162 ROYAL SOCIETY OF CANADA 



rent, the wire was allowed to undergo the full extension 

 caused by the heat generated by the current, and these exten- 

 sions were measured and plotted; then the stress-strain curve was 

 taken as in the case of no current in the wire. The current was 

 varied up to such a value that the heat generated by it was sufficient 

 to melt the wax pointer on the wire. This would be ait a temperature 

 not higher than 50° C, so that no annealing effect on the wire could 

 have been possible. The maximum current experimented with repre- 

 sented a high current density. Any heat generated by the stretching 

 of the material could not have been sufficient to cause appreciable 

 extension. The temperature of the room at the place of test was very 

 constant, never varying more than a degree during the test, and usually 

 varying by only a few tenths of a degree. 



By Hooke's law, if P is the total load on the wire of length L and 

 cross-sectional area A, and I is the extension caused by P; then, up to 

 the elastic limit. 



F = E A 



where E is the modulus of elasticity. 





Hence ?^ = E - -L 

 A L 



or p = E • e, 



p being the stress and e the strain. Hence E is proportional to the 

 slope of the stress-strain curve, and any change in the slope signifies 

 a change in E. 



In all cases the cufves obtained show that, taking account of the 

 extensions caused by the heat of the current, the stress-strain lines are 

 parallel to the stress-strain line when there is no current in the wire; 

 and when the extension caused by heat is deducted from the total 

 extension the stress-strain lines are identical. 



Constantin. 



This is one of tlie later resistance alloys. Its composition varies, 

 sometimes being given as 60 Cu, 40 Ni, and sometimes 58 Cu, 41 Ni, 

 1 Zn. It is a tough, fairly soft, and not very stiff material with 

 elastic limit high, but not very strong beyond that. 



The preliminary test for the stress-strain curve (Fig. 2), shows 

 the elastic limit of the specimen to be at a load of 4,650 gms., cor- 



