AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
95 
the wire may have been before stretching, the latter action is sure to diminish the 
diameter at some parts of the wire more than at others, and eventually the wire breaks 
at that part at which the greatest contraction has taken place. 
If a wire could be obtained of perfect uniformity of diameter and substance, the 
contraction would gradually increase from each end to the centre where it would be 
greatest, and at this point the wire would break. It is interesting to watch this 
gradual increase of contraction from the two ends to the middle in the case of a test- 
bar of ductile iron or steel, and through the kindness of Sir Joseph Whitworth 
1 was able to make the following experiment on such a bar of fluid-pressed steel, the 
specimen having been selected as suitable for this purpose in consequence of its great 
ductility. 
The bar had a total length of 6 inches before stretching, but a screw was formed, 
2 inches in length (see fig. 16), at each end for the purpose of securing the bar in the 
Fig. 16. 
framework of the hydraulic press employed to stretch it. The diameter of the bar 
between the two screws was before testing *7979 inch, and after each stretching the 
bar was removed from the press and the length and diameter re-determined: the 
results obtained are recorded in the next table. 
Table XIV. 
Stress in tons 
per square inch. 
Total 
permanent increase 
of length measured after 
the removal of 
the stress. 
Total 
permanent contraction 
of diameter at 
the centre. 
Ratio of 
permanent contraction 
of diameter to 
permanent elongation. 
20 
T060 
•025 
•236 
22 
•1560 
•030 
T92 
24 
•2380 
- 050 
•211 
25 
•3010 
•053 
•176 
26 
•5175 
•098 
T90 
26 Second time 
•7850 
•228 
•294 
23-g- Broke 
•8900 
•323 
•363 
It will be noticed that the ratio of permanent contraction of diameter to the 
permanent elongation is as sensibly constant as could have been expected from the 
way in which the diameter had to be measured, namely, by calipers,* until the 
* Whitworth’s measuring-machine would have been employed for this purpose as it was for measuring 
tlic increase of length, had not the curving of the bar produced by the stretching rendered such an 
instrument unsuitable, 
