1889 as an abbreviation of British Thermal Unit (1862), a commercial unit of electrical energy (the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit); the French Thermal Unit is the amount of heat required to raise 1 kilogram of water 1 degree centigrade. Also from 1889 as an abbreviation of Board of Trade Unit, in electicity "1,000 watt hours."
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