170 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
10. The alteration of the melting-point temperature of ice can be readily and 
accurately determined by observations of the change of resistance produced by fluid 
pressure on metal wires placed in the ice. 
11. Experiments on the permanent alteration of resistance of metal wires produced 
by stress furnish valuable information respecting the “ limit of elasticity ” of metals. 
12. There are two “ critical points” in every metal at which sudden changes occur in 
the ratio of the permanent extension produced by any load and the load itself, when 
the latter is gradually and carefully increased. The first of these two points fixes the 
true “ limit of elasticity,” and the second the true “ breaking-point ” of the metal. 
With iron there are three, and perhaps more “ critical points.” 
13. The “ critical points” are evidently in most cases closely related to the moduli of 
elasticity. 
14. The total resistance of most metals is permanently increased by permanent longi¬ 
tudinal extension, but with nickel the total resistance is permanently decreased, 
provided the extension does not pass a certain limit; beyond this limit further exten¬ 
sion produces increase of resistance. 
15. The rate at which a wire is “running down” under the influence of a load can 
be very advantageously studied by observing the permanent increase of resistance 
produced by tbe load. 
16. If P be the “ "breaking-load ” of a metal wire, and p be the load actually on the 
wire, the decrease per unit of the velocity of the increase of resistance is inversely 
proportional to P -p : so that the actual “ breaking-load ” of a wire can be calculated 
from observations of the rate of increase of resistance when a loaded wire is “ running 
down.” 
17. The above-mentioned proportion holds good not only for one and the same metal 
but for different metals. 
18. The result of experiments on the influence of permanent extension on the tem¬ 
porary alteration of resistance which can be produced by temporary longitudinal stress 
verifies the statement made in Part I. that “ the elasticity of a wire is diminished by 
permanent extension not exceeding a certain limit, but beyond this limit increased.” 
The effect of permanent extension on the alteration of resistance which can tem¬ 
porarily be produced in nickel by traction is very remarkable. 
19. Permanent extension, hammering and torsion produce, even when carried to 
excess, very small changes in the specific electrical resistances of metals. Most metals 
have their specific resistances increased by strain caused by the above-mentioned pro¬ 
cesses, provided the strain does not exceed a certain limit: beyond this limit further 
strain decreases the specific resistance. In the case of iron and nickel, on the contrary, 
the specific resistance is at first decreased and afterwards increased. 
20. The strain caused by heating annealed steel to a temperature slightly and very 
much in excess of that of the room produces effects on the specific resistance of the 
metal of a kind similar to those caused respectively by small and great mechanical 
strains. 
