514 On certain Effects of Stress on Soft Iron Wires. [June 10, 



from the results mentioned above, but nothing like so much as in 

 fig. 4. The difference can only be due to the long interval of no 

 stress. 



An investigation is now being made of the Hardening effects of 

 equal intervals of constant stress and no stress at different points along 

 the curve. So far as the experiments have yet gone, it appears that 

 outside the so-called elastic limit, that is, for stresses greater than 

 about 22 kilos., the effect of 19 hours of constant stress is not greatly 

 different, whether the stress is great or small. Inside the elastic limit 

 the effect of constant stress is to prolong that limit, that is, the wire 

 then requires a greater stress to make it begin to stretch considerably. 

 This effect is greater the nearer to the elastic limit the interval of 

 constant stress occurs. 



By intermitting the flow of water for a succession of short intervals 

 during one test we get a stepped curve, having a number of " knees " 

 upon it, like those in figs. 2 and 3. 



A wire to which a continuously but very slowly increased stress is 

 applied, gives a curve in which the elongation corresponding to each 

 stress (beyond the elastic limit) is less than when the increase of stress 

 is rapid. In other words, when the flow of water is very slow, 

 the curve is bent upwards to the right. The final elongation is 

 diminished, and the breaking strength is increased. It will be ob- 

 served that the last-mentioned results are in perfect agreement with 

 the remarkable experiments described by Mr. J. T. Bottomley, in the 

 Report of the British Association Committee for commencing secular 

 experiments upon the elasticity of wires (1879). 



When a stress is applied and maintained constant it produces two 

 effects. It produces a gradual viscous elongation, at first rapid, after- 

 wards slow. This I shall call for brevity the viscous effect. It also 

 produces the hardening effect described in this paper. The hardening 

 effect, like the viscous effect, is greatest at first. The viscous effect is 

 quite visible in the diagrams. Thus, in figs. 2 and 3, especially in 

 fig. 2, the wires have stretched considerably during the time that the 

 stress has been constant ; and in figs. 4 and 5 the wires have gone on 

 stretching during part of the time that the water took to run out of 

 the tank. 



If the viscous effect existed alone, and there were no hardening 

 effect, a wire would show a higher breaking strength and less final 

 elongation the more quickly the increase of stress took place. For, 

 with a quickly increased stress, the wire would not have time to take 

 the elongation, and consequently the diminished cross-section proper 

 to each value of the stress. 



If, on the other hand, the hardening effect existed alone, a wire 

 would show a higher breaking strength and less final elongation the 

 more slowly the increase of stress took place. 



