262 



HEAT. 



the maximum enables the engineer to determine how far the engine is 

 working economically. 



The steam engine is a complicated engine unsuitable for the investiga- 

 tion of the general theory of heat transformation. We shall use, instead, 

 a much simpler ideal machine, first imagined by Carnot, the founder 

 of this part of thermodynamics, and described in a celebrated essay, 

 Reflexions sur la Puissance Motrice du Feu, published in 1824. This 

 engine is outside the range of practical experience, but inasmuch as we 

 can see how it would work if the practical details of construction and 

 manipulation were surmounted, the abstract conception is perfectly 

 legitimate. 



Carnot's Reversible Heat Engine. Let us suppose that we have a 

 cylinder (Fig. 150) with absolutely non-conducting walls, and containing a 



Conducting 



Source 

 kept at f 



Variable 

 Load 



Working Substam V, 



A/o/7 Conducting 

 Table 



Conducting 



Refrigerate' 



kept el t'" 



FlG. 150. Carnot's Engine. Shaded parts non-conducting. 



non-conducting piston perfectly fitting yet without friction. Let the cylinder 

 contain air or other working substance under pressure applied by the 

 piston, which we may suppose loaded to any desired extent. Let the 

 bottom of the cylinder be perfectly conducting. As source and refrigera- 

 tor let us have two bodies arranged as stands, as in Fig. 150, with a non- 

 conducting table between them, their temperatures being maintained 

 constant somehow at t" and t' respectively. We shall take the working sub- 

 stance through a series of changes such that it ends in its initial condition. 

 This series, known as Carnot's Cycle, is represented on the indicator 

 diagram by the four-sided figure, ABCD (Fig. 151), bounded by BC and 

 AD, the t and t' isothermals, and two adiabatics BA and CD. We 

 suppose the substance in the cylinder initially in the condition represented 

 by the point A, Fig. 151, at the temperature '. Placing it on the central 

 table (Fig. 150), we gradually increase the load, decreasing the volume 

 adiabatically until we arrive at the point B, when the temperature has 

 risen to /. Now, sliding the cylinder along the table on to the source we 



