2 Wilson, Stress and Strain in Copper Bars, 



Professor Reynolds,* it is found that the points so 

 obtained all lie upon a straight line, inclined to the axis 

 at an angle whose tangent is "25, thus indicating a relation 

 of the form f= Ce^, where f is the stress in tons per square 

 inch of the reduced section of the bar, and e is the per- 

 manent strain and equals 



(stretched length — original lengthj/original length, 

 C being a constant. 



The object of using a separate bar for each point in 

 the curve was to eliminate the time-effect of the previous 

 loading, whilst the continuation of the loading for 30 

 minutes after its full value had been reached allowed the 

 bar to complete its extension before measurement. 



The stress-strain diagrams obtained in this manner 

 will differ from those usually drawn by automatic record- 

 ing instruments, inasmuch as the extension for a given load 

 will be less in the latter case than in the former. 



For the copper bars the law of extension has been 

 obtained for both cases, viz., for the extension under a 

 continuously-increasing load, with no period of rest ; and, 

 secondly, for bars loaded in a manner similar to that 

 adopted for the iron specimens already described. 



The copper bars used were 12 inches in length and 

 |-inch diameter. They were cut from drawn copper rods, 

 carefully annealed at the works of the manufacturers, 

 Messrs. the Broughton Copper Company who kindly 

 provided the bars, and were found to be sufficiently true 

 to do away with the necessity of turning them to a par- 

 ticular size. 



A length of 5^ inches, and in some cases 6 inches, was 

 set out upon a line scribed on each bar parallel to its 

 longitudinal axis. The ends of this length were indicated 

 by two small centre punch-marks, and into these the 



* Phil. Trans., 1879, p. 753. 



