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XIII..— The Effect of Permanent Elongation on the Specific Resistance of Metallic 
Wires. By Tuomas Gray, B.Sc., Demonstrator in Physics and In- 
structor in Telegraphy, Imperial College of Engineering, Tokio, Japan. 
(Plate XVIIIa.) 
(Received 23d October 1880.) 
The object of this investigation was to obtain information as to the change 
of specific resistance produced in wires of various metals by different amounts 
of elongation. ._The present paper refers to experiments on copper, iron, and 
German silver wires. 
Besides the effect of permanent elongation, I have added the results of a 
number of observations on the effect of elastic elongation. These results were 
obtained in the course of the other experiments, and, the two taken together, 
may serve to throw some light on the cause of the change of resistance. 
In order to render the effect of elastic elongation intelligible, it is necessary 
to form an estimate of the change of section due to the stretching. This can 
be readily done if we know the ratio of linear contraction, at right angles to 
the direction of pull, to the extension in the direction of pull. If we suppose 
this wire isotropic this can be obtained from the Youne’s and rigidity moduluses 
(THomson and Tarr’s “ Natural Philosophy,” § 684). I give a determination of 
these constants for several of the wires experimented on, both before and after 
a measured amount of elongation, from which an approximate estimate of the 
above ratio is derived. 
In reckoning the change of specific resistance, the resistance of 1 metre, 
weighing 1 gramme, has been taken as the specific resistance of the material. 
IT have thus left out of account the effect due to any change of density which 
may have resulted from the stretching of the wires. This change of density 
was in the later experiments carefully noted, and is recorded in the tables of 
results. It will be found, on examination, that the change of density was in 
every case small, that it sometimes increased and sometimes diminished, but 
that, in either case, there was little difference in the change of resistance. So 
far as these experiments go then, no effect, due to change of density simply, 
was discovered. 
The fact that the density remained almost constant all through the experi- 
ments, gives a very easy method of observing the effect of elastic elongation. 
Generally the density of a wire is diminished by elastic elongation, and there- 
fore, if change of form is sufficient to account for the change of resistance, the 
effect of elastic elongation should be somewhat /ess than that of permanent 
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