APPENDIX 



365 



the boiler till it is steam under the former conditions 

 of temperature and pressure. 



Therefore we must, in order to obtain a self-acting 

 engine, cause the working substance, and the mechanism 

 of the engine, to perform a series of cyclical operations. 



The Carnot engine is a cylinder containing a gas 

 called the working substance S, and this gas can 

 be brought into thermal contact with a source 

 of heat, or a refrigerator, that is, the gas can be 

 heated or cooled by a mechanism 

 outside itself. The walls of the 

 cylinder are made of some substance 

 which is a perfect non-conductor of 

 heat, but the bottom of the cylinder 

 is made of a substance which con- 

 ducts heat perfectly. There is a 

 piston in the cylinder which fits it 

 closely, but which moves up and down 

 without friction. At the bottom of 

 the latter is a valve which can be 

 turned so as to place the bottom of 

 the cylinder, and therefore the gas, 

 in thermal contact with a reservoir 

 of heat (+), or a refrigerator (-). 

 But when the valve is turned so that the non-con- 

 ducting part fills the bottom, the gas is perfectly 

 insulated, and heat can neither enter nor leave it. 



Such an engine is, of course, an imaginary one, since 

 there can be no mechanism in which there is not a 

 certain amount of friction between moving parts, and 

 there are no substances which conduct or insulate 

 heat perfectly. The engine is, in fact, the limit to a 

 series of engines each of which is supposed to be more 

 perfect than the last one. It is a fiction which is of 

 considerable use in theoretical work. 



Fig. 31. 



