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A non-SI unit of energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure of one atmosphere. International Table British thermal unit is based on the definition of the International Table calorie (exactly 4.1868 J) at the Fifth International Conference on the Properties of Steam (London, July 1956), and is equal to 1.05505585262 kJ.
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