James Prescott Joule ( / dʒ uː l / ; 24 December 1818 – 11 October 1889) was an English physicist. He studied how heat works and found how it connects to mechanical work. This discovery helped create the law of conservation of energy, which later led to the first law of thermodynamics. The SI unit of energy, the joule (J), is named after him.

He worked with Lord Kelvin to create a temperature scale that measures heat absolutely. This scale is now known as the Kelvin scale. Joule also observed magnetostriction and discovered the connection between the current in a resistor and the heat produced. This discovery is called Joule's first law. His experiments about energy changes were first published in 1843.

Early years

James Joule was born in 1818 in Salford, England, to Benjamin Joule, a wealthy brewer, and Alice Prescott. He was taught by the famous scientist John Dalton and influenced by chemist William Henry and engineers Peter Ewart and Eaton Hodgkinson. He was interested in electricity and conducted experiments with his brother, testing the effects of electricity on people, including family servants.

As an adult, Joule worked at the family brewery. His scientific work was a hobby he pursued seriously. Around 1840, he began studying whether electric motors could replace the brewery’s steam engines. He wrote his first scientific articles about this topic for Annals of Electricity, a journal edited by William Sturgeon. Joule was also a member of the London Electrical Society, which Sturgeon helped create.

Joule wanted to understand which power source was more efficient, driven by both his curiosity and a desire to measure economic value. In 1841, he discovered a rule now called Joule’s first law, which states that heat produced by an electric current is proportional to the square of the current’s strength and the resistance it faces. He later found that burning coal in a steam engine was more economical than using zinc in an electric battery. He measured energy outputs using a standard unit: the ability to lift a one-pound weight one foot, called a foot-pound.

Joule’s focus shifted from financial questions to understanding how much work could be obtained from energy sources. In 1843, he showed that the heat he had studied 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. Caloric theory, introduced by Antoine Lavoisier in 1783, had been widely accepted, especially after Sadi Carnot’s work on heat engines in 1824. Supporters of caloric theory argued that the Peltier–Seebeck effect proved 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 raise 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 continued his research to show how mechanical work can be converted into heat. By forcing water through a perforated cylinder, he measured the small amount of heat created by the water’s movement. He found a mechanical equivalent of 770 foot-pounds force per British thermal unit (4,140 J/Cal). The agreement between his electrical and mechanical results, accurate to at least two significant digits, supported his conclusion that work can be converted into heat.

— J.P. Joule, August, 1843

Joule then tested a third method by measuring the heat produced when compressing 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 their concerns through careful testing. He presented his findings to the Royal Society on June 20, 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 decision influenced in part by his beliefs.

Joule used the term 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 at a 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, raising the water’s temperature. He calculated a mechanical equivalent of 819 foot-pounds force per British thermal unit (4,404 J/Cal). He later wrote about this experiment in a letter published in the Philosophical Magazine in September 1845.

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 resistance to Joule's work came from his reliance on 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 experience in brewing and his access to tools used in that field. He was also supported by John Benjamin Dancer, a skilled maker of scientific instruments. Joule's experiments worked together with the theoretical ideas of Rudolf Clausius, who some people believe helped develop the concept of energy.

Joule proposed a kinetic theory of heat, believing it was a form of rotational, not translational, kinetic energy. This idea required a major shift in thinking: if heat was a type of molecular motion, why didn’t the motion of molecules slow down over time? Joule’s theory required the belief that molecular collisions were perfectly elastic. At the time, the existence of atoms and molecules was not widely accepted, though research into their discovery was ongoing from the 19th century through the early 20th century, 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 clear advantages at the time. Carnot’s successful theory of heat engines also relied on the caloric theory, but later, Lord Kelvin proved that Carnot’s math worked without assuming heat was a fluid.

In Germany, Hermann Helmholtz learned about Joule’s work and the similar 1842 research of Julius Robert von Mayer. Though both men had been ignored after publishing their findings, Helmholtz’s 1847 statement on energy conservation credited them both.

In 1847, Joule presented his work at the British Association in Oxford. Attendees included George Gabriel Stokes, Michael Faraday, and William Thomson (later Lord Kelvin), who had just become a professor at the University of Glasgow. Stokes supported Joule’s ideas, Faraday was impressed but unsure, and Thomson was interested but skeptical.

Later that year, Thomson and Joule met in Chamonix. Around the same time, Joule married Amelia Grimes and went on a honeymoon. Despite their happiness, they planned an experiment to measure the temperature difference at the Cascade de Sallanches waterfall, though the plan was later found to be impractical.

Thomson believed Joule’s results needed a theoretical explanation but defended the Carnot–Clapeyron theory. In his 1848 paper on absolute temperature, Thomson wrote that converting heat into mechanical work was likely impossible, though a footnote hinted at doubts about the caloric theory, mentioning Joule’s findings. Thomson did not send Joule a copy of his paper, but when Joule read it, he 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 more skeptical of Carnot’s theory and accepted Joule’s ideas. In his 1851 paper, Thomson stated that 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 sent him his thoughts and questions. This began a productive, mostly written collaboration between them. Joule performed experiments, while Thomson analyzed results and suggested further tests. Their work from 1852 to 1856 included discoveries like the Joule–Thomson effect. These findings helped gain wider acceptance for Joule’s work and the kinetic theory of heat.

Kinetic theory

Kinetics is the study of how things move. James Joule was a student of John Dalton and strongly believed in the atomic theory, even though many scientists at the 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. He was deeply 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 notebooks 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. In one of his personal notes, Joule claimed that Mayer's measurement was just as accurate as Rumford's, possibly hoping Mayer had not discovered the idea 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"
• The title page of Volume I of "The Scientific Papers"
• The preface of Volume I of "The Scientific Papers"
• A figure from Volume I of "The Scientific Papers"

Honours

James Prescott Joule died at his home in Sale and is 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 honors and commendations, including:

• Fellow of the Royal Society (1850), Royal Medal (1852), "For his paper on the mechanical equivalent of heat, printed in the Philosophical Transactions for 1850"
• Copley Medal (1870), "For his experimental researches on the dynamical theory of heat"
• President of the Manchester Literary and Philosophical Society (1860)
• President of the British Association for the Advancement of Science (1872, 1887)
• Honorary Membership of the Institution of Engineers and Shipbuilders in Scotland (1857)
• Honorary degrees: LL.D., Trinity College, Dublin (1857); DCL, University of Oxford (1860); LL.D., University of Edinburgh (1871)
• A civil list pension of £200 per year in 1878 for services to science
• Albert Medal of the Royal Society of Arts (1880), "for having established, after most laborious research, the true relation between heat, electricity and mechanical work, thus affording to the engineer a sure guide in the application of science to industrial pursuits"

There is a memorial to Joule in the north choir aisle of Westminster Abbey, though 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 was born in 1850 and died in 1922; a daughter named Alice Amelia, who was born in 1852 and died in 1899; and a second son named Joe, who was born in 1854 but died three weeks later.