688 LECTURE LV. 



becoming more powerful by the vicinity of the new south pole, and the 

 south pole being consequently strengthened in a certain degree; so that the 

 attractive power of the whole magnet is increased by the proximity of the 

 iron. A weak magnet is capable of receiving a temporary induction of a 

 contrary magnetism from the action of a more powerful one, its north pole 

 becoming a south pole on the approach of a stronger north pole; but the 

 original south pole still retains its situation at the opposite end, and 

 restores 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, as will produce an equilibrium of the attractions and repulsions of 

 the poles of a larger magnet; being a tangent to a certain oval figure, pass- 

 ino- through those poles, of which the properties have been calculated by 

 various mathematicians. This polarity may easily be imitated by electricity ; 

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

 conductor, which are placed parallel to each other, as in 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. (Plate XLI. Fig. 569. ) 



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 induced magnetism, 

 the north pole of each particle being attached to the south pole of the par- 

 ticle 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. (Plate XLI. Fig. .570.) 



The polarity of a needle may often be observed when it exhibits no sen- 

 sible attraction or repulsion as a whole ; and this may easily be understood 

 by considering that when one end of a needle is repelled 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 



