104 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
Table XVI.—Curves showing the alteration of specific resistance produced 
by hammering. 
Observations on the curves in Table XVI. 
Experiments XXXIII., XXXIV., and Table XVI. show that the effect of hammer¬ 
ing on the specific resistance is of a somewhat similar character to that of permanent 
extension. With all the metals, except iron, the specific resistance is at first increased, 
and this increase, after reaching a maximum, begins to diminish, but with iron the 
first effect is decrease of resistance which also, after attaining a maximum as the 
hammering is carried to a greater and greater extent, begins to diminish until finally 
there is a comparatively large increase of specific resistance. 
The changes produced, however, by hammering, though similar in kind to those 
produced by longitudinal extension, are very different in amount, and a comparison of 
the two sets of curves and the scales on which they are formed shows that the altera¬ 
tions in the former case are very much greater than those in the latter. 
The neutral points also, i.e., the points where the curves cut the axis of ordinates, 
are different in the two tables, being for copper and iron much higher for the extension 
than for the hammering, but with zinc and German-silver lower for the extension than 
the hammering. 
Torsion. 
As it seemed desirable to supplement the observations of extension and hammering 
with others on torsion, with the view of ascertaining whether the strain caused by 
