1880.] On certain Effects of Stress on Soft Iron Wires. 511 



Water was ran into the hanging tank from a cistern at a consider- 

 able height above it. This gave a convenient means of applying stress 

 slowly or quickly at will, and at a sensibly uniform rate. When not 

 otherwise specified, the rate of flow of water into the tank is to be 

 understood to have been about 1 kilo, in 15 seconds. 



The iron wires were 1*1 millim. in diameter, and the length under 

 test was 250 millims. 



Fig. 1 shows one of the diagrams of a continuous test, in which the 

 flow of water was continued uniformly at the above rate until the wire 

 broke. The horizontal ordinates give the stress in kilogrammes, the 

 vertical ordinates give the elongation in percentages of the original 

 length. The line is sensibly straight until the stress is about 21 kilos. ; 

 after that it curves rapidly but rather irregularly. The breaking stress 

 in this example was 37*5 kilos., and the final elongation 16*3 per cent. 



Fig. 2 shows the effect of maintaining a constant stress on the wire 

 for a considerable time. When the stress reached 35 kilos, the flow of 



Fig. 2. 



Showing effect of maintaining a stress of 35 kilogrammes constant for hours. 



water was stopped, and this load was allowed to remain on the wire 

 for an interval of 45J hours, after which the flow of water was resumed 

 at the same rate as before, and continued until the wire broke. The 

 effect was most remarkable. Instead of continuing to lengthen at 

 once when the flow of water was resumed, the wire refused to stretch 

 further until the stress rose to about 40 kilos. There was, in fact, a 

 new " limit of elasticity " at this point. When the stress exceeded 

 40 kilos, the wire ran down rapidly, and finally broke with a load 

 of 41 kilos, and a total elongation of 13J per cent. The breaking 

 strength was therefore greater, and the elongation less, than when the 

 wire was tested continuously. 



This effect, which I shall call the " hardening effect,'' depends on 

 the length of time during which the test is interrupted. Fig. 3 shows 



