This month marks the 150th anniversary of the lecture in which English physicist James Prescott Joule announced his profound discovery of how heat given off by one source is absorbed by another and never lost from a system.
Through a series of experiments, which included measuring temperature changes generated by the friction of a paddlewheel moving through water, Joule discovered the general law of energy conservation, at about the same time that German physicists Hermann von Helmholtz and Julius von Mayer and English physicist Lord Kelvin (William Thompson) reached similar conclusions. Joule also first determined the coefficient of equivalence--the relationship between heat and mechanical energy--and began measuring heat in units now called joules. In addition, he found that the temperature of a gas drops if it expands without doing work, the basis for modern refrigeration.
After his public lecture, delivered in the reading room of St. Ann's Church in Manchester, England, Joule published an essay, "On Matter, Living Force and Heat," in the local newspaper. He presented a report on his findings to the British Association meeting at Oxford the following month. Joule died in 1889 at the age of 71.