80 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
Critical points. 
Thalen lias also proved* that if a curve be drawn having for its ordinates and 
corresponding abscissae lines proportional respectively to the permanent extensions 
and the load producing them, there is a point of maximum curvature, where the 
increase of length becomes suddenly large compared “with the load, and suggests this 
point, which appears to be nearly at the same part of the curve for different specimens 
of iron and steel, as a suitable substitute for the so-called “ limit of elasticity.” 
My own investigations have shown that there exist in every metal two such points, 
which I have called critical points, and these are evidently very closely related to the 
moduli of elasticity. The first of these critical points is one that must of necessity 
have escaped the notice of most observers of the elongation of wires in the usual 
manner, as the load required to straighten the wire sufficiently for observations made 
in this way is beyond this point. Now in the present investigations it is not requisite 
to straighten the wire, and as the increase of resistance proves to be almost exactly 
proportional to the permanent elongation, it is evident that the curves showing the 
relations between permanent extension and load will be similar to those connecting 
permanent increase of resistance and load. 
Experiment XXI. furnishes a good example of the first critical point. It will be 
seen that the first critical point occurs at the seventh measure, this producing a 
permanent increase of resistance the ratio of which to that produced by the previous 
measure is appreciably greater than any of the other similar ratios. 
The following experiment is a sample of a series undertaken with a view to ascertain 
the position of the two critical points for the different metals. 
* Pliil. Mag., December, 1865. 
