282 DR E. G. COKER ON 



2. Effect of Tensional Stress on the Yield-point. 



The only experiments upon the yield -point appear to be those of M'Farlane.* These 

 showed that a tension lowered the yield-point. Reasoning from this result, Lord Kelvin 

 concludes that a compression stress would raise it, but no experiments appear to have 

 been made to verify this conclusion. 



In order to examine the effect of tension at or about the yield-point, a bar of wrought 

 iron was taken and cut into two parts ; one specimen was turned truly parallel to a 

 convenient diameter (0*424 inches), and the second was made exactly the same size. 

 Both specimens were tested and found to be perfectly cylindrical, as far as could be 

 ascertained by a micrometer gauge. 



The first specimen was then tested in the ordinary way, with the result shown in 

 column I., Table VII. The first noticeable deviation occurred when each pan was 

 loaded with a weight of 16 pounds — corresponding to a torque of 333 inch -pounds, the 

 maximum torque being 385 inch pounds. 



The second bar was then stressed ; but before the tension load was applied a pre- 

 liminary reading was taken — to see whether the readings agreed with those from the 

 first specimen, and, as will be seen (col. II.), the agreement is very close. 



The specimen was now loaded with an additional 2400 pounds — corresponding to 

 an increase of stress of 17,900 pounds per square inch — and a torque applied by incre- 

 ments. As shown by column III., a slight deviation was noticed at 333 inch-pounds, and 

 failure was accomplished by a torque of 360 inch-pounds. 



This result shows in a marked way the lowering of the yield-point by tension, and 

 confirms M'Farlane's experiments. 



* Loc. cit. ante. 



