MAGNETIC MOMENTS. 285 



forces, one applied at the centre of gravity of positive masses, and 

 the other at the centre of gravity of negative masses. These two 

 points of application are the poles of the magnet; the magnetic 

 axis of the magnet is the line joining the two poles, and the 

 direction of the magnetic axis is reckoned from the negative pole 

 towards the positive one. 



The magnet is evidently in stable equilibrium when its magnetic 

 axis is parallel to the direction of the field, and pointing in the same 

 way ; equilibrium is unstable if these two directions are parallel but 

 in contrary directions. 



296. THE MAGNETIC MASS OF A MAGNET is ZERO. The 

 vicinity of the Earth may be considered as a uniform magnetic 

 field. 



Experiment shows, in fact, that throughout a region whose extent 

 is considerable in reference to the dimensions of the magnet, but 

 small compared with the radius of the Earth, all magnets, when 

 under the influence of the Earth alone, tend to assume the same 

 direction. 



Coulomb showed, moreover, that the action of the terrestrial 

 field on any magnetised bar is purely directive that it has neither 

 vertical nor horizontal component ; it has no vertical component, for 

 the weight of a bar of steel is exactly the same before and after 

 magnetisation ; the horizontal component is also zero, for any 

 magnet which can move in a horizontal plane has no tendency 

 towards a motion of translation. The two forces of opposite directions 

 applied at the two poles are therefore equal, and constitute a couple. 

 From this follows this important conclusion that in any magnet 

 the sum of the positive masses is equal to the sum of the negative 

 masses ; in other words, the total sum of the magnetic masses is zero. 

 We have then always ^m = 0. 



From this point of view the state of a magnet is comparable with 

 that which a dielectric, or an insulated conductor, acquires by 

 induction. 



297. MAGNETIC MOMENTS. Let m be the absolute value of the 

 mass of each pole, and / the distance of the two poles ; the product 

 ml, of the mass by this distance, is called the magnetic moment M of 

 the magnet. 



This magnet may be represented by a straight line OA (Fig. 71) 

 having for direction, the magnetic axis, and for length the numerical 

 value of the magnetic moment M. 



This mode of representation amounts to supposing that all the 

 poles are identical, that their mass is equal to unity, for instance, 



