AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
17 
employed for testing increases , whereas with all the other annealed metals similarly 
treated, exactly the opposite effect is produced. This peculiarity of iron is no doubt 
to be attributed to its superior coercive force; and to the same cause must probably 
be assigned the difference between the effect of permanent extension on the elasticities 
of iron and copper when these metals are allowed to rest unloaded after the extension 
has taken place. 
In order to examine still further the effect of leaving a heavy weight on the wire 
for long periods, 20 kilogs. were, after the above experiments had been made, left on 
the wire for one day, and then for two more days. The average increases per 2 kilogs. 
after each of these periods, when tested with 20 kilogs., were respectively 1*474 and 
1*475 half-millims. Therefore the full effect of the loading must have been produced 
during the five days on which the previous trials had been made. 
Finally, it should be observed that during the whole of these last experiments the 
wire returned to its original length on the removal of the load. 
Effect of Permanent Torsion combined with Traction. 
The above are the only direct experiments which were made of the effect of per¬ 
manent extension on the value of “ Young’s modulus; ” but having ascertained 
indirectly that, at any rate in the case of some metals, permanent extension will, 
according to its amount, produce either decrease or increase of elasticity, I was induced 
to make a set of observations in which torsion was combined with longitudinal 
traction. 
Fig. 4. 
Let P Q B, S, fig. 4, “ represent a portion of the wire in the unstrained condition ; 
and suppose that, the upper end having been fixed, the lower end is twisted in the 
contrary direction to the hands of a watch, thus causing the portion A B C D to be 
extended along the diagonal A C and compressed along the diagonal B D ; if now a 
load be applied at the lower end S P, this will cause the wire to twist still further or 
to untwist, according as the extension produced by the load along A C is greater or 
less than that along B D. 
weight permanently on the wire, this weight may exceed that required for the above-mentioned limit. 
It is quite possible, also, that if the permanent extension and the heavy loading during rest be carried to 
very great excess, any load however small may exceed the first limit.] 
* ‘ Electricity and Magnetism,’ Clerk .Maxwell, vol. ii., p. 86. 
MDCCCLXXXIII, D 
