160 ROYAL SOCIETY OF CANADA 



refers to the question as follows : — " During the passage of a current 

 through metallic wires their cohesion is temporarily lessened, and there 

 also appears to be a decrease in their coefficient of elasticity. It was 

 thought by Edlund that a definite elongation could be observed in, 

 strained wires when a current was passed through them; but it has 

 not yet been satisfactorily shown that this elongation is independent 

 of the elongation due to the heating of the wire owing to the resistance 

 it opposes to the current." 



Acting from the suggestion of this last clause the present work 

 was undertaken. One would hardly expect that a tensile strain could 

 produce the effect mentioned, and the results obtained in this work 

 go to show that at least for the materials here tested no such effect 

 is produced. No other conclusion can be taken from the observations 

 obtained. , 



The present work was done on : 



(1) Pure metals, — Copper, aluminium, steel, which arc the metals 

 most used in electrical work; and (2) Alloys, — ^ German silver, man- 

 ganin, Constantin, rheotin, which are used for resistance purposes in 

 electrical laboratories. 



Arrangement of Apparatus and Method of Experiment. 



A rigid suspension for the test wire was made in this manner : 

 An iron plate, 6" x 1 and f" thick was tightly screwed to a heavy 

 oak beam. Through this plate protruded two studs with ends threaded. 

 Another iron plate, of dimensions same as the first, fitted exactly over 

 it, and the two plates were tightly secured to one another by means 

 o( bolts on the studs. By squeezing the wire between the plates the 

 suspension is made very rigid. 



The wire hanging from the plates was about ten feet long, and 

 carried at the bottom a scale-pan for holding the weights which made 

 up the load on the wire. 



A heavy copper wire was soldered to the suspension plates, and a 

 light flexible copper wire to the junction of the test wire and the soale- 

 pan. These ^vi^es conaucted the current to and away from the test wire. 



A thread of brittle wax on the wire, drawn out to a point when 

 soft, served as a very good pointer to indicate the extensions, which 

 yseve measured by a cathetometer giving readings to one-thousandth of 

 a millimetre. By keeping the cross-hairs of the cathetometer telescope 

 focussed on the wax pointer very small extensions could be observed on 

 the cathetometer scale. 



The first work done was to take the stress-strain curve of a wire, 

 of the same material and of the same dimensions, as the test wire. 



