AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
5 
been some slight slip of their ends through the vice. No sign, however, of anything 
of the kind could be detected, and on re-suspending and again testing them almost 
exactly the same results were obtained. Finally, the permanent loading was gradually 
increased in amount to determine if any error had arisen from the wires not having 
been sufficiently straight. It was hardly expected that this would prove to be the case, 
as before any of these trials had taken place the wires had been previously loaded for 
at least 24 hours to such an extent as to cause considerable permanent elongation. 
Nor indeed could any appreciable change in the measurements be detected until the 
weights used were such as to produce farther permanent elongation. 
When this happened, however, there was evidently an apparent alteration of 
elasticity. A reference to the notes made on previous occasions then revealed the fact 
that whenever the above-mentioned discrepancies had occurred a similar permanent 
elongation had been produced. A fresh series of experiments was therefore begun, 
which ended in the discovery of the following facts :— 
1. In the case of a wire which has suffered permanent extension the temporary 
elongation which can be produced by any load becomes less as the interval between the 
period of permanent extension and that of applying the load becomes greater.' 1 ' 
2. This increase of elasticity is greater in proportion for great loads than for small 
ones. 
3. The increase of elasticity takes place whether the wire is allowed to remain loaded 
or unloaded between the period of permanent extension and that of the testing for 
the elasticity. 
4. The rate of increase of the elasticity varies considerably with different metals; 
with some the maximum elasticity is apparently attained in a few minutes, and with 
others not till some days have elapsed—iron and steel are in this last respect very 
remarkable. 
5. The elasticity can also be increased by heavily loading and unloading several 
times, the rate of increase diminishing with each loading and unloading. 
6. A departure from “ Hooke’s law ” more or less decided always attends recent 
permanent extension, even when the loads employed to test the elasticity do not 
exceed one-tenth of the breaking-weight.! 
7. This departure is diminished very noticeably in the case of iron, and much less so 
with the other metals by allowing the wire to rest for some time either loaded or 
unloaded; it is also diminished by repeated loading and unloading. 
* Since writing the above I have found that the gradual increase of elasticity with time in the case of 
soft-iron wires has been also noticed by Ewing. (Proc. Roy. Soc., 1880, No. 205, vol. xxx., p. 510.) 
f G. Wiedemann and Webtheim have proved that there is a similar departure from “ Hooke’s law ” in 
the rigidity of metals when tested by the method of statical torsion. I shall have occasion in Part III. 
to refer further to Professor Wiedemann’s valuable researches relative to torsion and magnetism : but see 
Wiedemann’s ‘ Annalen,’ 1879, vol. vi.; Phil. Mag., vol. ix., Jan. and Feb., 1880; ‘ La Lumiere Electrique,’ 
vol. vi., Nos. 2, 3, and 4; or Wiedemann’s ‘ Galvanismus,’ 
