James Joule was perhaps the greatest British experimental physicist of the generation that immediately followed Faraday. He was born in Salford to a wealthy brewing family and was educated, with his brother, at home by private tutors. Joule was mostly self-taught in science and many of his early electrical experiments were done at his own expense in the laboratory of his parents' brewery. During these early experiments he invented an electromagnetic engine and he developed his prodigious practical skills, in particular, the ability to make extremely accurate measurements. Many of his later discoveries can be attributed to his extraordinary attention to detail. Joule believed that heat was derived from work, whether it was chemical, electrical or mechanical and in 1840 he showed that the heat produced in a wire by an electrical current was proportional to the resistance of the wire and the square of the current (Joule's law). He went on to determine the amount of work required to produce a unit of heat (i.e. the mechanical equivalent of heat) and in 1845 he demonstrated the conversion of work into heat by forcing paddles through a tank of water. This experiment was to be later viewed as a classic, although Joule had also used similar methods such as forcing water though capillary tubes and compressing air to show the same effect. These experiments impressed the young William Thomson and between 1852 and 1862 they worked together on 'porous plug' experiments that led to the discovery of the Joule-Thomson effect, the cooling of a gas as it expands freely. This discovery eventually led to the development of the refrigerator and to the science of cryogenics (low-temperature physics). The importance of Joule's work was recognized when his name was used for the SI unit of energy.
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