32 BELL SYSTEM TECHNICAL JOURNAL 



SO that we may write 



5 = S -\- tH 



A = A° + HX 



(10.1) 

 (10.3) 



This tells us that we may determine the temperature coefficients of the 

 elastic modulii by measuring the ejEfect of stress on the temperature ex- 

 pansion coefficients. 



In a similar way we find that if the isothermal elastic constant matrix at 

 temp. ^ + / is: 



C = C° + th (10.4) 



then the relation between temperature and stress at constant strain e is 



X = tB (10.5) 



where 



B = B" + he (10.6) 



The Difference between the Specific Heats at Constant Stress and Constant 

 Strain 



Writing for the specific heats at constant stress and at constant strain 

 o-P and a" , respectively, we can perform the following cycle: 



Equating the sum of the entropy changes to zero: 



..2 



Equating the work in to the heat out: 



P 



(10.7) 



