262 Mr. H. Tomiinson on raising Iron under Temporary 



wire was allowed to cool : first ensued a sudden momentary 

 untwist of the wire, whilst a few seconds afterwards a kick of the 

 galvanometer-needle in the opposite direction to that which 

 had occurred on heating, indicated that the iron had suddenly 

 regained its magnetic properties. This experiment was re- 

 peated several times ; and though the sudden jerk on heating 

 and cooling became less and less in intensity, and probably 

 would eventually have vanished, it did not do so after six 

 heatings and coolings*. 



Experiment V. — The last experiment had shown the great 

 persistency of the permanent torsional strain, even after the 

 wire had been several times heated to a white heat. A fresh 

 series of trials was made with iron which had suffered more 

 or less permanent torsion, for the purpose of examining this 

 persistency more closely. In these trials it was attempted 

 to reanneal the wire after the permanent torsion had been 

 imparted, by passing the flame of a Bunsen-burner very slowly 

 up and down it so as to heat all parts in turn to a white heat, 

 the wire being in some cases entirely free from any load on it, 

 and in others having merely the slight load due to the block 

 and its appendages. In no case could the sudden jerk at a 

 bright red heat be got rid of entirely ; though the same wire, 

 when tested previously to imparting permanent torsion, had 

 shown no trace whatever of the phenomenon. It is evident 

 that iron cannot be satisfactorily annealed by the process 

 mentioned above f. 



These trials also brought out distinctly the fact that at a 

 dull red heat the iron begins to permanently untwist rapidly, 

 so that a considerable amount of the permanent twist origi- 

 nally imparted can be got rid of by maintaining the tempera- 

 ture for some time between bright red and dull red. Directly, 

 however, the higher critical temperature is reached there is a 

 sudden twist, which in the case of very soft iron is partly 

 temporary and partly permanent, and in the case of steel wire 

 which is unannealed, or hard iron wire, is principally per- 

 manent %. 



Experiment VI. — A piece of the well-annealed iron wire, 

 about 16 inches long, was clamped at one end and sustained in 

 a horizontal position, save in so far as it was bent by its own 



* I had the pleasure of repeating the experiment before the Physical 

 Society. 



t I write this because some observers seem to think that iron can be 

 satisfactorily annealed in this way. In this, I believe, they are mistaken ; 

 the iron should be maintained at a high temperature for a considerable 

 period, find afterwards be allowed to cool very slowly. 



X With some unannealed specimens I found it difficult to detect any 

 trace whatever of untwisting temporarily when the iron cooled below the 

 critical temperature. 



