AND STRAIN ON THE ACTION OE PHYSICAL FORCES. 
23 
We observe that the annealed metals may be divided into two classes ; that iron, 
platinum, lead, and zinc, after suffering permanent torsion, twist temporarily* on 
loading, showing that the load produces greater temporary lengthening in the direction 
in which the torsion had produced extension than it does in the direction of compres¬ 
sion. It seems, however, that if the permanent extensions previous to the loading be 
carried to excess, the wire will begin to untwist on loading; thus in a we observed 
that after 260 turns of permanent torsion the twist produced by loading has begun to 
diminish and in o that, when the wire had been considerably stretched before torsion, 
after 70 turns the twist is changed to untwist. Moreover, the results in k on hard 
and soft steel, combined with those in o, clearly show us that whether the distance 
between the particles be increased by mechanical means or by hardening by the process 
of heating and suddenly coolingt we ultimately arrive at a point where the twist on 
loading is changed to untwist. 
The second class includes the metals, copper, silver, aluminium and tin; these, like 
the metals of the first class, at first twist temporarily on loading, but after a com¬ 
paratively small amount of permanent torsion has been applied begin to untwist and 
continue to do so, as far as could be ascertained, until the wires will not bear any 
further twisting without instantly breaking ; indeed, the copper in c was broken 
several times, and became so brittle and hard as to snap, like steel that has been 
heated to a white heat and suddenly cooled, and yet the wire apparently showed no 
decrease in the amount of untwisting on loading. 
It will further be noticed that the average twist or untwist per kilogramme is nearly 
the same for the different weights employed, but other experiments on metals of the 
first class showed that when the load became excessive the average twist became less 
and less, and was finally converted into untwist when the load approached the breaking 
stress ; and moreover whereas with smaller loads the -permanent effect was in the case 
of both classes to cause untwist , with these larger stresses the wire commenced to 
twist permanently: this latter point is most easily proved with lead or tin, as com¬ 
paratively small weights are required to break the wires made of these metals. 
It may, I think, be fairly concluded from these and the previous experiments, that 
with all metals the longitudinal elasticity is diminished by permanent extension carried 
to a certain point; but beyond this point increased. 
Analogous results have been obtained by Thomson with respect to the torsional 
rigidity of metals^ and as we shall see latter on, the action of all physical forces is 
* It is perhaps as well to observe here that only the variations of torsion produced by unloading are 
recorded in the table. 
t Sir W. Thomson has already Brit. Encyc.,’ Art. : “ Elasticity,” § 81) proved that hard steel wire 
untwists on loading, after suffering permanent torsion ; but we see that if the steel be softened it acts like 
iron and twists on loading. 
+ Proc. Roy. Soc., vol. xiv., p. 289, and ‘ Brit. Encyc ,’ Art. : “ Elasticity,” § 78. 
