102 
MR. II. TOMLINSON ON THE INFLUENCE OF STRESS 
which we shall learn presently that rest causes after strain, since the resistance was 
decreased by the removal and replacing of the pulley in one experiment 14 hours after 
the permanent extension had been made; a much longer time, of course, elapsing in 
this case than was required in the above mentioned operations. 
Again, though the curves are not capable of showing any sudden changes in the 
ratio of the alteration of the specific resistance, and the extension at the two critical 
points before alluded to, inasmuch as with most metals these points occur when a 
comparatively small amount of extension has taken place, yet it will be seen from 
Experiment XXVIII. that with the increase of length caused by the load of 6f kilogs. 
there is a sudden increase of the ratio B : A, and this load corresponds very closely 
with that at the second critical point of silver. 
Hammering. 
W. Thomson, in 1857, experimented on the effects of hammering and permanent 
extension on the electrical conductivity of copper, and though no actual numbers are 
given in his paper, states* that “ the greatest degree of brittleness produced by tension 
does not alter the conductivity of the metal by as much as one-half per cent.” He, 
moreover, adds: “ A similar experiment showed no more sensible effect on the con¬ 
ductivity of copper wire to be produced by hammering.” The foregoing experiments, 
it will be seen, fairly bear out Thomson’s statement with reference to the small amount 
of alteration produced in the specific resistance of copper by longitudinal extension, and 
show a still smaller change in the resistances of silver and platinum. But since these 
experiments at the same time showed that the alteration after increasing to a maximum 
in one direction began to decrease, and in certain cases finally set in in the opposite 
direction, it seemed desirable to ascertain whether hammering would produce like 
effects on the specific resistance. 
The third of the methods used in determining the change of specific resistance by 
extension was here employed, except that the wires were now lengthened by hammer¬ 
ing them transversely. The last process was accomplished rather by a great number 
of comparatively small blows than by a less number of large ones, so as to hammer the 
wire as uniformly as possible throughout its whole length. The following ate examples 
of the results arrived at:— 
* 
Proc. Roy. Soc., vol. viii., p. 553. 
