Magnetisation in the Length of t Metal Rods. 



125 



I have also made a great number of experiments with steel. The 

 results obtained appeared at first to be of the most inconsistent 

 character, and it was with difficulty that I finally succeeded in evolv- 

 ing order out of them. The fact clearly is that the point of maximum 

 elongation (when there is one) depends in a very remarkable manner 

 upon the hardness or temper of the steel. Like Joule, I found that a 

 soft steel rod which had been neither annealed nor tempered behaved 

 in very much the same manner as iron, though the effects were 

 smaller. There was a point of maximum elongation which was well 

 defined, but I was not able by any current at my command to produce 

 actual retraction. A rod which was made exceedingly hard by being 

 dropped into cold water when at a bright red heat, had no point of 

 maximum elongation within the limits of my magnetising currents, 

 the temporary elongation continually increasing with increasing 

 magnetisation, and giving no evidence of an approach to a limit. 

 But when the same rod was let down to a yellow temper its behaviour 

 was altogether different. With a very small magnetising force it 

 showed signs of retraction, and the retraction increased with stronger 

 magnetising currents, ultimately becoming very considerable. A rod 

 tempered blue also retracted when magnetised, but the effect did not 

 begin to appear until the magnetising force was much greater than 

 was necessary when the temper was yellow, and after the rod had 

 been still further let down by heating, a measurable elongation 

 occurred before the magnetising force was sufficient to cause retrac- 

 tion. 



Again, another piece of steel was hardened by raising it to a dull 

 red heat and dropping it into cold water. It could easily be marked 

 with a file, and was certainly softer than the last -mentioned rod before 

 it was tempered, though it appeared to be harder than the same rod 

 in the yellow condition. The new hard rod was slightly elongated by 

 feeble magnetisation, and after passing a maximum retracted at about 

 the same rate as iron. 



All these various results, which at first sight appeared to be discon- 

 nected and inharmonious, point to the following conclusion : — The 

 critical value of the magnetising force for a steel rod diminishes as the 

 hardness becomes greater up to a certain point corresponding to a 

 yellow temper, after which it increases, and, with very hard steel, 

 becomes very high. There is therefore a critical degree of hardness 

 for which the value of the critical magnetising force is a minimum ; 

 in steel of a yellow temper the value of the critical magnetising force 

 is lower than in steel which is either softer or harder. 



Some careful experiments were made with the hard steel rod last 

 referred to. The results are contained in Table II, and the corre- 

 sponding curve in fig. 2. As in the case of iron, the rod was first 

 permanently magnetised by a current equal to the strongest subse- 



