654 CONSTANTS OF MAGNETISATION. 



The investigation of the external field by one of the methods 

 previously indicated, will give us the potential on the surface to 

 within a constant, starting from any given equipotential surface V 15 



and taking the integral Fdn, which is equal to V 1 - V along a 



line of force to the surface S. In the case of a bar magnet, for 

 instance, two level surfaces V : and V 2 will be chosen which 

 surround the extremities ; the difference of potentials Vj - V 2 , and, 

 starting from one or the other, the potential over the whole surface 

 will be determined. 



1216. POLES OF MAGNETS. The poles of magnets, or the 

 centres of gravity of the positive and negative masses which they 

 contain (295), may be determined if we know their magnetic 

 structure, or simply the distribution of the surface fictive layer. 



In the case of cylinders of small diameter, the centre of gravity 

 of the surface bounded by the curve of lateral distribution will give 

 the position of the pole, provided always that the elementary surface 

 near the end comprises also the magnetism which is found on the 

 terminal faces. 



From the experiments of Coulomb (419) the distance x l of the 



pole from the end will be - of the length for short magnets with a 



linear distribution, or -- for magnets whose length is 50 times the 



o 

 diameter of d. 



If the curve of distribution were replaced by a parabola tangential 

 to the direction of the magnet at the distance / from the ends, we 



shall have x l = -. 

 4 

 Biot's law, finally, A = #/**, where the constant p is smaller than 



unity, gives x l = -- . For bars magnetised to saturation, the con- 



stant a, according to Jamin's experiments,* would only depend on 

 the nature of the steel, and the constant //. on its tempering or 

 annealing. 



These various methods all fail precisely from the manner in 

 which the law of distribution is determined. 



1217. Measurement of magnetic moments gives the best results, 

 but it is still necessary to have recourse to some hypotheses. 



JAMIN. Comftes rendus, Vol. LXXX., p. 1553. 1875. 



