APPARATUS FOR MEASURING STRAIN AND APPLYING STRESS. 



273 



It will be seen at once that the bar exhibits quite different qualities from that shown 

 before. The stress is now no longer proportional to the strain, and the curve showing 

 the relation between the two no longer returns upon itself, but forms a looped figure. 

 Similar results are obtainable from bars subjected to tensional stress.* t 

 If the bar be tested again after a short interval of time, recovery will be found to be 

 very marked, At the end of one hour a test of the bar gave the results shown by 

 column III., there being a marked falling off of the increments at the higher loads. 

 Thus the strain caused by increasing the torque from 300 inch pounds to 375 inch 

 pounds now caused only 272 units of strain, instead of 289 ; and similarly at the end 

 of three hours we find a further decrease to 268 units. The recovery of the bar was 

 tested at suitable intervals of time, as shown in the annexed table, the effect becoming 

 less apparent as the time increased ; but practically perfect recovery was reached at the 

 end of two days, and very little change was noticeable after this time. 



Table III. — continued. 

 Column III. (One hour afterwards.) 



Torque in inch lbs. 



Reading. 



A 











246 



75 



246 



252 



150 



498 



255 



225 



753 



266 



300 



1019 



272 



375 



1291 



248 



300 



1043 



250 



225 



793 



257 



150 



536 



260 



75 



276 



269 







7 





The results may be shown graphically by direct plotting, but it is more convenient 

 to adopt the plan of subtracting from each reading a number proportional to the torque, 

 and plot the new set of readings thus obtained. The method is due to Prof. Ewing, 

 and is used in fig. 16, the diminution in the case being 200 units of scale reading for 



* Ewing, " On the Measurement of Small Strains in the Testing of Materials and Structures." Proc. Royal Society, 

 May 1898. 

 t Muir, " On the Recovery of Iron from Overstrain." Phil, Trans., 1899. 



