4 
MR, H. TOMLINSON ON THE INFLUENCE OE STRESS 
half of the breaking-weight. During this time the relative positions of the scale and 
vernier were viewed from day to day ; and when there seemed to be no alteration of these 
positions, the wire sustaining the heavier load was relieved from stress and allowed to 
rest for a period of from one to six days. Weights were then very carefully* put on 
to or removed from the pan attached to this wire, and the changes thereby produced 
in the length determined, the wire having in the meantime been permanently loaded 
to a sufficient extent to keep it quite straight. The softer metals—lead, tin, and even 
zinc and aluminium—were subjected to stresses much less in proportion to their 
tenacities than those used with the other metals. In their case each of the pair of 
wires was elongated permanently about 10 per cent, by weights, half the weight was 
then removed from the comparison-wire, and the two allowed to rest for four or five 
days. After this interval, the other wire was entirely relieved from stress, and finally 
was permanently loaded with a weight equal to that on the comparison-wire. As 
with the harder metals, the real testing did not begin till time wrought no appreciable 
change in the relative lengths of the two wires. 
Reasons for the above precautions. 
In the case of all the substances employed very great care was taken with the 
determinations, which often numbered more than one hundred for each of two or more 
different weights, and it was noticed after the experiments had been carried on for 
some time that the results obtained on different days frequently varied to a greater 
extent than could be accounted for by mere errors of observation. At first it was 
suspected that, in spite of the mode of suspension, a slight yielding of the upper sup¬ 
port might be the cause of these discrepancies. Two additional wires were therefore 
suspended to this same support and loaded alternately with weights of 14 lbs. so as to 
tend to turn it about axes, respectively parallel and perpendicular to the line joining 
the points of attachment of the first two wires. Not the slightest effect, however, could 
be detected on the relative positions of the scale and vernier. The latter were then 
both shifted several time to parts of the wires a few inches above or below their 
previous positions to ascertain if any sticking between them could be observed— 
though in all cases care had been previously taken to adjust them so that their planes 
were parallel to that of the wires. But again these alterations did not seem to affect 
the results. The wires were now taken down and examined in case there might have 
* Several methods were tried for loading and unloading ; amongst other the stress was applied by 
allowing measured amounts of water to flow slowly into a large but comparatively light pail attached to 
the wire instead of the scale-pan, the removal of the stress being accomplished by suffering the water to 
pass out through a piece of caoutchouc tubing attached to an orifice at the bottom of the pail, and which 
during the process of loading was hitched up by the side of the pail. I found, however, that quite as 
good results could be obtained by putting on the weights by small amounts at a time by hand; a little 
practice, as far as my experience goes, enables the experimenter, especially if a support be provided for 
the elbow, to load or unload without causing any appreciable shock. 
