lO ROYAL SOCIETY OF CANADA 



tional to the difference of temperature is correct. Hence at any points 

 1, 2, we shall have the relations 



or k = loge -^ (^2-^) 



In the present example the value of k was found to he ,00276, the 

 time being measured in seconds, and the corrected curve is shown on 

 Figure 1, from which it will he seen that the relation of thermal change 

 to stress is a linear one, except at the commencement, where the inertia 

 of the moving coil and the lag of the thermopile itself causes a disturb- 

 ing influence. There axe other corrections due to (1) the change in 

 the resistance of the circuit owing to change in temperature, (2) the 

 galvanometer deflections being piopoitional to cp / cos cp where ^ is the 

 angular displacement of the coil, and also due to (3) the scale being 

 straight; of these (1) was negligible, as the temperature was very con- 

 stant and about 68° Fahr., and in (2) and (3) the value of cp was 

 rarely greater than 5°, so that all these latter corrections are negligible 

 usually. 



It has been supposed by Turner ^ that the relation between thermal 

 change and stress does not follow the same law as the relation between 

 stress and strain for the range of stress up to the yield point, but that 

 there is a marked deviation from linearity for the thermal change long 

 before the yield point is reached, and hence it is inferred that the 

 elastic limit, as determined by thermal measurements, is much lower 

 than if determined by strain measurements. If, however, due correc- 

 tion is made for the effects of conduction and radiation, it can be shown 

 that this is not the case, and that both strain and thermal effect bear a 

 linear relation to the stress for approximately the same range of stress. 



In the case of the specimen mentioned above, this was tested by 

 applying an extensometer of the Unwin type as well as a thermo-pile, 

 and observing the readings of both simultaneously up to the yield point 

 of the material. The experiment gave the following results: — 



^ IjOC. cit ante. 



