Robertson-Thompson Steam Engine Indicator - ca 1900

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 James L. Robertson & Sons of New York, NY, manufactured this steam indicator about 1900. The indicator is based on a design patented by Joseph W. Thompson which made improvements in the mechanisms driving the recording stylus thus allowing improved measurements of higher speed steam engines. The design also includes elements from another patent by Mr. Alpheus O. Lippincott and assigned to Mr. Robertson. It dealt with the reduction wheel mechanism below the recording drum. The reduction mechanism allowed for measuring engines with a variety of piston throw lengths.
This indicator set contains within the mahogany box the indicator itself; extra springs of varying stiffness for different steam pressures; a reducing wheel to decrease the piston motion to that required by the indicator drum; sized wooden pulleys for different piston stroke lengths; an extra indicator piston of small diameter for very high pressures; a planimeter for measuring the area of the diagram; servicing tools; and extra blanks. 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 Robertson-Thompson steam indicator represented over one hundred years of evolution and improvement of the devices. Its ability to make recordings for a wide range of engine speeds, pressures and piston stroke lengths was a significant improvement for many applications.
Object Name
indicator, steam engine
date made
ca 1900
Physical Description
wood (case material)
case: 6 1/2 in x 9 1/2 in x 10 1/2 in; 16.51 cm x 24.13 cm x 26.67 cm
place made
United States: New York
ID Number
catalog number
accession number
Steam Engines
See more items in
Work and Industry: Mechanical and Civil Engineering
Measuring & Mapping
Engineering, Building, and Architecture
Power Machinery
Exhibition Location
National Museum of American History
Data Source
National Museum of American History, Kenneth E. Behring Center


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