338 Mr. J. Muir. 



summarised here, is to show the effect of moderate temperature, of 

 mechanical vibration, and of magnetic agitation on this slow return to 

 the elastic state ; and further to illustrate this recovery by means of 

 compression tests. One section of the paper deals with the phenomenon 

 of hysteresis in the relation of extension to stress which is exhibited 

 in a marked degree by iron in the overstrained state. Incidentally 

 attention is called to subsidiary points of interest. 



The experiments were carried out in the Engineering Laboratory of 

 Cambridge University, and were the outcome of suggestions by Pro- 

 fessor Ewing. It was on his suggestion that the effect of moderate 

 temperature on recovery from overstrain was tried, and the result of 

 that trial led to much of the work incorporated in the paper. 



The straining and testing were done by means of the laboratory 50- 

 ton testing machine, the specimens employed for the most part being 

 taken from steel rods one inch in diameter, of a quality which may be 

 described as semi-mild. The small strains of extension were measured 

 by Professor Ewing's extensometer.* 



After referring to the apparatus and the material employed, and 

 describing the method of experimenting, there are first given in the 

 paper examples of the slow recovery of elasticity with lapse of time. 

 These examples are illustrated by stress-strain curves obtained, at 

 succeeding intervals of time, from extensometer readings similar to 

 those tabulated by Professor Ewing in his paper, referred to above, 

 " On Measurements of Small Strains in the Testing of Materials and 

 Structures." Recovery is shown to be at first comparatively rapid ; 

 but latterly very slow progress is made, and weeks or months may be 

 required before an approximately perfect restoration of elasticity is 

 effected. When this is brought about, the specimen may be subjected 

 to a stress a few tons per square inch higher than that at which 

 the virgin material yielded, before a yield-point is passed and the 

 material once more brought into a semi-plastic state. If sufficient 

 time be allowed to elapse after passing this second yield-point, an 

 elastic state will again be assumed, and a third yield-point may be 

 obtained about as far above the second yield-point as the second was 

 above the first. In this manner four or five yield-points may be 

 obtained with the same specimen before fracture occurs. A specimen 

 broken in this manner shows greater ultimate strength, but less ulti- 

 mate elongation than would have been obtained had fracture been 

 brought about in the usual fashion, that is, without allowing inter- 

 mediate recoveries of elasticity to take place. 



Keference might also be made to Lord Kelvin's discovery of tlie effect of a Sunday's 

 rest on wires which had been subjected to torsional vibrations throughout the pre- 

 ceding week. 



* For description see paper already cited, " On Measurements of Small Strains, 

 &c.," 'Boy. Soc. Proc.,' vol. 58, April, 1895. 



