Stress or Permanent Strain to a Bright-red Heat, 257 



twisting in the same direction as that in which it had pre- 

 viously suffered permanent torsion. When the burner was 

 removed and the wire was cooled to a temperature which 

 seemed slightly lower than that at which the phenomenon 

 occurred in the first instance, it as suddenly untwisted ; and 

 when it had again attained the temperature of the room, there 

 remained a decided permanent twist in addition to that which 

 had been originally imparted*. This additional permanent 

 twist is rendered more remarkable by the fact that rise of tem- 

 perature beyond a dull red, but not extending to a bright 

 red, is attended with a considerable permanent untwist. The 

 phenomenon is evidently closely associated with one discovered 

 by Mr. Gore so far back as 1869, and described in the ' Pro- 

 ceedings of the Royal Society ' for that year. Mr. Gore's 

 experiments were conducted in the following manner : — A 

 thin iron wire, fixed at one end to a binding- screw, is attached 

 at the other to an index which multiplies any motion of the 

 wire ; the wire is stretched horizontally by a feeble spring, 

 and is heated by an electric current or by a row of gas-jets. 

 According to Mr. Gore, no anomalous action is observed on 

 heating the wire to bright incandescence ; but when cooling 

 begins, the index moves back until a moderate red heat is 

 attained, when suddenly the pointer gives a jerk, indicating 

 a momentary elongation of the wire during the progress of its 

 contraction. In 1873 Professor Barrett extended Mr. Gore's 

 researches ; and in a paper full of interest f, entitled " Certain 

 remarkable Molecular Changes occurring in Iron Wire at a 

 low red Heat," showed that, under suitable conditions, the 

 sudden change could be observed not only on cooling, but on 

 heating. It is apparent, from the description given above, 

 that the phenomenon observed by Mr. Gore resembles the one 

 observed by myself, in that there is a sudden change in the 

 iron when it has been raised to a bright red and afterwards 

 cooled to a certain temperature. But in Mr. Gore's experi- 

 ment the wire was under temporary stress, whilst in mine it 

 was permanently strained; and whereas in the one case, on 

 cooling below a certain temperature, there was a sudden yield- 

 ing to the stress, in the other there was a sudden diminution 

 of permanent strain. A careful consideration of the results 

 of my own experiments, and of those of Mr. Gore and Pro- 

 fessor Barrett, satisfied me that in this, as in many other 

 instances, temporary stress and permanent strain act in oppo- 



* This is not so if the wire be heated rather slowly ; but, on the con- 

 trary, in this case there is a very decided permanent untwist (see experi- 

 ment V.). 



t Phil. Mag. ser. 4, vol. xlvi. p. 472. 



Phil. Mag. S. 5. Vol. 24, No. 148. Sept. 1887, S 



