356 Profs. Gray and Mees on the Effect of Permanent 



hard-drawn iron wire increased in total rigidity by being 

 slightly stretched above its elastic limit. Slight changes in 

 the modulus of rigidity, due to stretching, are of common 

 occurrence ; but we had not previously noticed, nor heard of 

 anyone else having noticed, such a change as to overbalance 

 the effect of increased length and probably diminished section. 

 The result seemed to make it probable that there was an 

 actual increase of diameter produced by pull when the elastic 

 limit is reached. As to whether actual permanent stretch is 

 necessary to produce this effect we are not yet certain ; but 

 there seems, from subsequent experiments, little doubt but 

 that, for several metals, hard-drawn wires do expand laterally 

 when they begin to take a permanent longitudinal set. 



The observation here referred to seemed so interesting, 

 that we instituted careful observations on the effect of slight 

 stretching (that is to say, permanent stretching of from -^ to 

 j^j of 1 per cent, of the original length) on the density of 

 hard-drawn iron, brass, German silver, and pianoforte-steel 

 wires. The results indicate that for an elongation of less than 

 £ per cent, the section of the wire is slightly increased, above 

 which the section diminishes. The experiment is, however, 

 by this method exceedingly tedious and difficult, and hence 

 the exact elongation which gives the greatest increase of sec- 

 tion has not been determined. One or two of the results are 

 quoted below mainly for the purpose of indicating the mag- 

 nitude of the quantities concerned. These changes are very 

 small ; and as they enter into the experiments on density 

 determined by weighing in water as the differences between 

 quantities of comparatively large magnitude, we cannot 

 assume the results to be more than an indication confirmatory 

 of the result obtained from the elasticity experiments. It 

 may be remarked, however, that experiments on this subject 

 were made by several members of the senior class both on 

 soft and hard wires, with the consistent result that they ob- 

 tained sometimes a gain, sometimes a loss of density for the 

 soft wire, but always a loss of density in the hard wires. 

 The specimens which showed most decided results were 

 cut from two coils of very hard wire, one brass and the 

 other German silver. The brass is so hard as to render 

 it difficult to obtain permanent stretch without fracture. 

 The results with iron and steel have not been quite so 

 conclusive, owing to difficulties with oxidation during the 

 density -determinations. A pianoforte-steel wire does, how- 

 ever, show signs of slightly increasing in volume just before 

 breaking. The difference between the elastic strength and 

 the rupturing strength is not great in pianoforte-wire, and it 



