282 PRELIMINARY. 



Magnetic actions appear, therefore, to be concentrated at the 

 ends of regular magnets. We shall call these ends the poles of the 

 magnet, and shall afterwards define this term with more precision. 



291. THE Two KINDS OF MAGNETISM. The two ends of the 

 magnet are not of the same kind; any magnet free to turn in a 

 horizontal plane takes up a fixed direction in space. This direction 

 is nearly north and south. When a magnet is displaced from this 

 position it reverts to it when left to itself, and it is always the same 

 end which points to the north. That end of the magnet which 

 points to the geographical north is called the north pole, and that 

 end which points to the south is the south pole. These poles may 

 be marked once for all on permanent magnets. 



292. LAW OF MAGNETIC ACTIONS. Magnets act on each other' 

 The north pole of a magnet when presented to the north pole of 

 another magnet repels it, but on the contrary attracts a south pole. 

 In like manner two south poles repel each other. The phenomena 

 are therefore analogous to those of electrical action : Two poles of 

 the same kind repel, and two poles of opposite kinds attract, each other. 



Hence there are four actions between two magnets which are 

 near each other : two, which are repulsive, between poles of the same 

 kind, and two attractive between different poles. If the magnets are 

 very long as compared with their transverse dimensions, and are 

 situated at a considerable distance in reference to these same dimen- 

 sions, the action of each end may be considered as concentrated in 

 a point. The reciprocal action of these two magnets consists, then, 

 of four forces directed along the straight line which join in pairs the 

 centres of action or the poles of the two magnets, and it is impossible 

 in experiments to reduce them to a smaller number. 



Yet, by causing the poles of two very long magnets to act on 

 each other at distances, and in positions, such that the actions of the 

 two other poles may be neglected, Coulomb experimentally estab- 

 lished the law that the attractive or repulsive actions between two poles 

 are inversely as the square of their distance. 



It may, however, be remarked that nothing proves the existence 

 of these elementary forces. Experiment shows, indeed, that the 

 reciprocal action of the systems which constitute the magnets, and 

 which are probably very complicated, may be reduced to attractive 

 or repulsive forces directed along the right line joining the poles ; 

 but as one pole can never be separated from its fellow pole, the 

 action of poles is a purely mental conception, advantageous no 

 doubt in representing and calculating phenomena, but without any 

 real existence as demonstrated by experiment. If it should happen 



