On the Recovery of Iron from Overstrain. 339 



The question of recovery of elasticity under stress is next con- 

 sidered in the paper, and it is shown that the process of recovery pro- 

 ceeds at practically the same rate whether the material is kept stressed 

 or is allowed to rest free from load. A slight difference, however, is 

 shown in the two cases, as restoration of elasticity takes place about 

 the position of continued stress. 



After this, the phenomenon of hysteresis in the relation of extension 

 to stress is considered, and a closed cycle is shown, having features 

 analogous to those exhibited by a magnetic hysteresis cycle.* 



The effect of moderate temperature on recovery from overstrain is 

 next treated of, and it is shown that a slight increase in temperature 

 hastens the restoration of elasticity to a remarkable extent. Three or 

 four minutes at 100 C. proved to be more efficient than a fortnight's 

 rest at the normal atmospheric temperature. The effect of various 

 temperatures below 100 C. is then investigated, and so moderate a 

 temperature as 50 C. is shown to have a large influence in hastening 

 recovery from overstrain. The manner in which recovery proceeds with 

 time when the specimen is kept at a constant temperature is shown in 

 the paper by means of curves. These curves show that at first that is, 

 before elasticity is fairly well restored the amount of recovery, 

 measured by the diminution in the elongation produced by a maximum 

 load, is proportional to the square root of the time. For example, the 

 effect produced by, say, four minutes at 80 C. was approximately 

 double of that produced by one minute at the same temperature. 



By subjecting an overstrained specimen to temperatures above 100 

 C., no effect (other than the recovery from the temporary effect of over- 

 strain) was found to be produced until a red heat was almost attained. 

 When the specimen had been subjected to an annealing temperature, 

 of course the whole effect of overstrain was removed, and the material 

 assumed its virgin state, f 



After the effect of temperature is discussed, that of mechanical 

 vibration is next recorded in the paper ; and it is shown that by 

 striking a recently overstrained specimen with a hammer, so as to 

 make it ring, the material of the specimen is made less elastic. That 

 is, the effect of mechanical vibration is opposite to that of increase of 

 temperature ; recovery of elasticity is not hastened, but the material 

 becomes more semi-plastic after mechanical vibration than it was 

 before. 



The influence of magnetic agitation is next described. A recently 

 overstrained specimen was subjected to magnetic reversals by means of 

 a coil giving a field strength of 140 C.G.S. units at its centre, but no 



* Ewing, " Experimental Researches in Magnetism," ' Phil. Trans.,' 1885, or 

 book on ' Magnetic Induction in Iron and other Metals.' 



t See paper by Unwin, " On the Yield-point of Iron and Steel, and the Effect of 

 repeated Straining and Annealing," ' Eoy. Soc. Proc.,' vol. 57, 1895. 



VOL. LXIV. 2 D 



