Notes: "A steam engine indicator is an instrument for creating a plot of pressure vs. piston displacement in a steam engine. A chart is wrapped around a cylinder which can rotate against a spring in one direction. A string connected to the piston rod is wrapped around that cylinder and rotates it as the piston moves. The pencil is driven by a piston which is exposed to the steam pressure. There is usually a series of levers between the piston and pencil to amplify its motion and draw a larger trace. The resulting plot can be measured to find the horsepower and efficiency of the steam engine. The Trill indicator was patented in 1912, and made in Corry, Pennsylvania."