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



tient of the quantity m by the square of the diameter preserves, 

 for one and the same value of the current-intensity, a constant 

 value. The difficulty of ascertaining this resides in the in- 

 equality of the hardening communicated to needles of different 

 diameters placed in the same external conditions — for instance, 

 on immersing them in water when they exhibit the same 

 bright red*. 



On the other hand, if we compare the phenomena of the 

 steeping of steel with those of the steeping of iron, we are led 

 to attribute to the steeped steel a certain degree of heteroge- 

 neity from the surface to the centre ; and if it really existed, 

 one could no longer talk of a magnetizing function of steeped 

 steel. Therefore, upon the advice of M. Jamin, I tried dissol- 

 ving in an acid f the superficial layer of steeped needles, so as 

 to reduce them to less diameters, and afterwards magnetizing 

 the needles thus obtained. In this way I proved, not without 

 some surprise, the existence of a perfectly determinate niag- 

 netizing-function. The steeped steel of my thin needles is, 

 then, very sensibly homogeneous X ; and the experiment in 

 question permits us to state precisely what should be under- 

 stood by the rather vague expression identical temper when 

 applied to needles of different diameters. The following Table 

 is intended to prove the existence, in the present case, of a 

 magnetizing-function ; it relates to the magnetism acquired 

 permanently by needles thinned with acid from an initial dia- 

 meter equal to 1*178 millim. 



losopliical Magazine 5' the numbers of both columns are expressed in 

 absolute units : — 



Intensities. Quantities. 



2-756 26880 



3-219 34200 



3-551 40320 



4-469 52940 



5-698 61920 



11-44 91530 



20-69 96940 



38-99 100770 



* In this case it is the thickest needles which are the most hardened ; 

 they approach more slowly than thin needles the maximum of magnetism 

 of which they are susceptible. Nevertheless, as I have demonstrated 

 {Ann. de VEcole Normale, 2 e ser. t. iii. p. 40), this maximum verifies the 

 law of the diameters. 



t Boiling chlorhydric acid, or aqua regia. 



% The outermost layer, to a depth of 0-05 millim. at the most, possessed 

 perhaps different properties; but to decide this question requires new 

 experiments. 



Intensities. 



Quantities. 



0-1356 .... 



18 



0-2793 .... 



80 



0-5287 .... 



.... 255 



0-9398 .... 



.... 727 



1-421 



.... 2526 



1-880 .... 



. . . . 5108 



1-947 



.... 6482 



2-300 



.... 13510 



