English physicist who was initially interested in the efficiency of electric motors. He discovered the heat dissipated by a resistor is given by Q = I²Rt (where I is the current, R the resistance, and t the time ), a result now known as Joule's law.

Motivated by theological beliefs, Joule began attempting to demonstrate the unity of forces in nature. He determined the mechanical equivalent of heat by measuring change in temperature produced by the friction of a paddlewheel attached to a falling weight in the 1840s. He made a series of measurements and found that, on average, a weight of 772 pounds falling through a distance of one foot would raise the temperature of one pound of water by 1° F. This corresponds to (772 ft lbs)(1.356 J/ft lb) = 59 453.6 Calories, or 1 cal = 4.15 Joules, in close agreement with the current accepted value of 1 cal = 4.184 J. Joule was not the first person to establish the mechanical equivalence of heat, but it was his demonstration that eventually came to be accepted. He did not claim, however, to have formulated a general Law of Conservation of Energy. Nevertheless, his experiments were certainly fundamental in bringing that formulation about. In addition, Joule's experiments showed that heat is produced by motion, contradicting the caloric theory.