Section III., 1906. [ 171 ] Trans. R. S. C. 



XI. — Tke Effect of Tensile stress on Electrical Resistance. 

 By 11. VV. Boyle, M.Sc, Demonstrator of Physics McGill University. 



(Communicated by Dr. H. T. Barnes, and read May 23rd, 1906.) 



The object of t-his investigation was to lind out if a tensile stress is 

 a wire produced changes in its electrical resistance, beyond what we know 

 to follow from its changes of dimensions, which can be calculated for. 



W.ork of this nature has been done before, but not for resistance 

 alloys such as constantjin, manganm, and rheotin, which are now used 

 for electrical work. 



W lUiam Thompson (Lord Kelvin) in 1856 stated that " he had verj' 

 nearly established, for the case of iron, at least, that the augumented 

 resistance due to tension, either temporary or permanent, is very little 

 more than can be accounted for by the change of form." 



The whole problem is, of course, one of change of Specific Kesis- 

 tance. 



it was conclusively established by Lord Kelvin and others that the 

 specific resistance always increases with the decrease of density, so that 

 if the density decreases we may expect an increase of specific resistance 

 and hence a greater change of resistarce than we can calculate for from 

 cliaiige of length and diameter. 



Tomlinson, working on tjhis subject under " Influence of Stress and 

 Strain on the action of Physical Forces'' (Phil. Trans., Vol. 174, 1883), 

 found that permanent extension by longitudinal traction produces a 

 slight alteration of density. In his experiments this change was a de- 

 crease of density which never reached as much as .5 per cent although 

 some of his wires were strained to breaking. 



Previous to this Werthein had obtained similar results (Ann. de 

 Chemie, 1884, tom XVI) ; and Gray and Henderson working on this 

 subject under Lord Kelvin in 1887, found a change in density amounting 

 in the case of copper to fully .5 per cent. 



The present work was on the resistance alloys, Constantin, Man- 

 ganin, Kheottin, and on thie pure metals. Copper, Aluminium. No work 

 of this kind, so far as we know, has been done on the first three, but was 

 done for the last two by Tomlinson and by Gray and Henderson. 



In this work the method of measuring the resistance was by the fall 

 of potential method, the potential drop being measured on a 100,000 — 

 ohm Thompson-Variey slide. 



Arrangement and Description of Apparatus. 



A rigid suspension for the test wire was made by squeezing it 

 between two iron plates, each G" x 1" and f" thick. One of these plates 



