Entropy Meters. 93 



for solids and liquids, the. other for gases. Moreover, if the two 

 instruments suited to the particular state of matter under 

 consideration be employed together, the algebraic sum of their 

 indications will give the change of entropy during any transfor- 

 mation, whether the pressure, volume, or temperature be 

 changing, or all three change together ; provided that no such 

 change of physical state as fusion or condensation occur during 

 the process. 



About two years ago I worked out the first rough ideas of the 

 designs for two entropy meters, working models of which are 

 on the table to-night ; but the detailed calculations and the 

 construction of the apparatus were only completed lately. 



(2) Entropy Meter for Constant Pressure or Constant 



Volume. 



The construction of this instrument is based on the following 

 considerations : — 



If we denote volume by v, absolute temperature by T, pressure 

 by/, quantity of heat by Q, and entropy by </>, the definition of 

 entropy gives us at once 



= t(^—\ — (^\ 

 V6T/p ■ ^8t/i. 



K.VSTv'p 

 where K^, denotes the specific heat at constant joressure 

 . (H\ _K, /STx 

 "\^v)p T'VS^'A. 



■^^P* 1X1 



Dropping the sufiixes and integrating, we have 



fk-fXq 



