- Indicator diagram
of engines.
The diagram is simply a
chart of thepressure ofsteam in a cylinder against the steam's volume. In1796 , Southern developed the simple, but critical, technique to generate the diagram by fixing a board so as to move with thepiston , thereby tracing the "volume" axis, while apencil , attached to apressure gauge , moved at right angles to the piston, tracing "pressure".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 byDavies Gilbert as early as1792 and used byJonathan Hornblower inlitigation against Watt overpatent s 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 learning of it on seeing it used, probably by Watt's men, when he visitedRussia in1826 .In
1834 ,Émile Clapeyron used a diagram of pressure against volume to illustrate and elucidate theCarnot cycle , elevating it to a central position in the study ofthermodynamics .Bibliography
*cite book | author=Cardwell, D.S.L. | title=From Watt to Clausius: The Rise of Thermodynamics in the Early Industrial Age | location=Heinemann | publisher=London | year=1971 | id=ISBN 0-435-54150-1 | pages=pp. 79-81
*Pacey, A.J. & Fisher, S.J. (1967) "Daniel Bernoulli and the "vis viva" of compressed air", "British Journal for the History of Science" 3, 388-92
*"Handbook for Railway Steam Locomotive Enginemen", British Transport Commission (1957), p81
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