James Prescott Joule

New Bailey Street

James Prescott Joule was born on Christmas Eve, 1818, on New Bailey Street in Salford. His father Benjamin was a wealthy brewer. His mother Alice came from the Prescott family, which is where the middle name came from. The Joules were comfortable. They had money, they had a business, and they had the kind of freedom that money buys: the freedom to let a clever son pursue whatever interested him.

What interested him was electricity. As a boy, he and his brother experimented with electric shocks, testing them on each other and, with the casual cruelty of the era, on the family's servants. It was curiosity, not kindness, but it was the beginning of something. He wanted to know what electricity could do. He wanted to measure it.

In 1834, the Joule family arranged for James and his brother to be tutored by John Dalton, the man who had given the world atomic theory and who was by then the most famous scientist in Manchester. Dalton was 67. He taught the boys natural philosophy, and the younger Joule listened. The tuition ended when Dalton suffered a stroke in 1837. Dalton died in 1844. By then, Joule was already doing work that would change how we understand the universe.

The Brewery and the Question

Joule was never a professional scientist. He never held a university position. He never drew a salary for research. He ran the family brewery in Salford, and science was what he did alongside it. That distinction matters, because it means every experiment he conducted, every measurement he made, every paper he published was the work of a man who had a brewery to manage and chose to spend his remaining hours asking questions about energy.

The first question was practical. Around 1840, electric motors were a new invention, and Joule wanted to know whether they could replace the steam engines that powered the brewery. Could electricity do the same work more efficiently? He started measuring. What he found was that electric motors were not yet competitive with steam, but the measurements themselves led him somewhere far more interesting.

In 1841, he published his first major finding, now known as Joule's Law: the heat produced by an electric current is proportional to the resistance of the conductor multiplied by the square of the current. It was a law about the relationship between electricity and heat, and it raised a deeper question. If electricity could produce heat, and mechanical work could produce electricity, then were heat and work somehow the same thing? Was energy just energy, regardless of what form it took?

The entire science of thermodynamics was hiding inside that question. And a brewer from Salford was the one who asked it.

The Paddle Wheel

The experiment that made Joule famous involved falling weights, a paddle wheel, and an insulated vessel of water. The weights fell on a cord, which turned the paddle wheel inside the vessel. The paddle wheel churned the water. The water got warmer. Joule measured how much warmer, with a precision that bordered on obsessive: he claimed accuracy to one two-hundredth of a degree Fahrenheit.

The point was to establish a number: how much mechanical work, measured in foot-pounds, was needed to raise the temperature of one pound of water by one degree Fahrenheit? If you could pin that down, you could prove that heat and work were interchangeable, that energy could be converted from one form to another but never created or destroyed.

He presented the results at the British Association meeting in Cambridge in June 1845. The scientific establishment was not immediately convinced. The dominant theory at the time, the caloric theory, held that heat was a fluid substance that flowed between objects. Joule was saying that heat was not a substance at all, that it was a form of motion, and that he had the numbers to prove it.

He refined the experiment over years. His 1850 measurement, 772.692 foot-pounds per British thermal unit, was extraordinarily close to the modern accepted value. The precision was remarkable. The implication was revolutionary. Energy is conserved. It changes form, but it does not disappear. That principle, the first law of thermodynamics, underpins every engine, every power station, every machine, and every physical process in the universe. A brewer from Salford measured it with a paddle wheel and a thermometer.

The Waterfall and the Scotsman

In 1847, Joule presented his findings at the British Association meeting in Oxford. Among the audience was a young Scottish physicist called William Thomson, later Lord Kelvin. Thomson was sceptical. He believed in caloric theory and was not ready to accept that a brewer with a paddle wheel had overturned it.

A few months later, Joule married Amelia Grimes, on 18 August 1847, and the couple honeymooned in the French Alps. By chance, he bumped into Thomson in Chamonix. The story goes that Joule had brought a thermometer on his honeymoon, because of course he had, and that the two men arranged to meet at the Cascade de Sallanches to test whether the water at the bottom of a waterfall was measurably warmer than the water at the top. If energy was conserved, the mechanical energy of the falling water should convert to heat on impact.

The experiment was attempted and it didn't work. The conditions were too uncontrolled, the temperature differences too small to measure reliably against the noise of a real waterfall. But the meeting mattered. Thomson came away convinced that Joule was onto something, and the two men began a collaboration that lasted from 1852 to 1856, mostly conducted by correspondence. Joule did the experiments. Thomson analysed the results and suggested new ones.

Together they discovered the Joule-Thomson effect: the temperature change of a real gas when it expands through a restriction without exchanging heat with its surroundings. It is the principle behind liquefying gases, and it turns up in the expansion valve of every fridge in the country. A brewer from Salford met a Scotsman in the Alps, they started arguing about waterfalls, and the result ended up in your kitchen.

The Unit

Joule's wife Amelia died in 1854, seven years after their marriage. They had three children; the youngest, a boy, was born and died at three weeks old the same year Amelia did. The brewery was sold around the same time. The comfortable life that had funded his science was gone, and what replaced it was quieter and harder. He continued his research, but the resources were not what they had been.

In 1878, the government awarded him a Civil List pension of £200 a year, in recognition of his services to science. It was not wealth. It was acknowledgement. The man who had established that energy could be neither created nor destroyed had spent most of his own.

He spent his final years in Sale, in what was then Cheshire and is now Greater Manchester. He died on 11 October 1889, aged 70. He was buried at Brooklands Cemetery in Sale. A memorial tablet was placed in the north choir aisle of Westminster Abbey, though he is not buried there.

Six weeks before he died, on 31 August 1889, the International Electrical Congress formally adopted the joule as a unit of energy. It is now the standard SI unit. Every time anyone measures energy, anywhere in the world, they use a unit named after a brewer from Salford who wanted to know whether an electric motor could replace a steam engine.

His statue stands in Manchester Town Hall, alongside John Dalton's. The teacher and the pupil, both in the same building, both from the same city, both responsible for ideas that changed how we understand the physical world. Dalton gave us the atom. Joule gave us the conservation of energy. Between them, they account for a fair chunk of modern science, and both of them did it within walking distance of the Lit & Phil on George Street.

A brewer's son from New Bailey Street who measured everything. That's a Manchester legend.