Thompson Matched Pair, Steam Engine Indicators

An engine indicator is an instrument for graphically recording the pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine. The American Steam Gauge Company of Boston, MA manufactured these indicators. They are a matched pair of internal spring indicators based on a design by Joseph W. Thompson which he patented on August 31, 1875 . This design reduced the size and weight of the recording mechanism in order to allow more accurate measurements of high speed steam engines. Each of the indicators consists of a brass cylinder with internal spring and piston connected to a recording stylus and a recording drum. The piston causes the stylus to rise and fall with pressure changes in the engine under measurement thereby directly recording the indicator’s output on the paper. Around the drum’s base is wound a cord that is attached to the connecting rod of the piston on the steam engine being measured. This causes the drum to rotate as the engine’s piston moves. An internal coil spring causes the cord to retract and the drum to counter rotate back to its original position as the connecting rod returns. 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 the steam indicator in the late 1790s and early 1800s by James Watt and others 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. This Thompson steam indicator set represented over one hundred years of evolution and improvement of the devices. Its ability to make simultaneous recordings of two cylinders on a high speed steam engine was a significant improvement for many applications.
Currently not on view
Object Name
indicator, steam engine
overall: 4 1/2 in x 12 1/4 in x 8 5/8 in; 11.43 cm x 31.115 cm x 21.9075 cm
ID Number
catalog number
accession number
Engineering, Building, and Architecture
Measuring & Mapping
Steam Engines
See more items in
Work and Industry: Mechanical and Civil Engineering
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of Allen-Mitchell & Co., Washington, D. C.

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