Mechanical equivalent of heat

Joule presents his first quantified mechanical equivalent of heat at Cork

Joule carried a numerical conversion between work and heat to Cork, and later remembered the room's response chiefly as silence.

21 August 1843, at the British Association

James Prescott Joule read "On the Calorific Effects of Magneto-Electricity, and on the Mechanical Value of Heat" to the chemical section of the British Association meeting at Cork, Ireland, on 21 August 1843.

He reported experiments made at Broom Hill near Manchester, in which a small electromagnet revolved in water between a stationary magnet's poles while he measured the generated current and the water's temperature change.

That original revolving-electromagnet calorimeter survives in the Science Museum Group collection.

838 foot-pounds

He concluded quantitatively that mechanical work could be converted into heat, announcing an equivalent of 838 foot-pounds for the heat needed to warm one pound of water by one degree Fahrenheit.

A separate estimate, obtained by forcing water through narrow tubes, gave about 770 foot-pounds per pound-degree.

Neither figure was his final or most accurate value; his title also spoke of the "mechanical value of heat," not the later-standardised "mechanical equivalent."

The famous paddle wheel came later

The falling-weight paddle-wheel determinations that usually illustrate this work belong to later years. The 1843 apparatus was the electromagnet turning in water.

Joule's own memoir recalled that the Cork presentation attracted little general attention; John Apjohn, the section president, was among the few he remembered taking an interest. The full paper appeared in the Philosophical Magazine in instalments later that year.

He was not working alone in the field either: Julius Robert Mayer had independently published related energy-conservation reasoning in 1842.

Sources

Researched 24 Aug 2026 6 sources not yet audited Date corrected

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

Date corrected. The research places this at 1843-08-21.

What the sources leave uncertain

The experiments preceded August 21; that date is the presentation of the result, not the instant at which Joule first calculated it.
“Mechanical equivalent of heat” is a later-standardized description. Joule’s 1843 title used “mechanical value of heat.”