9 



dicating the repulsion which takes place 

 between the adjacent ends of the wires, 

 in consequence of their being similarly 

 affected by the inductive power of the 

 magnet ; the lower ends of both being 

 rendered north poles, and the upper ends 

 south poles. This divergency of the 

 wires will continue to increase until the 

 magnet has approached to a certain 

 limit. But if the magnet is brought 

 nearer than this limit, its own attractive 

 force becomes so strong as to overpower 

 the repulsion that exists between the 

 lower ends of the wire ; and therefore 

 brings them nearer to each other, as 

 shewn in fig. 1 7 ; while the repulsion 

 of the upper ends s, s, still conti- 

 nues to manifest itself, by keeping them 

 remote from one another. On removing 

 the magnet entirely, the wires imme- 

 diately collapse, their magnetism being 

 only of a transitory nature. But if the 

 same experiment be made with sewing 

 needles, instead of soft iron wires, the 

 needles will often continue to repel each 

 other after the removal of the magnet, 

 having acquired some degree of perma- 

 nent magnetism by the circumstances in 

 which they have been placed. 



(36.) If four wires be suspended in a 

 manner similar to those in the last ex- 



