MI;. .1. Mill; ON Till: i;i:rovi:i;\ oi- II;ON n;o.\i OYKIISTKAIN. 3 



sions cun IK.- plotted to a inucli larger s.-ale. sli<>\v the imperfect elasticity <i' recently 

 overstrained iron which -lias leen referred to above; that is, they slmw tliat the 

 material is semi plastic. It' time l>e allowed to elapse between unloading and 

 reloading, tin- recovery l'n>m the effect of overstrain may be shown in a diagram like 

 the present, liy some such curve as ef. When no interval of time is allowed to elap^- 

 U'tween the removal and the replacement of the load, then the stress-strain curve i> 

 continued in the manner shown by /</, until a j>oint g is readied, at which local 



Diagram No. I. 



Extensions in tenths of An inch. 



sets in. When this hap]>en8 the stress may be diminished, and fracture 

 may take place at a load lower than that at which local extension occurred. The 

 stress per square inch of fractured area is, however, found to In- much greater than 

 the stress per square inch of the actual area when local elongation began. 



Tin- A/1/inratnjs and (lie Material. 



The straining and testing in the following experiments was done by means of the 

 50-ton Wicksteed single-lever hydraulic testing machine of the Cambridge Engineer 

 ing Laboratory. With this machine the magnitude of the load applied could In- read 

 in tons to a second decimal place by means of a vernier, and to a third decimal place 

 roughly by estimation. Thus a load could be applied accurately if necessary to, it 

 may he said, 5 ,',,,th of a ton. 



The small strains of extension \\eiv measured by Professor EWI\I;'S extensometer.* 



* Km- ;i full description of this instrument sec the jcinci already cited "On the Meiwuruniuiit of Snmll 

 stiains, \c.," ' Proe. Koy. Soc.,' vol. 3*, April, 1695. 



Ji 'I 



