84 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
Remarks on Table VIII. 
It is evident from the last table that the loads both at the first and second critical 
points, in the case of each substance, bear a constant ratio to the corresponding value 
of “ Young’s modulus,” there being quite as much difference between the several 
c c 
ratios - and - for the three specimens of copper as for the various metals. The time 
during which the load was allowed to act was in all cases the same, namely, three 
minutes,'" and the wire was relieved of all weight except that of the scale-pan before 
determining its alteration of resistance. 
Experiment XXI. having shown that the position of the critical points must be 
altered by the process of wire-drawing, it is necessary that very great care should be 
taken in annealing the wiret if we wish to determine the true position of these 
points for any substance whose particles are to be free from mutual strain previous to 
beginning the experiments. The theoretically correct definition of the ‘‘limit of elas¬ 
ticity ” would be the highest load per unit of surface which a wire can bear without 
undergoing the slightest permanent elongation. It is clear, however, that we cannot, 
even with the utmost care, obtain a substance which will have its particles in a 
'perfectly homogeneous condition, and it will of necessity happen that some of the par¬ 
ticles are on the point of passing the elastic limit or, at any rate, that some are much 
nearer this limit than others, before any external stress has been applied. Conse¬ 
quently we can never hope to obtain the true value of the elastic limit by merely 
endeavouring to observe the first trace of a permanent elongation, our power to do so 
depending upon the delicacy of our instruments ; but we can accomplish our object in 
all probability by determining the point at which the ratio of the elongation and the 
stress producing it reaches its first maximum. I would venture, therefore, to suggest, 
tliat the first\ of the two so-called critical points be taken to represent the true “ limit 
of elasticity ” of a well-annealed substance, and to agree with Thaler that neither the 
method of measuring the limit of elasticity by the greatest load which will produce 
sensible permanent elongation, nor the purely arbitrary one adopted by Wertheim and 
others of fixing an elongation of ’0005 of the unit length as corresponding to this limit, 
is desirable. 
* Except in the case of copper (3) where five minutes was allowed. 
f It is certainly not sufficient to heat a wire to redness for a few minutes, and then allow it to cool 
slowly; the high temperature must be maintained in some cases for a considerable time. 
J The second of the critical points is evidently the same as that suggested by Thalen as being suitable 
for measuring the limit of elasticity (Phil. Mag., December, 1865), but I think that the second point is 
rather the precui-sor of breakage than of the passage of the elastic limit. 
