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



Hence arises the necessity of placing the helix at a great 

 distance from the galvanometer-needle, and consequently the 

 obligation to be contented with moderate or small deflections 

 in order to estimate the moments. I have recently obviated 

 this inconvenience by rendering the galvanometer-needle 

 nearly astatic with the aid of a compensating bar placed above 

 the needle in an invariable situation. 



There are strong reasons for admitting, a priori, that the 

 distance of the temporary poles of the needles from their ex- 

 tremities is independent of the intensity of the current. In 

 fact, during temporary magnetization the needles are sub- 

 mitted to a magnetic force which has the same value in all 

 points of them — that is to say, to a force analogous to the 

 coercive force introduced by Green into his calculations upon 

 saturated needles*; the distribution of the magnetism and the 

 situation of the poles will be the same in both cases. The same 

 observation was applicable to rupture-needles f, for which we 

 have directly verified that the poles occupy an invariable 

 situation|. 



We Avill provisionally accept this conclusion, which I have 

 verified most completely in the case of bars. Thus the deter- 

 mination of the temporary moments is equivalent to that of the 



cV 

 quantities of magnetism when the distance -= from one tem- 



Z 



porary pole to the nearer extremity is known from experiments 

 on saturated needles. It has appeared convenient, in order to 

 eliminate as far as possible every cause of error, to compare 

 the deflection produced by the temporarily magnetized needle 

 with that which it produces in the same situation by its resi- 

 dual magnetism after the suppression of the current. 



Let p be the ratio of these deflections, w! and m the quanti- 

 ties of temporary and permanent magnetism, 8 the double dis- 

 tance from one temporary pole to the nearer extremity, d the 

 same quantity for the permanent magnetization (determined 



by formula 3 of the preceding section), and r the ratio — ; we 



have as a very close approximation, 



m'(l-ir> l-B 

 P m(l-d) l-d <■ > 



* I have verified that the formula of Green is conformable with , the 

 results of experiment, as regards saturated needles : see Annates de VJEcole 

 N&rmaie, 2 e serie, t. iii. p. 37. 



t At the moment when a needle is detached from the centre of a much 

 longer needle, it is found to be subject to the same magnetic force in every 

 one of its points. 



X Annates de VEcole Nor male, 2 e serie, t. iii. p. 43. 



