12 6 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
diminished the elasticity, and therefore we have evidence here that very slight extension 
permanently increases "the elasticity of iron ; whilst in Part I. we have seen that 
moderate permanent extension decreases, and excessive permanent extension increases 
the elasticity of iron. It is evident, therefore, that we have three critical points in 
iron wire, and that the elasticity is first increased to a maximum, then decreased to a 
minimum, and finally begins to increase again as the permanent extension is gradually 
increased from exceedingly small amounts to the breaking point of the wire. 
Again, the alteration of resistance produced by all the loads is now greater at 100° 
than at 12° by amounts varying from about 5 to 6 per cent. 
Experiment XLVIXI. also shows that when loads from 6 to 3 kilogs. are left per¬ 
manently on the wire when cooling from 100° C. to the temperature of the room, the 
alteration of resistance decreases as we decrease the load, and bears out the previous 
observations that the elasticity is less at the higher than at the lower temperatures. 
In Experiment XLIX. we have evidence that the slight annealing caused by raising 
the temperature of the copper to 100° C. permanently decreases the elasticity, and that 
the alteration of resistance at 100° C. is for the load employed about 2 per cent, 
greater than at 12° C. It is, however, when we come to Experiment L. that the most 
noticeable changes are seen to be produced, and when we consider the results recorded 
in this experiment and in Table XXII. we are led to the conclusion that the tem¬ 
porary alteration of susceptibility to change of resistance from changes of stress 
effected by raising to 100° C. is, with nickel, as remarkable as we have seen this 
susceptibility itself to be. The curves are drawn on the same scale as the curves for 
nickel at 22° C., and it will be observed that not only is the maximum diminution of 
resistance lessened by raising to 100° C. to an amount which is less than one-lialf of the 
maximum diminution at the lower temperature, but that also the load at which this 
maximum diminution occurs is much less ; and it may well be that at a sufficiently 
high temperature the decrease of resistance which is observed to be produced by 
moderate loads at the lower temperatures would be changed to an increase. In fact, 
I am inclined very strongly to believe that there exists with all metals a critical 
temperature below which temporary stress will produce temporary decrease of resist¬ 
ance, and that above this temperature there is an opposite effect caused by the stress. 
The value of “ Young’s modulus ” was determined for this piece of nickel by the 
method of static extension, in a manner to be presently described, and was equal to 
2480 X 10° at the temperature of 16° C. and to 2280 X 10 6 at the temperature of 100° C. 
It will be observed that the maximum decrease of resistance as shown in Table XXIII., 
is appreciably greater than the maximum decrease observed with the other specimens,"' 
and still further shows how the amount of this decrease depends upon the elasticity. 
As regards the question whether the thermo-electric properties of the metals, as 
* These two specimens were received at different times from Messrs. Johnson, Matthey, & Co., and the 
latter shows still more remarkably than the former the large elasticity which can be obtained from nickel 
even in a well-annealed condition. 
