[coKER A m'kekgow] THERMAL CHANGE TO TENSION 



13 



As an example of a compression test, the case of a wrought iron 

 specimen of square section and one inch side may be quoted. This 

 specimen was 4f inches long, just sufficient to accommodate the ther- 

 mopile and strain measuring instrument. Preliminary experiments 

 showed that the value of k under the given conditions was '00985; the 

 curve I from which the value was obtained is plotted on Figure 3. 



In the final test, beyond the yield point, the readings obtained 

 were as follows: — 



TABLE IIL 



Resistance of pile = 5 55 ohms. 

 Resistance of galvanometer = 52 ohms.* 

 Resistance of leads - 181 )hms. 

 Scale distance = 10'. .04." 

 One division on scale = 5 inches. 



These results show that the yield point, as determined by strain 

 measurements, coincides with the point where the thermal change shows 

 a marked deviation from linearity. 



The thermal change is, moreover, proportional to the stress up to 

 this point, and this is clearly indicated in Figure 3, where the thermal 

 time curve IV is plotted from the observed readings, and from this 

 curve V has been derived, giving a linear relation to near the yield 

 point, like the stress-strain relation curve III. 



Similar experiments upon other compression members confirmed 

 these results, and hence it appears that the thermal change for both 



