the Limit of Elasticity in Metals. 199 



going show sufficiently that, by stretching the bars, a raising of 

 the limit of elasticity is possible. 



§ 5. On the Diminution of the Limit of Elasticity by Heat. 



By heating and by subsequent cooling, the limit of elasticity 

 of metals can be very considerably diminished. This has been 

 shown by Wertheim in his experiments on elasticity. For when 

 he investigated the drawn wire without any previous heating, 

 and at another time after heating, he found the limit of elasticity 

 so changed that, after heating, it was 



In iron . \ 



„ cast steel -fa 



English steel (pianoforte-wire) 



of what it was before heating*. 



Although Wertheim's experiments show completely the power 

 of heat to lower the limit of elasticity, I may be allowed to ad- 

 duce an example from my experiments which also affords an 

 illustration. 



A bar of hard steel was placed in a Fuller's machine and sur- 

 rounded by a thick cast-iron tube (internal diameter =2*25 

 inches, length = 4 feet) in which three thermometers were in- 

 serted, in order to indicate the temperature of the air in the 

 tube which surrounded the bar. The cast-iron tube, suitably 

 supported, was closed at its ends by covers provided with a suf- 

 ficiently wide hole for the bar to move freely. Above the cast- 

 iron tube was a gas-tube which served as heating-apparatus. 



By carefully reading the thermometer and corresponding 

 regulation of the gas-tap, the temperature could be kept tole- 

 rably constant. That the mean temperature of the mass of air, 

 as indicated by the thermometers, was sufficiently near the tem- 

 perature of the bar, is shown by the fact that, from the observa- 

 tions, the coefficient of expansion of steel was obtained as fol- 

 lows : — 



1 0-0000106, 



II 0-0000105, 



which numbers differ little from those of Lavoisier and Laplace. 

 The permanent elongations obtained by stretching the bar in 

 question are given in Table II. For the sake of comparison, 

 the experiments are given which were made at the ordinary 

 temperature with another bar of the same kind of steel. See 

 Table III. and the corresponding figs. 2 and 3 on Plate III. 



* Loc. cit. pp. 40 & 55. 



