VARIATION OF MAGNETISATION WITH THE MAGNETISING FORCE. 635 



for it enables us to obtain for all degrees of magnetisation very 

 regular finite curves, by the aid of which we may determine with 

 more precision the value of the maximum. It consists in taking 

 as abscissa the intensity of magnetisation I a , or the magnetic in- 

 duction Fj = < + 47rl a = </> ( i + 47r) = p, and as ordinates the mag- 

 netic permeability /*, or the quantity A = 47r/x. 



The curves thus obtained (Fig. 247) have the form of inclined 

 parabolas, and are well represented by the formula 



X = A sin 



in which a, b, and c are constants depending on the metal ob- 

 served, and A is the maximum value of A. 



The figure refers to a series of observations on wrought iron 

 of good quality. The constant A is equal to 30860, which gives 



- 1250 



1000 



750 



500 



250 



Iff 



n 



10 



20 



30 



Fig. 246. 



^ = 2456, or =195, for Fj = 5968 or =2*43. The point where 

 the curve meets the x axis, corresponds to the maximum of mag- 

 netisation, which will be 1400 for F x = 17500. 



The Norwegian iron already mentioned gave A = 57820, or 

 ^ = 4602, and = 366, for F 1 = 538o or <= 1*169. 



1206. It is worthy of notice that these high values of the 

 coefficient of magnetisation were obtained by the method of in- 

 duction applied to ring-shaped pieces of soft iron, while the direct 

 measurement of magnetic moments by deflections does not give 



