1880.] On certain Effects of Stress on Soft Iron Wires. 513 



ceptible "knee" in the curve similar to, but of course much smaller 

 than, that shown in fig. 3. 



Experiments have also been made to determine the effect produced 

 by completely relieving the wire of stress for a time, and then con- 

 tinuing the test. Fig. 4 is an example of the effect observed. In this 

 example, when the stress reached 35 kilos., the flow of water into the 

 tank was stopped, and the tank was emptied by opening a stop-cock 

 at the bottom. This brought the line in the diagram back to the line 

 of no stress. The wire was then left without stress for 19 J hours, 

 after which water was run into the tank again, at the usual rate, until 

 the wire broke. The result, as may be seen from the diagram, was 

 that no considerable lengthening took place until a stress far exceeding 

 35 kilos, .was reached. It was not until the stress reached nearly 

 40 kilos, that the wire began to stretch again, and then it stretched 

 rapidly, and broke with the comparatively small final elongation of 

 less than 13 per cent. 



Here the interval of no stress had an hardening effect exceedingly 

 similar to the effect produced by an interval of constant stress. 



In the test shown in fig. 4 there was a gradually diminishing stress 

 on the wire while the tank was being emptied, and a gradually in- 

 creasing stress on it while the tank was being refilled. To show that 

 the hardening effect observed was not due to these stresses, I made 

 the experiment whose diagram is gi ven in fig. 5. Here the wire was 



Fig. 5. 



Showing effect of removing the load when it reached 35 kilogrammes, and replacing 

 it without leaving any sensible interval during which the wire was free from stress. 



stressed up to the same point as before (35 kilos.), the water was then 

 run out of the tank at the same rate as in fig. 4, and immediately run 

 in again. The circumstances of this experiment were, therefore, 

 identical with those of the preceding one, except that now there was 

 no sensible interval during which the stress was nil. There was in 

 this case a small hardening effect, such as might have been anticipated 



