64 BUTLER art. d 



In 1824 S.Carnot made use of such a process to determine 

 the amount of work obtainable by an ideal heat engine, drawing 

 heat from a heat reservoir at a temperature t' and giving it out 

 at a lower temperature t". In this process, the body or "work- 

 ing substance" is put through a cyclic series of operations, 

 consisting of two isothermal and two adiabatic stages : 



(1) The working substance is put in contact with the heat 

 reservoir at the temperature t' and is allowed to expand, thereby 

 performing work against the opposing forces and, since its 

 temperature remains constant, absorbing a quantity of heat 

 Q' from the heat reservoir. 



(2) The working substance is thermally insulated so that it 

 cannot receive or give up heat to its surroundings, and allowed 

 to expand further, whereby work is obtained and the tempera- 

 ture falls to t". 



(3) The working substance is put in contact with a heat 

 reservoir at t", and is compressed until it reaches a state from 

 which it can be brought into its original state without communi- 

 cation of heat. In this stage work is expended on the substance 

 and a quantity of heat —Q" passes from it to the heat reservoir. 



(4) The working substance is thermally insulated, and 

 brought into its original state by the expenditure of work. 



In this process a quantity of heat Q' has been taken from 

 the heat reservoir at t' and a quantity of heat — Q" given to the 

 heat reservoir at t". Since the working substance has been 

 returned into its original state the total work obtained is equal 

 to the sum of the quantities of heat absorbed, i.e. 



W = Q' + Q". 



The ratio of the work obtained to the heat absorbed at the 



Q' + Q" 

 higher temperature, i.e. ^ is termed the efficiency of 



the process. 



Carnot postulated, (1) that a cyclic process, in which every 

 stage is carried out reversibly, must be more efficient than any 

 irreversible cycle working between the same temperature 

 limits can be, and (2) that all reversible cycles working between 

 the same temperature limits must be equally efficient, whatever 



