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ArrowJames Joule (1818-1889)

James Joule

James Prescott Joule experimented with engines, electricity and heat throughout his life. Joule’s findings resulted in his development of the mechanical theory of heat and Joule’s law, which quantitatively describes the rate at which heat energy is produced from electric energy by the resistance in a circuit. Initially many 19th century scientists were skeptical of Joule’s work, but his efforts proved fundamental to the modern understanding of thermodynamics. The SI unit of work, the joule, was named in honor of his significant scientific contributions.

A native of England, Joule was born on December 24, 1818, in Salford, Lancashire. His family was quite wealthy due to the success of the family brewery business. As a teenager, Joule began studying with the renowned chemist John Dalton at the University of Manchester, but a sudden change for the worse in Dalton’s health prematurely ended the tutelage. Despite their short time together, Dalton’s emphasis on quantitative experiments had a lasting effect on Joule’s scientific techniques. Joule continued his education under the guidance of John Davis, who co-founded the Royal Victoria Gallery for the Encouragement and Illustration of Practical Science.

Joule’s earliest interests in science concerned electromagnetic engines and their strength and weaknesses as compared to steam engines. His involvement with the family brewery helped direct his attention to this area because more efficient engines could substantially improve the profitability of the business. Joule’s first published paper was “Description of an electro-magnetic engine”, which appeared in 1838 in Annals of Electricity. Displeased by his discovery that steam engines appeared to be much better at work production than the electromagnetic engines available at the time, he embarked on an experimental journey in which he tried to improve the performance of the electromagnetic engine by varying the arrangement of iron in the device.

Joule’s dedication to experimentation eventually led to his formulation of the law that now bears his name in 1840. According to Joule’s law, the heat generated in an electric wire is proportional to the current squared multiplied by the resistance. The law is often written as P=I2R, where P equals power loss, I is the current in amperes, and R is the resistance given in ohms. Joule included the law within the treatise “On the Production of Heat by Voltaic Electricity”, which was published in abbreviated form in the Proceedings of the Royal Society. However, the youth of its author and the perception of others that he was merely a hobbyist prevented many members of the Society from immediately realizing the importance of Joule’s work.

The cool reception he received did not deter Joule, who forged ahead with additional experiments involving electricity and heat. Soon after, he began giving lectures at the Royal Victoria Gallery, and by 1843 began publicly speculating on the convertibility of energy. That same year he published his early calculations on the amount of work needed to generate a unit of heat that he termed the mechanical equivalent of heat. Over the course of his career, Joule continually improved his methods of determining the mechanical equivalent of heat and repeatedly refined the value of the unit. His final calculation for the unit was about 772 foot pound force, that is, according to Joule, the heat required to raise the temperature of one pound of water a single degree Fahrenheit was equivalent to (and convertible into) a mechanical force that could lift approximately 772 pounds one foot high.

In the late 1840s, Joule wed Amelia Grimes. Not long after, Joule’s contemporaries became somewhat more receptive of his work, largely due to his acquaintance and collaboration with William Thomson, later better known as Lord Kelvin. Thomson, who had witnessed one of Joule’s lectures to the British Association for the Advancement of Science, was very interested in his thermodynamic experiments. Yet he was also concerned with their apparent conflict with the then widely accepted caloric theory originally proposed by Antoine Lavoisier, which stated that heat existed as a type of fluid that flowed from warmer to cooler bodies. Correspondence between Joule and Thomson over the course of many years, as well as additional experiments (some suggested by Thomson) and refinements in technique, eventually led Thomson to support fully Joule’s mechanical theory of heat. Their association with one another also resulted in the discovery of the Joule-Thomson effect, a phenomenon in which a gas allowed to expand freely experiences a change in temperature. The Joule-Thomson effect laid the foundation for the emergence of the refrigeration business.

During the latter part of his career, Joule received much of the attention and accolades withheld from him in his earlier years. In 1850, following his publication of one of his most consistent calculations of the mechanical equivalent of heat, Joule became a fellow of the Royal Society, from which he received the Royal Medal in 1852 and the Copley Medal in 1870. Various institutions of higher learning, including Trinity College Dublin, University of Oxford and University of Edinburgh, also awarded him with honorary degrees, and the Royal Society of Arts bestowed to him the Albert Medal (1880).

Nevertheless, some controversy surrounded the priority of Joule’s thermodynamic work. A German physicist, Julius Robert von Mayer, claimed that he proposed the equivalency of heat and work at least a year before Joule announced his mechanical theory of heat. In order to help resolve the issue with as little enmity as possible, Joule eventually conceded Mayer’s claim for earlier development of the theory while asserting his own priority in experimentally confirming the theory. Despite the concession, however, most publications have since given primary recognition to Joule, who passed away at his home on October 11, 1889.


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