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



of d, becomes almost illusory. Relying, however, upon a 

 result of experiment which I have already indicated else- 

 where*, we may determine m and d by means of only one 

 needle. When a regular magnetized needle, tempered hard, 

 is broken, the different fragments taken from its middle 

 portion, and of sufficient length, have magnetic moments 

 represented by 



y-m (•'-*) (2) 



The quantity 8 is independent of the intensity of the magneti- 

 zation ; and it is almost evident a priori that the quantity m 

 is the same as in the mother needle. Besides, we demonstrate 

 it experimentally; remarking that equations (1) and (2), in 

 which x and y are considered as current coordinates, represent 

 two straight lines, we have only to trace these by a sufficient 

 number of observations, and prove that they are parallel — 

 that is, that the two quantities m are equal. 



This being admitted, let us determine the length and the 

 magnetic moment, x and y, of a needle, then reduce it by 

 ablation of the two ends to a suitable length, and determine 

 again the magnetic moment y' and the corresponding length 



x r . The distance 5 of the pole of a rupture-needle from its 



extremity is known beforehand, and is equal, for example, to 

 2*75 millims. for a needle of 0*55 millim. diameter. Equa- 

 tions (1) and (2) therefore completely determine m and d by 

 means of one needle only. Let us add that we can further 

 shorten the primitive fragment by successive breakings, and 

 obtain as many points as we please, in order to determine 

 better the straight line (2) — that is, the value of m. The 

 quantity of magnetism will thus be ascertained to a very 

 great nicety f . 



* Amiales de VEcole Normale, 2 e serie, t. iii. p. 43. 



t Let B C and AD (fig. 2) be the straight lines represented by equa- 



l J 





Fig. 2. 



/?'/ 









' s y / 



'X>/ 











A 









/ y / / 



1 









/ ' ! s ** 



1 









/ ''' S /' 









3 / 



y//y 



1 









< -X" 



.--'<B Id 



p 



M 



tions (1) and (2). The problem of the determination of d comes geo- 

 metrically to the following — through a given point C to draw B 



