AND STRAIN ON THE ACTION OE PHYSICAL FORCES. 
119 
the strained wire is most increased in resistance by rise of temperature up to a certain 
limit of straining ; whilst beyond this limit further strain diminishes the first effect. 
In the second class, which comprises copper, silver, platinum and German-silver, the 
strained wire is least increased in resistance by rise of temperature, but that, here 
again, after a certain limit of strain has been reached the first effect begins to 
be diminished. Now it will be shown in Part IV. that the metals of the first class, 
when subjected to a moderate amount of strain, whether the latter be produced by 
permanent extension, hammering, or torsion, or by other than purely mechanical means, 
are thermo-electrically positive to pieces of the same metal unstrained, and that the 
first effect is ultimately reversed if the strain be carried to excess .; whilst the metals 
of the second class are, when moderately strained, thermo-electrically negative to 
unstrained pieces of the same metal, but when subjected to excessive strain become 
thermo-electrically positive. We are therefore led to the conclusion that there must 
be some close relationship between the thermo-electrical properties of metals, and their 
! temporary alteration of resistance from change of temperature, and that strain renders 
a piece of metal thermo-electrically positive or negative to a piece of the same metal 
unstrained according as the strained piece is caused to he less or more increased in 
electrical resistance hy rise of temperature. 
Another point to be considered is that the metal which is increased in specific 
resistance by the strain is not always rendered less susceptible to change of resistance 
from alteration of temperature, for, as we have seen, zinc and platinum-silver are 
increased in specific resistance by moderate strain, and are yet at the same time 
rendered more susceptible to alteration of resistance from variations of temperature. 
With the former of the above-mentioned metals, the alteration of susceptibility to 
change of resistance from change of temperature is comparatively much larger than 
is the case of any of the other metals ; whilst with platinum-silver the effect of 
strain in this respect is comparatively slight, and of the opposite kind to that which is 
produced on its two constituents. 
Again, the third column in the experiments shows that the permanent effect 
j produced by the change of temperature is not of the same nature with iron as with 
copper ; with the former metal the resistance is in some cases actually increased by the 
annealing when the strain is moderate, and in those cases where a permanent decrease 
[ of resistance is caused this increases with increased strain; with the latter, on the 
contrary, permanent decrease of resistance after moderate straining is the result of the 
I annealing, and this decrease after increasing to a maximum begins to become less with 
further strain. The difference between iron and copper in the above-mentioned respect 
is readily intelligible when we remember that these metals differ as regards the kind 
of alteration of specific resistance caused by strain, as we have seen that with iron the 
finer gauges was found to be to diminish the temperature coefficient, but it seems that Mr. Taylor 
annealed the wires after the process of drawing. 
