Entropy Meters. 95 



point the logarithm becomes 0. For temperatures between 1° 

 and 0° (absolute) the readings will be negative, and pass rapidly 

 from to - cx.y . It may be objected that entropy is an 

 essentially positive quantity ; I would answer, so for that 

 matter is temperature, but no one objects to reckon temperatures 

 negatively below the melting point of ice. 



Entropy INIeters for Constant Temperature. 



The principle of these is obtained in the following way : — 

 Employing our old notation, and using in addition the symbol 

 w to denote the pressure coefficient, and e to denote the 

 isothermal elasticity, we may write the second and fourth 

 thermo-dynamic relations thus : — 





Dividing (1) by (2), we obtain 

 '8p^ 



but - v[ ; 



substitute in (1), we get 





^ / — a e 



.'.d'^cf> = a . e . d'l'V. (3). 



For all sul)stances a is constant so long as the temperature 

 does not vary ; e is, under similar circumstances, constant for 

 solids and liquids, but numerically equal to the pressure for 

 a gas. 



Dropping the suffixes and integrating, we obtain for a solid 

 or a liquid 



i>.,-cf>,=-a.e.(v.,-V,) 



Hence an instrument which measures change of volume can 

 measure the entropy changes of solids and liquids. I have not 



