6 Wilson, Stj^ess and Strain in Copper Bars. 



law was obtained, it is evident that the difference cannot 

 be due to variation in speed. 



On referring to the Records of tests made for the 

 pubHc in the Whitworth Laboratory, to see if a stress of 

 19 tons per square inch had been obtained, the highest 

 recorded stress for annealed copper rods was found to be 

 i6'97 tons per square inch, with a corresponding measured 

 elongation of nearly 37^ %. This bar curiously happened 

 to come from the same makers as the bars in question, 

 and the results obtained from it correspond very fairly 

 with the above formula, the theoretical stress for an 

 elongation of 37^ % being i6"5 tons per square inch. 



It is evident, therefore, that the annealed copper and 

 iron both refuse to follow the law any further when the 

 elongation has reached a certain value, about 33% for iron 

 and between 30 % and 40 % for the copper. 



In the case of the iron this occurs after the theoretical 

 maximum load has been passed, thus enabling the 

 theoretical maximum stress to coincide with the actual 

 maximum stress as determined from experiment, whilst 

 for the copper the theoretical elongation is never reached, 

 and hence the maximum stress from experiment is less 

 than its theoretical value. 



To eliminate the time-effect of the load on the exten- 

 sions, a set of bars were loaded in turn to pre-arranged 

 amounts, each bar being allowed to stand under its load 

 for 30 minutes and being then removed from the machine. 

 This ensured that the viscous extension might have 

 time to cease before measurement. 



After the 30 minutes' interval the extension had 

 practically ceased, and each bar was removed and 

 measured. The result of these tests are shown in the 

 appendix. (Tests 7-15 ; tests 13, 14, 15 being repetitions 

 of Nos. 7, 8, and 12). 



