Prof. E. Bouty's Studies on Magnetism. 187 



If a needle slightly tempered be grasped on both sides of and 

 very near its middle by means of two pincers, so that only a 

 very thin section on each side of the plane of separation takes 

 part in the flexions which precede rupture, the two halves of the 

 needle present very nearly equal magnetic moments. Therefore 

 the difference above found is due to the flexions which precede 

 the fracture of needles that are tempered soft. It has moreover 

 long been known that mechanical actions of this sort, when sub- 

 sequent to magnetization, diminish the magnetic moment of the 

 needles submitted to them. 



In needles that break like glass, the mechanical act of break- 

 ing concerns only an infinitely thin layer of molecules on each 

 side of the plane of separation ; it must be presumed that the effect 

 of such an action is infinitesimal. I have ascertained that, in 

 the case of hard-tempering, the magnetic moment of a fragment 

 depends neither on the number nor the mode of the breakings 

 by which it has been detached from the mother needle — which 

 would be very difficult to account for if any peculiar appreciable 

 influence were exerted by the act of breaking. In a word, in 

 none of my experiments have I found a weakening of the mag- 

 netic moment that could be attributed to such an influence. 

 But proofs still more conclusive will present themselves in the 

 course of this Part. 



For all the following experiments I employed only needles of 

 hard and nearly invariable temper — obtained by heating a recti- 

 linear steel wire, of greater length than the needle required, in a 

 gas-flame supplied with air from a bellows, and dipping it when 

 bright-red hot in an earthen pan full of water — the ends being 

 then detached, so as to reserve only the middle portion of the 

 wire, the temper of which is very regular. The length of the 

 needles obtained was, at the most, 150 millims. 



2. Saturated cylindrical Needles broken perpendicularly to the 



axis. 



In his f Essay on the Application of Mathematical Analysis to 

 the Theories of Electricity and Magnetism' (Nottingham, 1828), 

 Green deduced from the hypothesis of coercive force the follow- 

 ing formula, which gives the magnetic moment y of a needle of 

 length x magnetized to saturation* : — 



* Beer (Elelctrostatik) has demonstrated that Green's formula applies 

 to a needle placed in a magnetizing spiral, provided that the turns are of 

 large dimensions in proportion to the diameter of the needle. In that case 

 A is proportional to the magnetizing force/. 



-*(-? 



