Dr. E. Bouty on the Magnetization of Steel by Currents. 129 



ployed were obtained by erosion from the same mother needles, 

 there is a magnetizing-function (f)(x) ; and the magnetic mo- 

 ment of a needle of length I and diameter D is represented by 

 the formula 



y- ^V-OP-fcrfKl-jw)], • • • (5) 



up to values of y very near to the limit Y corresponding to 

 saturation. The limit itself is given by the formula 



Y= 7 J 2 L(?-2*D), (6) 



L being the limiting value of the magnetizing-function for 

 #=oo . 



Repetition of passing the Needle to the Helix. — I have already 

 pointed out * that, when the passage of the needle into the 

 magnetizing helix is repeated, its magnetic moment increases 

 so as to be represented by the formula 



y= A ~l co 



where A and B are constants depending on the intensity of the 

 magnetizing force, and n indicates the number of the passages. 

 It was of importance to seek to ascertain if the augmentation 

 of the magnetic moment in question proceeded from a mere 

 change in the distribution of the magnetism (that is to say, a 

 displacement of the poles towards the extremities), or if there 

 was a real augmentation of the quantity of permanent magne- 

 tism preserved by the needle. 



The method above employed permitted me to solve this 

 question in a satisfactory manner. I found that in these cir- 

 cumstances the quantity of magnetism and the situation of the 

 poles change simultaneously ; but the alteration of the latter 

 was scarcely perceptible. Formula (7) applies very well to 

 the quantities of magnetism considered separately. 



If, starting from zero, the intensity of the magnetizing force 



A 

 x be progressively increased, the ratio -r ~ of the quantity 



of magnetism for co passages, to the quantity corresponding 

 to the first passage, diminishes and tends toward unity f; but 



* Annates de VEcole Kormale, 2 e ser. t. iii. p. 22. 



t In a recent paper (Pogg. Ann. Erganzungsband vii.) M. Fromme ex- 

 presses doubts concerning' this result. According to liim, the number of 

 impulsions required by a needle in order to acquire all the magnetism which 

 can be communicated to it by a given force increases with the intensity of 

 that force, and saturation can only be obtained by an infinity of impulsions. 



This, to say the least, strange result was obtained by employing very 



Phil. Mag. S. 5. Vol. 3. No. 16. Feb. 1877. K 



