Page:Elementary Text-book of Physics (Anthony, 1897).djvu/259

§ 228] increases in volume on the introduction of heat, and does work by expansion, under a pressure which is the same for all points on its surface.

Construct two rectangular axes of Tolume and pressure as in Fig. 72. Let the pressure and corresponding volume of the body, when its temperature $$R$$ is that of the refrigerator, be represented by the point $$A.$$ The operation of transforming the heat received from the source into work is then performed as follows: The body, being so placed that it can neither receive nor emit heat, is compressed adiabatically; its volume diminishes and its temperature rises, and the operation is continued until its temperature becomes $$S,$$ equal to that of the source. Its pressure and volume in this state are represented by the point $$B.$$ It is then put in contact with the source and allowed to expand; as soon as its expansion begins, its temperature falls and heat enters from the source. The expansion may be so regulated that the difference of temperature between the body and the source never exceeds an infinitesimal, so that the heat which enters the body during this part of the process enters it at the constant temperature $$S.$$ The expansion may be allowed to continue until any desired quantity of heat $$H$$ is taken from the source. The pressure and corresponding volume attained by this isothermal expansion are represented by the point $$C.$$ The body is then removed from the source and allowed to continue its expansion under such conditions that it neither receives nor emits heat. Its volume will increase and its temperature will fall. This adiabatic expansion is allowed to continue until the temperature of the body becomes $$R,$$ that of the refrigerator. The body is then placed in contact with the refrigerator and compressed. As its volume begins to diminish, its temperature rises, and heat passes out from it into the refrigerator. The compression may be so regulated that the difference of temperature between the body and the refrigerator never exceeds an infinitesimal, so that the heat which leaves the body during this part of the process leaves it at the constant