The indicator diagram was developed by
James Watt and his employee
John Southern to help understand how to improve the
efficiency of
steam engines. In 1796, Southern developed the simple, but critical, technique to generate the diagram by fixing a board so as to move with the piston, thereby tracing the "volume" axis, while a
pencil, attached to a
pressure gauge, moved at right angles to the piston, tracing "pressure". The indicator diagram constitutes one of the earliest examples of
statistical graphics. It may be significant that Watt and Southern developed the indicator diagram at roughly the same time that
William Playfair (a former Boulton & Watt employee who continued an amicable correspondence with Watt) published
The Commercial and Political Atlas, a book often cited as the first to employ statistical graphics. The gauge enabled Watt to calculate the work done by the steam while ensuring that its pressure had dropped to zero by the end of the stroke, thereby ensuring that all useful
energy had been extracted. The total work could be calculated from the area between the "volume" axis and the traced line. The latter fact had been realised by
Davies Gilbert as early as 1792 and used by
Jonathan Hornblower in
litigation against Watt over
patents on various designs.
Daniel Bernoulli had also had the insight about how to calculate work. Watt used the diagram to make radical improvements to steam engine performance and long kept it a trade secret. Though it was made public in a letter to the
Quarterly Journal of Science in 1822, it remained somewhat obscure,
John Farey, Jr. only learned of it on seeing it used, probably by Watt's men, when he visited Russia in 1826. In 1834,
Émile Clapeyron used
a diagram of pressure against volume to illustrate and elucidate the
Carnot cycle, elevating it to a central position in the study of
thermodynamics. Later instruments for steam engine (
illus.) used paper wrapped around a cylindrical barrel with a pressure piston inside it, the rotation of the barrel coupled to the piston crosshead by a weight- or spring-tensioned wire. In 1869 the British marine engineer
Nicholas Procter Burgh wrote a full book on the indicator diagram explaining the device step by step. He had noticed that "a very large proportion of the young members of the engineering profession look at an indicator diagram as a mysterious production." Indicators developed for steam engines were improved for internal combustion engines with their rapid changes in pressure, resulting from combustion, and higher speeds. In addition to using indicator diagrams for calculating power they are used to understand the ignition, injection timing and combustion events which occur near dead-center, when the engine piston and indicator drum are hardly moving. Much better information during this part of the cycle is obtained by offsetting the indicator motion by 90 degrees to the engine crank, giving an offset indicator diagram. The events are recorded when the velocity of the drum is near its maximum and are shown against crank-angle instead of stroke. , with an indicator shelter, == See also ==