Manchester Memoirs, Vol. xliii. (1899), No. 10. 7 



The logarithmic plotting is shown in Fig. 2, the 

 points obtained being indicated by the circles. 



The mean value of ;/ is "512 with a corresponding 

 value of C=40'25. 



In finding n, only those points which lie on the line 



have been used ; with respect to the points Nos. 8, 13, 15, 



an error of 'Oi inch in measuring the extension would 



account for their variation from the mean line. Thus the 

 formula becomes /'=40"25^'s^2. 



In the expression /= Ce^, C represents the stress 

 which will extend the bar to twice its original length. If 

 n be put equal to unity then the law represents the elastic 

 extension, C having a different value, viz., the modulus of 

 direct elasticity, but still being the stress necessary to 

 double the length of the bar. 



The result of plotting the logarithmic stress-strain 

 curve for the elastic portion would evidently be a straight 

 line inclined at 45'' to the axes of coordinates. The point 

 of intersection of this line with that previously obtained 

 would mark the yield-point. 



Being desirous of obtaining a complete diagram before 

 and after the yield -point, and not possessing the means of 

 measuring the elastic extensions of the |-inch diameter 

 bars, a series of copper wires were tested up to a stress 

 somewhat beyond the elastic limit. 



These wires, which finally were cut in lengths of about 

 20 inches, were suspended vertically and loaded by 

 weights applied to the lower end of the wires by means of 

 a hanger. On the under side of the hanger was a projec- 

 tion, which could be brought into contact with the upper 

 end of a vertical screw working in a nut attached to 

 a disc divided circumferentially into 100 equal parts. 

 The screw was chased with 10 threads to the inch, so that 

 extension of "ooi inch could be measured, and of "oooi 

 inch estimated. 



