VARIATION OF MAGNETISATION WITH THE MAGNETISING FORCE. 631 



Some experiments of Joule,* although limited to permanent 

 magnetisation, show that for very small forces the magnetisation 

 increases more rapidly than the magnetising force. 



This result has been neatly demonstrated by the experiments 

 of Professor G. Wiedemannf on the magnetisation of cylindrical 

 bars by currents, by using the methods of deflection and determining 

 separately the total magnetisation and the residual magnetisation, 

 from which the temporary magnetisation is deduced by difference. 

 It follows that the curve of the intensity of magnetisation (Fig. 244) 

 has no rectilinear part (OA) : it begins to turn its convexity towards 

 the axis of abscissae, has a point of inflection at B, and then tends 

 towards a maximum. The curve of total and that of permanent 

 magnetisation have the same features. The intensity of magnetisa- 

 tion begins then to increase more rapidly than the magnetising force. 



1204. VARIATION OF MAGNETISATION WITH THE MAGNETISING 

 FORCE. Several methods may be used to express the results of 

 experiments. The method which most directly answers to practical 

 requirements consists in expressing the relation between the mag- 

 netic moment, or the intensity of magnetisation, and the magnetising 

 force by a curve or by a formula. 



Lamontt obtained a tolerably rational expression by assuming 

 that the increase dm of the magnetic moment, produced by an 

 increase d<f> of the magnetising force, is at each instant propor- 

 tional to the excess of the maximum moment M over the actual 

 value m that is to say, to the total increase of magnetisation 

 which the bar is still capable of receiving. If we put 



dm ' dm 



= aU-m, or 



M.- m 

 it follows that 



M - m A<?- a< ; 



as M = A for < = 0, we get 



m 



The curve which gives the ratio , as a function of the mag- 

 netising force <, starts from the origin, where the angular coefficient 



* JOULE. Phil. Trans, for 1856, p. 287. 



f WIEDEMANN. Galvanismus, 1st Edition, Vol. II., p. 297. 



LAMONT. Handbuch des Magnetismus, p. 407. 1867. 



