83 



The results are shown in Table III. These results show that the be- 

 havior of nickel steel under overstrain is like to that of ordinary steel, 

 namely: 



Overstrain destroys the P-liniit, and elevates the Y-limit. 



The P-limit may be restored liy annealing for a few moments in a 

 bath above 212° F. The P-limit may be also restored by a period of rest. 

 By a process of overstrain and subsequent annealing, the P-limit may be 

 •elevated to nearly the ultimate strength. 



The decrease of diameter was also measured. The ratio of side con- 

 traction to longitudinal extension was found to be nearly y^, which is 

 the value of Poisson's ratio for this metal. 



TABLE I. 



Value of E. in 100,000 Units as Derived from Tests on Bars of Nickel Sleel at 



Three Laboratories. 



T.\BLE II. 



Comparison of Extensometers 



