PRELIMINARY. 



masses ; we shall agree to give the sign + to the magnetic mass of a 

 north pole, and the sign - to that of a south pole. 



The action of two poles, expressed by the formula /= - , will 



be positive in the case of repulsion, the masses being of the same 

 sign, and negative in the case of an attraction. 



The law of elementary actions being the same as that for elec- 

 trical phenomena, we may apply all the theorems relative to electrical 

 potential, at any rate as regards fixed masses, and disregard, for 

 the present, phenomena relative to conductors. The consideration 

 relative to lines of force, to tubes and flows of force, are more 

 particularly directly applicable to magnetism. 



294. MAGNETIC FIELD.- A magnetic field is a space in which 

 magnetic phenomena are produced. The direction and strength of 

 the field at a point, are the direction and intensity of the force 

 which would act on a positive magnetic mass equal to unity placed 

 at this point. 



295. DEFINITION OF POLES. MAGNETIC Axis OF A MAGNET. 

 We have assumed in the foregoing remarks, that the actions of a 

 magnet reduce to that of two magnetic centres situate at the 

 ends; this is the case, but then only approximately, when we are 

 dealing with long cylindrical magnets, at a very great distance in 

 reference to their transverse dimensions. Magnetic properties are 

 really perceptible throughout the whole extent of the magnet, and 

 only exhibit a very marked maximum near the ends. This is readily 

 seen from the manner in which filings attach themselves to the 

 magnet. We must admit, therefore, that in the magnet there is a 

 series of magnetic masses, some positive and others negative, which 

 are distributed according to a certain law, and the whole of which 

 constitutes the total magnetic mass. 



This being assumed, we may define more precisely what are 

 called the poles of a magnet. 



Let us suppose the magnet placed in a uniform magnetic field. 

 The actions exerted by the field on the different points of the 

 magnet are parallel to each other, and for each volume-element 

 are proportional to the mass present there. All those which act on 

 the positive masses are in the same direction : they have a resultant 

 equal to their sum, and parallel to their direction, which is applied 

 at the centre of mass, or the centre of gravity of the positive masses. 

 The same is the case for negative masses, on which the field pro- 

 duces actions parallel to the preceding, but in the opposite direction. 

 The magnet is submitted to the action of two parallel and opposite 



