EXPEETMENTAL EESEAECHES ON DRAm^ STEEL. 
38 
modulus 111 these wires is — 0'00045,* whilst for resistivity it is + 0 '0035 approxi¬ 
mately. 
Density. 
6. When a steel rod is drawn into wire the variation of density as the drawing 
proceeds may not progress uniformly, for it is not unlikely that the stress required to 
force the rod through the draw plate may so far separate the molecules longitudinally 
that the lateral compression does not compensate for the extension. In this case the 
density will diminish, and, in short, density will depend upon the ratio of extension 
to compression. It is, therefore, not only interesting, but of some importance to 
trace the change of density at each stage of manufacture and to compare this with 
the change of the properties already examined. 
The method adopted was to weigh a suitable length of the wire in air and after¬ 
wards in distilled water, at the same time noting the data necessary for corrections 
on account of density of the water, the buoyancy of the air, the weight of the 
suspending fibre, etc., the most important correction being the one wdiich makes allow¬ 
ance for the temperature of the water differing from that of its greatest density, 
approximately 4°. 
For the sake of confirmation an entirely independent duplicate set oi observations 
was carried out, and the agreement between the two sets was very satisfactory, 
especially in the earlier specimens, which, being of greater size and mass, could be 
weighed with relatively higher accuracy. The results are given in the last column of 
Table IX. It will be seen that they end at the eleventh wire, as the twelfth was 
accidentally mislaid; this is particularly unfortunate, as it would have been useful to 
know whether No. 12 exhibited a density greater or less than the abnormally high 
density of No. 11. 
The feature which first claims notice is the diminution of density, although only 
slight, which takes place when the rod is first subjected to traction, namely, between 
the 2nd and 3rd, and between the 4th and 5th stages. 
This appears to be an illustration of the remarks at the beginning of this section. 
Afterwards, as the drawing progresses, tliere is a steady increase of density, with a 
large increment between the 10th and 11th stages. The entire variation of density is 
about 2 5 per cent., the least and greatest values lying respectively at the beginning 
and end of the list. 
As a general rule, it has been observed that density and Young’s modulus in steel 
vary directly together, and this leads to a comparison of the present results with the 
curve of the modulus. The similarity is not immediately obvious, but, in both, 
annealing and tempering produce an upward movement, whilst the first drawing after 
either of these operations produces very little change, subsequent drawings, however, 
* According to Styfte the average value for ordinary steel is 0-0003. “ Strength of Iron and Steel,” 
by Knut Styffe, p. 122. Vide also Tomlinson, ‘Roy. Soc. Phil. Trans.,’ vol. 179, p. 23 ^ vol. 174, 
pp. 132-133. 
VOL. CCI. — A. 
F 
