MAGNETISM. 



settle in a position directed nearly north 

 and south. If it be disturbed from this 

 situation and placed in any other direc- 

 tion, it will, as soon as it is again at li- 

 berty to move, resume its former posi- 

 tion. The end of the bar which points 

 to the north, is that which was lower- 

 most at the time it acquired its magnet- 

 ism by hammering: the end which, 

 during that operation, had been the 

 upper one, is consequently that which, 

 when the magnet is free to move, directs 

 itself to the south. The two ends of a 

 magnet of this form are called its poles : 

 the one which spontaneously turns to the 

 north, being distinguished as the north, 

 and the other as the south pole: and 

 the tendency of the magnet to assume 

 the above described position is called its 

 Polarity. The straight line joining the 

 two poles of a magnet is called its axis. 

 (7.) There are several ways of sup- 

 porting a magnet so as to enable it to 

 manifest its polarity. The readiest mode 

 is to suspend it by a thread, fastened 



Fig. 1. 



Fig. 2. 



round it at the middle, so that it 

 may be sufficiently balanced to pre- 

 serve its horizontal position as it turns 

 freely round its centre. It cannot, in- 

 deed, turn thus, without, at the same time, 

 either twisting or untwisting the thread 

 by which it hangs ; and the reaction of the 

 thread, the fibres of which tend to resume 

 their original situation, or the force of 

 torsion, as it is called, may prevent the 

 magnet from assuming the precise po- 

 sition to which its polarity would have 

 brought it. But by employing a very 

 slender thread, and taking it of sufficient 

 length, the force of torsion may be so 

 much reduced as to be quite insensible 

 in the experiments about to be described. 

 (8.) Another convenient mode of ex- 

 amining the horizontal movements of 

 the magnetized bar is to poise it on its 

 centre, hollowed into a cap, which is 

 made to rest on a fine point fixed in a stand. 



When thus fitted up, it acts like the 

 needle of a mariner's compass ; and in 

 its principle is identical with that in- 

 strument; 



(9.) We may sometimes find it more 

 expedient to fix the magnet on a piece 

 of cork, and thus make it float on water, 



Fig. 3. 



in a basin. In this case, we must take 

 care, however, that it be kept at a suffi- 

 cient distance from the sides of the 

 vessel to prevent its being affected by 

 the capillary attractions of the water. 



The same precaution must be used if 

 the magnet be made to float on the sur- 

 face of mercury, which is an excellent 

 mode of giving it complete liberty of 

 motion. But the vessel containing the 

 mercury should be at least six inches in 

 diameter, in order to guard against the 

 effects of the curvature of the surface 

 of the mercury near the sides. When 

 the surface of the mercury is very clean 

 and bright, which happens only when 

 the metal is very pure, it allows of the 

 ready motion of pieces of iron floating 

 upon it. But it soon tarnishes, and the 

 film of oxide which forms on the surface, 

 becomes a great impediment to the 

 freedom of motion of the floating body. 

 The best way of rendering it clean, is to 

 strain it through a funnel of paper, 

 rolled up into a cone, having a small 

 aperture at the point, of about the for- 

 tieth of an inch in diameter. 



2. Attraction of Iron. 



(10.) If either pole of a magnet be 

 brought near any small piece 'of soft 

 unmagnetic iron, it will be found to at- 

 tract it. Iron filings, for instance, are 

 immediately collected together when a 

 magnet is placed among them ; and 

 they adhere more especially to the poles 

 (as shewn k in fig. 4, A), from which, 



