Watt Steam Engine Indicator (Replica, 1927)

An engine indicator is an instrument for graphically recording the cylinder pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine. This is a replica of the original steam indicator invented in the late 18th Century by James Watt of Scotland. This was the first device intended to measure the varying pressures within a stem engine’s cylinder as it was working.

Originally consisting of only the brass cylinder and piston, Watt’s assistant (John Southern) made the important improvement of the recording tablet and pencil that resulted in the ability to make a lasting recording of a complete cycle of the engine under measurement. The piston of the engine moved the tablet horizontally via an attached cord, and the indicator’s piston moved the pencil vertically. A weight attached to the tablet via a pulley caused the tablet to move back horizontally as the engine’s piston returned to its original position. The result is a steam pressure-volume diagram which is used to measure the efficiency and other attributes of the steam engine.

The introduction of this steam indicator in the late 1790s by James Watt had a great impact on the understanding of how the steam behaved inside the engine's cylinder and thereby enabled much more exacting and sophisticated designs. The devices also changed how the economics and efficiency of steam engines were portrayed and marketed. They helped the prospective owner of a machine better understand how much his fuel costs would be for a given amount of work performed.

Measurement of fuel consumed and work delivered by the engine was begun by Watt, who in part justified the selling price of his engines on the amount of fuel cost the purchaser might save compared to an alternate engine. In the early days of steam power, the method to compare engine performance was based on a concept termed the engine’s “duty”. It originally was calculated as the number of pounds of water raised one foot high per one bushel of coal consumed. The duty method was open to criticism due to its inability to take into consideration finer points of efficiency in real world applications of engines. Accurate determination of fuel used in relation to work performed has been fundamental to the design and improvement of all steam-driven prime movers ever since Watt’s time. And, the steam indicators’ key contribution was the accurate measurements of performance while the engine was actually doing the work it was designed to do.