56 MAGNETISM. 



the magnet nearly to its original condition, after the removal of the 

 disturbing cause. 



The polarity of magnets, or their disposition to assume a certain 

 direction, is of still greater importance than their attractive power. 

 If a small magnet, or simply a soft wire, be poised on a centre, it 

 will arrange itself in such a direction, us will produce an equilibrium 

 of the attractions and repulsions of the poles of a larger magnet ; 

 being a tangent to a certain oval figure, passing through those poles, 

 of which the properties have been calculated by various mathema- 

 ticians. This polarity may easily be imitated by electricity ; a sus- 

 peuded wire being brought near to the ends of a positive and negative 

 conductor, which are placed parallel to each other, as iu Nairne's 

 electrical machine, its position is perfectly similar to that of a needle 

 attracted by a magnet, of which those conductors represent the 

 poles. 



The same effect is observable in iron filings placed near a magnet, 

 and they adhere to each other in curved lines, by virtue of their in- 

 duced magnetism, the north pole of each particle being attached to 

 the south pole of the particle next it. This arrangement may be 

 seen by placing the filings either on clean mercury, or on any surface 

 that can be agitated ; and it may be imitated by strewing powder 

 on a plate of glass, supported by two balls, which are contrarily 

 electrified. 



The polarity of a needle may often be observed when it exhibits 

 no sensible attraction or repulsion as a whole ; and this may easily 

 le understood by considering that when one end of a needle is re- 

 pelled from a given point, and the other is attracted towards it, the 

 two forces, if equal, will tend to turn it round its centre, but will 

 wholly destroy each other's effects with respect to any progressive 

 motion of the whole needle. Thus, when the end of a magnet is 

 placed under a surface on which iron filings are spread, and the sur- 

 face is shaken, so as to leave the particles fur a moment in the air, 

 they are not drawn sensibly towards the magnet, but their ends, 

 which are nearest to the point over the magnet, are turned a little 

 downwards, so that they strike the paper further and further from the 

 magnet, and then tall outwards, as if they were repelled by it. 



The ni.igi.i-t-, \\hicli we have hitherto considered, are sut h as have 

 a simple and well determined form ; but the great compound mag. 

 net, which directs the mariner's compass, and which appears to con. 

 .-ist principally of the metallic and slightly oxidated iron, contained 



