182 Prof. W. C. Unwin. On the Yield Point of Iron [Dec. 13, 



a fire-clay jacket; the chamber was heated by a row of Bunsen 

 burners, the products of combustion passing round the annealing 

 box. When the bar to be annealed was red-hot it was turned end 

 for end, and shortly after the gas was extinguished, and the chamber 

 allowed to cool down slowly with the bar in it. Every time the bar 

 was strained beyond the yield point, its diameter sensibly decreased 

 from lateral contraction. Some further diminution of section was 

 due to oxidation in the annealing oven. After each annealing the 

 bar was re-measured before re-testing. In each test an autographic 

 load- strain diagram was taken. In the final loading the bar was 

 broken. 



Experiment I. 

 Mild Steel Bar, STo. 1431. 



Original diameter = '750 in. Area = -4418 sq. in. Clips 8 in. apart. 



In annealing, the bar was put into the furnace cold, the gas was 

 then lit, and the bar left until it became dull red in colour. It was 

 then changed end for end, and after a few minutes the gas was turned 

 off, and the bar allowed to cool in the furnace. 



The bar was laid upon the bottom of the furnace, and not pro- 

 tected in any way. 



On examining the autograph diagrams (fig. 6) it will be seen that 

 the yield point was reproduced after every annealing in as marked a 

 way as in the first testing. Reckoning on the area at the beginning 

 of each test, there is a rise in the stress at the yield point from 17'45 

 tons per sq. in. to 18'71 tons per sq. in. in seven successive loadings 



