James Prescott Joule (born December 24, 1818; died October 11, 1889) was an English physicist. He studied how heat works and found out how heat connects to mechanical work. This discovery helped create the law of conservation of energy, which later became part of the first law of thermodynamics. The unit of energy called the joule (J) is named after him.

He worked with Lord Kelvin to create a temperature scale known as the Kelvin scale. Joule also observed magnetostriction and discovered how electricity passing through a resistor produces heat. This principle is called Joule's first law. His experiments about energy transformations were first published in 1843.

Early years

James Joule was born in 1818 to Benjamin Joule, a wealthy brewer, and his wife, Alice Prescott, on New Bailey Street in Salford. As a young man, he was taught by the famous scientist John Dalton and influenced by chemist William Henry and engineers Peter Ewart and Eaton Hodgkinson. He became very interested in electricity. He and his brother conducted experiments, including giving electric shocks to themselves and the family's servants.

As an adult, Joule managed his father’s brewery. He studied science as a serious hobby. Around 1840, he began exploring whether the brewery’s steam engines could be replaced with the newly developed electric motor. His first scientific writings on this topic were published in William Sturgeon’s Annals of Electricity. Joule was also a member of the London Electrical Society, which Sturgeon and others founded.

Joule was motivated by both a desire to understand the cost-effectiveness of energy sources and a curiosity about science. He investigated which energy source was more efficient. In 1841, he discovered a rule now called Joule’s first law, which states that the heat produced by an electric current is proportional to the square of the current’s strength multiplied by the resistance it faces. He later found that burning a pound of coal in a steam engine was more economical than using a pound of zinc in an electric battery. To compare different energy sources, Joule measured their ability to lift a one-pound weight one foot high, a unit called the foot-pound.

Joule’s focus shifted from financial questions to understanding how much work could be obtained from energy sources. This led him to explore whether energy could be converted from one form to another. In 1843, he published results showing that the heat he had measured in 1841 came from the conductor itself, not from other parts of the equipment. This challenged the caloric theory, which claimed heat could not be created or destroyed. The caloric theory, introduced by Antoine Lavoisier in 1783, had been widely accepted. Sadi Carnot’s work on heat engines, starting in 1824, further supported this theory. Because Joule was not part of academia or the engineering profession, he faced challenges proving his ideas. Supporters of the caloric theory argued that the Peltier–Seebeck effect showed heat and electricity could be converted in a reversible process.

The mechanical equivalent of heat

Further experiments with his electric motor helped Joule calculate the mechanical equivalent of heat as 4.1868 joules per calorie of work needed to increase the temperature of one gram of water by one kelvin. He shared his findings at a meeting of the chemical section of the British Association for the Advancement of Science in Cork in August 1843, but no one responded.

Joule remained determined and worked to create a mechanical demonstration of how work can be converted into heat. By forcing water through a perforated cylinder, he measured the small amount of heat generated by the fluid’s viscosity. He found a mechanical equivalent of 770 foot-pounds force per British thermal unit (4,140 J/Cal). The agreement between values from electrical and mechanical methods to at least two significant digits strongly supported the idea that work can be converted into heat.

— J.P. Joule, August, 1843

Joule then tested a third method by measuring the heat produced during the compression of a gas. He calculated a mechanical equivalent of 798 foot-pounds force per British thermal unit (4,290 J/Cal). Although this experiment was easy for critics to challenge, Joule addressed these challenges through careful experiments. He presented his findings to the Royal Society on 20 June 1844, but the society refused to publish his paper. He later published his work in the Philosophical Magazine in 1845. In this paper, he clearly rejected the caloric theory proposed by Carnot and Émile Clapeyron, a rejection influenced by theological beliefs.

Joule used the language of vis viva (energy), possibly because Hodgkinson had shared a review of Ewart’s On the measure of moving force with the Literary and Philosophical Society in April 1844.

In June 1845, Joule presented his paper On the Mechanical Equivalent of Heat to the British Association meeting in Cambridge. This work described his most famous experiment, which used a falling weight to spin a paddle wheel inside an insulated barrel of water, causing the water’s temperature to rise. He calculated a mechanical equivalent of 819 foot-pounds force per British thermal unit (4,404 J/Cal). He also wrote a letter to the Philosophical Magazine in September 1845, describing his experiment.

In 1850, Joule published a more precise measurement of 772.692 foot-pounds force per British thermal unit (4,150 J/Cal), a value closer to modern estimates.

Reception and priority

Much of the early opposition to James Joule's research came from the need for very accurate measurements. He claimed he could measure temperatures to within 1/200 of a degree Fahrenheit (3 mK). Such accuracy was rare in physics at the time, but his critics may have overlooked his skills in brewing and his access to tools used in that field. He was also helped by John Benjamin Dancer, a maker of scientific instruments. Joule's experiments worked together with the theories of Rudolf Clausius, who is sometimes called a co-creator of the energy concept.

Joule proposed a kinetic theory of heat, believing it was a form of rotational, not translational, kinetic energy. This idea required a new way of thinking: if heat was movement of molecules, why didn’t that motion stop over time? His theory depended on the idea that molecular collisions were perfectly elastic. At the time, many scientists did not believe atoms or molecules existed, though research on their existence began in the 19th and early 20th centuries, starting with John Dalton and continuing with Ernest Rutherford. A collection of Dalton’s writings was published in 1893, 49 years after his death.

Although the caloric theory of heat may seem strange today, it had advantages at the time. Carnot’s theory of heat engines also relied on the caloric idea, but later work by Lord Kelvin showed that Carnot’s math worked without assuming heat was a fluid.

In Germany, Hermann Helmholtz learned about Joule’s work and the similar research of Julius Robert von Mayer from 1842. Though both men were ignored at first, Helmholtz’s 1847 paper on energy conservation credited them both.

In 1847, Joule presented his work at the British Association in Oxford, where George Gabriel Stokes, Michael Faraday, and William Thomson (later Lord Kelvin) attended. Stokes supported Joule’s ideas, Faraday was impressed but unsure, and Thomson was curious but skeptical.

Later that year, Thomson and Joule met in Chamonix. Joule married Amelia Grimes on August 18, and the couple went on a honeymoon. Despite their happiness, they planned an experiment to measure the temperature difference at the Cascade de Sallanches waterfall, though it proved too difficult.

Thomson believed Joule’s findings needed a theory to explain them but defended the Carnot–Clapeyron school. In his 1848 paper on absolute temperature, Thomson wrote that converting heat into mechanical energy was likely impossible, though a footnote hinted at doubts about the caloric theory, citing Joule’s work. Thomson did not send Joule a copy of his paper, but after reading it, Joule wrote to Thomson on October 6, stating he had shown heat could be converted into work and planned more experiments. Thomson replied on October 27, saying he would conduct his own experiments and hoped to reconcile their views. Over two years, Thomson grew less confident in Carnot’s theory and more convinced by Joule’s. In his 1851 paper, Thomson said the theory of heat’s power relied on two ideas: one from Joule and one from Carnot and Clausius.

After reading Thomson’s paper, Joule wrote with his thoughts and questions. This began a long, mostly written collaboration between the two scientists, with Joule doing experiments and Thomson analyzing results and suggesting new tests. Their work from 1852 to 1856 included discoveries like the Joule–Thomson effect, and their findings helped others accept Joule’s work and the kinetic theory of heat.

Kinetic theory

Kinetics is the study of motion. James Prescott Joule was a student of John Dalton and strongly believed in the atomic theory, even though many scientists of his time were unsure about it. He was also one of the few people who recognized the importance of John Herapath’s earlier work on the kinetic theory of gases. Joule was greatly influenced by Peter Ewart’s 1813 paper titled "On the measure of moving force."

Joule saw the connection between his discoveries and the kinetic theory of heat. His laboratory notes show that he believed heat was a type of rotational motion, not translational motion.

Joule looked for earlier ideas that supported his views in the work of Francis Bacon, Sir Isaac Newton, John Locke, Benjamin Thompson (Count Rumford), and Sir Humphry Davy. Although these connections were reasonable, Joule used Rumford’s publications to estimate the mechanical equivalent of heat as 1,034 foot-pound. Some modern writers have criticized this method because Rumford’s experiments were not organized or precise measurements. In one of his personal notes, Joule argued that Mayer’s measurement was just as inaccurate as Rumford’s, possibly hoping Mayer had not discovered his findings first.

Joule is sometimes credited with explaining the sunset green flash phenomenon in a letter to the Manchester Literary and Philosophical Society in 1869. However, he only described the last glimpse of the sun as bluish green and did not attempt to explain the phenomenon.

Published work

• Volumes I and II of "The Scientific Papers"
• Title page of volume I of "The Scientific Papers"
• Preface to volume I of "The Scientific Papers"
• Illustration from volume I of "The Scientific Papers"

Honours

James Prescott Joule passed away at his home in Sale and was buried in Brooklands Cemetery there. A pub in Sale, the town where he died, is named "The J. P. Joule" in his honor.

Joule received many awards and recognitions throughout his life, including:
– Becoming a Fellow of the Royal Society in 1850. He was awarded the Royal Medal in 1852 for his research on the mechanical equivalent of heat, and the Copley Medal in 1870 for his experiments on the dynamical theory of heat.
– Serving as President of the Manchester Literary and Philosophical Society in 1860 and as President of the British Association for the Advancement of Science in 1872 and 1887.
– Receiving Honorary Membership in the Institution of Engineers and Shipbuilders in Scotland in 1857.
– Earning honorary degrees from Trinity College, Dublin (LL.D. in 1857), the University of Oxford (DCL in 1860), and the University of Edinburgh (LL.D. in 1871).
– Being granted a civil list pension of £200 per year in 1878 for his contributions to science.
– Receiving the Albert Medal of the Royal Society of Arts in 1880 for his research on the relationship between heat, electricity, and mechanical work.

There is a memorial to Joule in the north choir aisle of Westminster Abbey, but he is not buried there, as some biographies incorrectly state. A statue of Joule, created by Alfred Gilbert, is displayed in Manchester Town Hall.

Family

James Joule married Amelia Grimes in 1847. Amelia passed away in 1854, seven years after their marriage. Together, they had three children: a son named Benjamin Arthur Joule, who lived from 1850 to 1922; a daughter named Alice Amelia, who lived from 1852 to 1899; and a second son named Joe, who was born in 1854 but died three weeks later.