36 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
Critical points. 
Several determinations of tlie permanent increase of length produced by loading 
were made, and led to the discovery that in all well and carefully annealed metals 
there are at least two points at which a sudden change takes place in the ratio of the 
load to the permanent extension produced thereby; these points have been called 
critical points, and it appears that changes more or less profound take place in most 
if not all of the physical properties of the substance when these points are attained. 
As, however, the subject will be fully discussed in Part II., where it will be shown 
how these points can be indirectly determined more accurately than in the ordinary 
manner, it will suffice here to say that the existence of the first of these critical points 
seems to prove beyond a doubt that there is a true limit of elasticity for each sub¬ 
stance, and that this is intimately connected with the value of “ Young’s modulus.” 
It would also appear 4 ' that at these two critical points sudden changes take place 
in the density of the substance. 
Summary of Part I. 
1. The magnitude of the temporary elongation which can be produced by any load 
on a wire which has experienced permanent extension is reduced by simply allowing 
the wire to rest either loaded or unloaded for some time after the permanent extension 
has taken place. 
2. The length of the period of rest necessary to produce the maximum of the effect 
mentioned in 1 varies considerably with the nature of the metal of which the wire is 
made ; with some metals a few minutes suffice, whilst with others, such as iron or 
steel, many hours are required. 
3. The effect of rest mentioned in 1 is greater in proportion for large loads than for 
small ones, and apparently vanishes in the case of such small temporary alterations of 
length as are produced by causing the wire to vibrate longitudinally. 
4. The magnitude of the temporary elongation which can be produced by any load 
on a wire which has suffered recent permanent extension is also reduced by heavily 
loading and unloading the w T ire, the rate of reduction diminishing with each loading 
and unloading. 
5. A departure, as far as temporary elongation is concerned, from “ Hooke’s law,” 
more or less decided, always ensues after recent permanent extension, even when the 
weights employed to produce the temporary elongation do not exceed one-tenth of 
the breaking-load of the wire. 
6. This departure is diminished very noticeably in the case of iron, and to a greater 
or less extent with all metals by allowing the wire to rest for some time either loaded 
or unloaded; it is also diminished by repeated loading and unloading. 
7. With aluminium and zinc, and probably with the more viscous metals tin and 
* See Experiments XXI.-XXIII. inclusive. 
