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Joule
built 654 days ago
The only surviving part of Joule’s apparatus is the paddle wheel, now displayed in the Science Museum, London. Initially, Joule’s ideas about energy were not accepted, partly because they contradicted the commonly held belief at that time that heat was a fluid, and partly because of the extreme precise measurements that were necessary. His contemporaries could not believe that he was able to measure temperatures to an accuracy of 1/200th of a degree of Fahrenheit. Modern reworking of Joule’s paddle-wheel experiment has demonstrated the extraordinary difficulties he faced and that the accuracies required meant that he could not repeat his experiment in public. Among his first supporters were William Thomson (later Baron Kelvin) and Michael Faraday.
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While Joule was not the first scientist to suggest this principle, he was the first to demonstrate its validity. Although Thomson and a number of other scientists later made significant contributions to thermodynamics, Joule is correctly recognized as the chief founder of thermodynamics. He showed that ‘work can be converted into heat with a fixed ratio of one to the other, and that heat can be converted into work.’3
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By the beginning of 1841 Joule realized that there were limits on the power of the electromagnetic engine. While previously he had believed that they could reduce the cost of work ad finitum, he later noted that the resistance within the electromagnet and the battery slowed down the efficiency. As the force went down, the speed went up. The cost of materials, zinc and the necessary fluids, were ... much too high for the results obtained. The electromagnetic engine did not give the maximum effect for the minimum amount of current and work. Steam engines remained the most efficient way of producing mechanical power.
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Joule's study of the interrelation of heat and electrical energy may have stimulated his study of the relationship between heat and mechanical work. His approach was direct: he used the mechanical energy provided by falling weights to heat water by stirring it and made precise measurements of the heat produced and the energy lost by these weights. The results provided the first value of the mechanical equivalent of heat, corresponding to a temperature increase of 1F of 1 pound of water for the expenditure of 838 foot-pounds of work. The apparent simplicity of Joule's experiment is quite misleading, for enormous experimental skill, great care, and limitless patience were needed to get repeatable results; experts regard his work as demonstrating exceptional skill.
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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.
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In the 1850s Joule and William Thomson discovered that the temperature of a gas falls when it expands without doing work, which became known as the Joule-Thomson effect, and underlies the operation of common refrigeration and air-conditioning systems. A few years previously, in 1846, Joule discovered that an iron bar fractionally changed its length when magnetized. This phenomenon was later called magnetostriction and became important in the study of ultrasonics.
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