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British thermal unit (Btu)
1. The quantity of heat required to raise the temperature of one pound of water from 60 degress to 61 degrees Fahrenheit (3.9 C to 4.4C) at a constant pressure of one atmosphere.
2. The quantity of heat equal to 1/180 of the heat required to raise the temperature of one pound of water from 32 degrees to 212 degrees Fahrenheit at a constant pressure of one atmosphere, equal to approximately 1055.056 joules.

A joule is the International System unit of energy or work, equal to the work done when the application point of one newton force moves one meter in the direction of application. Symbol J [Named for the British physicist James Prescott Joule, 1818-1889, noted for his research on the mechanical equivalent of heat].

Word Entries at Get Words: “british thermal unit
British thermal unit, Btu
An imperial unit of heat, now replaced in the SI system by the joule (one British thermal unit is approximately 1,055 joules).

Burning one cubic foot of natural gas releases about 1,000 Btu of heat.

This entry is located in the following unit: Measurements and Mathematics Terms (page 3)
British Thermal Unit; BTU, Btu
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.
This entry is located in the following unit: Photovoltaic Conversion Efficiency Terms + (page 3)