Dr. E. Bouty on the Magnetization of Steel by Curi*ents. 121 



theory of the coercive force *) the laws of the distribution in 

 a series of saturated cylindrical bars with the magnetic mo- 

 ments of the bars. The portion of my memoir which relates 

 to large bars is only a wider development of Biot's method ; 

 that relative to long and thin needles is more original, and its 

 purpose is to determine independently of any hypothesis the 

 two factors m and X of the magnetic moment. 



Given that we possess a series of magnetized needles in 

 which the quantity of magnetism m is the same, as well as 



the distance ~ from one pole to the nearer extremity. This 



will be the case when the needles are obtained, by breaking, 

 from the middle of one and the same strongly hardened 

 needle f; and we shall see in the sequel that it is a very 

 general one. Then let y be the magnetic moment of a needle 

 of which the length is x ; we have 



y = m (x—d); (1) 



and, theoretically, two measurements, made upon needles of 

 different lengths, will be sufficient for determining the two 

 quantities m and d. 



This, I believe, is the first time that determinations have 

 been published relative to the situation of the poles in needles 

 magnetized by currents. As regards the quantities of 

 magnetism, their investigation has been the object of the 

 researches of a great number of physicists, of which I have 

 elsewhere % cited the principal and most recent. The appli- 

 cation of the methods employed requires great delicacy, and 

 supposes the possession of considerable homogeneous masses 

 of the metal under investigation — a condition very difficult 

 to fulfil. Moreover these masses must take the form of 

 ellipsoids§ or of rings || for the magnetization they receive to 

 be the same in all their points ; then the quotient of their 

 magnetic moment by their volume gives the quantity of 

 magnetism //, referred to the unit of volume. 



* Annates de VEcole Normale SupSrieure, 2 e serie, t. iii. p. 34. 



t Ibid. pp. 36, 43. % Ibid. p. 10. 



§ Poisson has demonstrated (Memoires de VAcademie des Sciences, 

 t. vii.) that, in an ellipsoid submitted to the action of a constant magnetic 

 force acting in the direction of its major axis, the magnetization is iden- 

 tical in all the points of the mass, and equal to the value which it has at 

 the centre of an infinitely long cylindrical needle submitted to the same 

 force. Quintus Icilius employed ellipsoids. 



|| Stoletow and Rowland made use of rings. The magnetization is the 

 same in all the points by reason of symmetry ; but, a closed solenoid being 

 without action upon any exterior point, they were obliged, in order to 

 accomplish their measurements, to produce induced currents accompany- 

 ing the magnetization or demagnetization of the metallic ring. 



