Short answers, pulled from the story.
Joule discovered that heat and mechanical work are interconvertible, establishing the mechanical equivalent of heat. His experiments showed that a measurable, consistent amount of mechanical work always produces an equivalent amount of heat, laying the groundwork for the law of conservation of energy and the first law of thermodynamics.
Joule's first law states that the heat produced by an electric current is proportional to the square of the current's intensity multiplied by the resistance of the conductor. Joule discovered this relationship in 1841 while investigating electric motors at his family's brewery.
In the paddle wheel experiment, reported at the British Association meeting in Cambridge in June 1845, Joule used a falling weight to spin a paddle wheel inside an insulated barrel of water. The work done by gravity raised the water's temperature, allowing him to calculate the mechanical equivalent of heat as 819 ftlbf/BTU.
Joule's work challenged the caloric theory, the dominant model since Lavoisier's 1783 formulation, which held that heat was an indestructible fluid. His experiments required extremely precise temperature measurements that contemporaries doubted, and he worked entirely outside academia and the engineering profession. The Royal Society rejected his 1844 paper for publication.
Joule and William Thomson (later Lord Kelvin) began corresponding in October 1848 and formally collaborated from 1852 to 1856. Joule conducted experiments while Thomson analysed results and proposed further tests. Their work produced the Joule-Thomson effect and brought broad acceptance of Joule's measurements and the kinetic theory of heat.
Joule was elected a Fellow of the Royal Society in 1850 and received the Royal Medal in 1852 and the Copley Medal in 1870. He served twice as President of the British Association for the Advancement of Science, in 1872 and 1887, and received a civil list pension of two hundred pounds per annum in 1878. The Albert Medal of the Royal Society of Arts followed in 1880.