12 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
ancl that the stress thus produced had been acting for several days previous to these 
trials. 
When lesser weights were now used and only the scale-pan left on permanently the 
maximum alterations took place much more quickly, and the departure from “ Hooke’s 
law,” which had before been very considerable, became comparatively slight. 
The modulus was calculated from these last results. 
Cases of tin and lead. 
These metals are remarkable for the manner in which they run down under the 
influence of the slightest stress, and also for the persistence of this running down ; in 
this latter respect they surpass aluminium and zinc. It was necessary, therefore, to 
keep them loaded for many days before attempting to determine their elasticity. The 
weights also employed for this latter purpose were very small, and, in consequence, 
the values of the modulus obtained by using them cannot be considered as accurate as 
those of the other metals ; I believe, however, that they are correct within 2 per cent. 
“Hooke’s Law.” 
We have seen that there is a very appreciable departure from this law shortly after 
permanent extension has taken place ; but a departure also exists when a long rest 
has been allowed, both when the wire has in the meantime been heavily weighted and 
when it has not, even though only moderate loads be employed. 
Thus, in the case of a soft copper wire capable of bearing a load of 18 kilogs., and 
which had been heavily loaded and frequently tested during a period of three weeks, 
the following' observations were made :— 
O 
Experiment VIII. 
Number of kilogs. in load. 
Average alteration per kilog. 
Millimetre. 
2 
•808 
4 
•811 
6 
•816 
The values here recorded are the means of about 20 observations on each weight 
made during the last three days of observation, and show small but decided differences. 
Again, another piece of copper similar to the above was treated in the same manner, 
but for a period of six days, and when examined on the last day, with loads up to 
12 kilogs., gave the following results as the means of seven trials with each weight :— 
