Heat and its relationship to mechanical work

Heat and its relationship to mechanical Work in the brewery (Joule).

Brewer, son and grandson of a brewer in Manchester, James Prescott Joule (1818-1889) conducted experiments in the family brewery that were decisive for modern science. His research and the way he conducted it were so intimately intertwined with theoretical tools, brewing techniques and industrial equipment that one wonders whether Joule's brilliant work could have been carried out anywhere else but in the shade of a brewing vat.

In 1840 he formulated the relationship between the heat released (Q) by an electric current of intensity I conducted in a resistance wire R for a time interval t (Q = I² R t). He determines this relationship experimentally, taking hundreds of measurements with calorimeters, thermometers, galvanometers, dynamo and conductors of various qualities. The dynamo that Hippolyte Pixii and the dynamo has just invented in 1831 produces a current thanks to a crank turned by the human arm. It converts mechanical work into electric current. This brings Joule back to the brewery where he plans to replace his steam engine by an electric motor, bringing about the reverse conversion done by the dynamo: electric current ⇒ mechanical work. This electric motor has to turn the blades of the mash tun to mix the hot water and the crushed malt[1].

William Thomson (later Lord Kelvin) had already discovered that the temperature of the air drops when it can spread through a porous membrane. With the help of his friend Joule and his brewery, they discovered that the difference in temperature is four times greater with the carbon dioxide that is collected pure and at will above the fermentation vats (Gall 2004[2]). The Joule-Thomson effect, a manifestation of the heat ⇔ energy equivalence, will be used as a basis for refrigeration appliances, via the liquefaction of gases. In return, the introduction of cold in breweries will give birth at the end of the century to beers fermented with bottom-fermenting yeasts that can be sold and kept throughout the summer, a major industrial revolution.

Without ceasing his own research, Joule runs his brewery since 1844, one of the largest in the city of Manchester, which is also one of the centers of the Industrial Revolution, with Birmingham and Liverpool. The conversion of mechanical work into heat and vice versa is at the very heart of his professional universe. A brewery is a large thermodynamic device. In Joule's time, all the brewing implements are visible inside the buildings: vats, boilers, fireplaces, tanks, pipes, pulleys, gears, etc. From start to finish, the production of beer converts energy: a steam engine make rotating the mechanical mixer inside the mash tun, the brewing material is heated, cooled, or pumped up, the fluids run down floor by floor to the tanks in the cellar, vapors are rising from boilers or steam engines. What a playground for a thermodynamics scientist !

Joule imagines an experiment to measure the heat created by a mechanical work, and to discover a relationship between them he could write down with a mathematical equation. On the same time, Joule and most of his scholarly contemporaries suspect that this equivalence hide a fundamental reality, what we today call energy, an informal concept of Joule's time. His experimental device resembles very closely a miniaturised brewery mash tun[3].

Scheme of his experience published by Joule in Philosophical Transaction 1850.  In the center, the closed container made of brass, provided with small blades rotating around an axis, and filled with water. It is placed on an insulating support to limit heat loss. Above, the detachable pulley mechanism that lets you raise up quickly the two 13 kg weights with the crank.

A small brass container is filled with a defined volume of water. It is provided with a central vertical axis and equipped with a small wheel, with brass blades, and a lid with two holes. A rod passes through the central orifice, that fits on the axis of the blades. The second hole, slightly eccentric, lets a thermometer plunge into the water. The rod is connected by two cords wrapped on it and tied to a couple of 13 kg weights, whose fall causes the rotation of the blades of the wheel, via a mechanism of symmetrical pulleys. The experiment runs like that : let the two weights falling down 20 times from a height of 63 inches (1.60 m) in a 35 minutes span, and measure the temperature of the water at the beginning and the end of the complete experiment. The mechanical work (falling weights) is converted into heat captured by the water. Joule takes into account all the parameters, including the temperature of the room, a cool and insulated cellar to avoid temperature fluctuations during the experiment. All the expertises of a brewer, the brewing equipments, the thermometer, a sensitive balance[4], the mounted pulleys, and the vats with blades, everything belonging to the brewery world is gathered in the Joule's experiment. Many times repeated with his brewer assistant, Joule finally reports his results in June 1849 to his respective Learned Society (Joule's file[5]).

Joule reads with the naked eye on the graduations of his thermometer a temperature rise of 0.5°F. He converts the T work into feet/pounds. The most refined value of the ratio T/Δθ = 772.692, later 772.55, given in the units used by Joule. He wants to show that this value is independent of the nature of the work (mechanical, electrical, chemical).

The experimental and material features of Joule's experiments carried out in a cellar of his brewery were concealed and then forgotten. Only the theoretical benefits for pure science mattered. Julius von Mayer (1814-1878) achieved by calculation alone the same results as Joule did as early as 1842, postulating the general principle of energy conservation. But a physical theory without experimental proof is worthless.

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