Graphs of pressure as a function of volume for complete thermodynamic cycles are called indicator diagrams. The area enclosed by the figure is a measure of the work done during the cycle. 

   The problem for the engineer is to devise a mechanism to draw the pressure-volume graph. This is done with the device at the left, called an indicator. 

   Paper is wrapped around the nickel-plated cylinders, which are connected, by way of the pulleys, to the piston rod. Thus, as the piston goes through one complete cycle, the cylinder rotates back and forth and  then returns to its starting point, thus giving a measure of  the volume in the cylinder.. 

   The pressure is recorded by letting the steam into a small cylinder in the unit, where it compresses springs. A pencil attached to the moving end of the spring then traces the pressure-volume graph on the paper.

   This double-indicator apparatus I found at an antique shop in Lambertville, New Jersey in April 2000. 

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