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LXXII. Intelligence and Miscellaneous Articles. 



ON MAGNETIZATION. BY M. MASCART. 



T^HEN an isotropic and feebly magnetic substance is placed in 

 ^ * a uniform field, it acquires a magnetization parallel to the field, 

 and its coefficient of magnetization is the ratio of the magnetic 

 moment for unit volume, or intensity of magnetization, to the 

 intensity of the field. 



With highly magnetic substances, on the contrary, such as iron, 

 nickel, cobalt, the reaction due to the induced magnetism must be 

 allowed for, and the preceding definition is only applicable for 

 infinitely long cylinders magnetized longitudinally or for closed 

 rings. 



The calculation of the magnetizing force as a function of the 

 external field is very simple in the case of the sphere or the 

 ellipsoid, or of an infinitely long cylinder magnetized transversely ; 

 but except in the case of very elongated ellipsoids, the co- 

 efficient of magnetization may vary within very extensive limits 

 without the magnetic moment of the body being sensibly modified. 

 The slightest want of homogeneity has in that case a considerable 

 influence. 



The coefficient of magnetization is often determined by using 

 cylinders arranged parallel to the field, and which are assimilated to 

 infinite cylinders, or to ellipsoids of the same length and the same 

 median section ; the magnetic moment is then measured, or the 

 discharge, induced in a coil surrounding the central section, when 

 the magnetization is reversed. With rings, induced charges can 

 alone be used, and the coefficients of magnetization are generally 

 found too high. It may then be asked if one of the methods is at 

 fault ; and if, for instance, a special phenomenon is produced in the 

 case of closed rings which exaggerates the effects of induction. In 

 order to solve this question, I used with the same metal closed 

 rings, and a series of cylinders in which the ratio of the length to 

 the diameter varied within very wide limits. 



The cylinders were placed in the axis of a cylindrical coil of 

 1-20 metre in length, and 0-03 metre in diameter, so that the internal 

 field of the current may be considered as sensibly uniform over a 

 length of more than 0*80 metre. This coil was at right angles to 

 the magnetic meridian, and its action on an adjacent declinometer 

 was compensated by that of an external coil. The magnetic moment 

 of the cylinder is deduced from the deflection produced in the 

 declinometer. Further, a coil of several turns, wound on the 

 mean portion of the magnetized cylinder, was connected with a 

 ballistic galvanometer, and the induced charge produced by the 

 inversion of the magnetization, eliminating the effect produced by 

 the field itself. 



