page_title: Imperial and US customary measurement systems
section_title: Energy, power, and temperature
hierarchical_section_title: Imperial and US customary measurement systems / Energy, power, and temperature
caption_attribution_description: English: Watt's beam engine Old Bess, dating from 1777 and built by Boulton and Watt
context_page_description: The Imperial and US customary measurement systems are both derived from an earlier English system of measurement which in turn can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure.
  The US Customary system of units was developed and used in the United States after the American Revolution, based on a subset of the English units used in the Thirteen Colonies. The Imperial system of units was developed and used in the United Kingdom and its empire beginning in 1826. US Customary units are the predominant system of units in the United States and the metric system has, to varying degrees, replaced the imperial system in the countries that used it.
  Most of the units of measure have been adapted in one way or another since the Norman Conquest. The units of linear measure have changed the least – the yard and the chain were measures derived in England. The foot used by craftsman supplanted the longer foot used in agriculture. The agricultural foot was reduced to ​¹⁰⁄₁₁ of its former size, causing the rod, pole or perch to become ​16 ¹⁄₂ agricultural feet.
context_section_description: Imperial and US customary units have long been used in many branches of engineering. 
  Two of the earliest such units of measure to come into use were the horsepower and the degree Fahrenheit. The horsepower was defined by James Watt in 1782 as the power required to raise 33,000 pounds of water through a height of one foot in one minute and the degree Fahrenheit was first defined by Daniel Fahrenheit in about 1713 as being a temperature scale having its lower calibration point (0 °F) at temperature where a supersaturated salt/ice mixture froze and its upper calibration point at body temperature (96 °F). In 1777 the Royal Society, under the chairmanship of Henry Cavendish, proposed the definition of the Fahrenheit scale be modified such that the temperature corresponding to the melting point of ice be 32 °F and the boiling point of water under standard atmospheric conditions be 212 °F.
  The British thermal unit (Btu) is defined as the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It was in use before 1859 as a unit of heat based on imperial units rather than the metric units used by the French—Clément-Desormes having defined the calorie in terms of the kilogram and degrees centigrade in 1824.
  In 1873 a committee of the British Association for the Advancement of Science under the chairmanship of William Thomson (Lord Kelvin) introduced the concept of coherence into units of measure and proposed the names dyne and erg as the units of force and work in the CGS system of units. Two years later James Thomson, older brother of William Thomson, introduced the term poundal as a coherent unit of force in the Foot–pound–second system (FPS) of measurement. The FPS unit of work is the foot-poundal.
  Other systems for the measurement of dynamic quantities that used imperial and US customary units are the British Gravitational System (BG) proposed by Arthur Mason Worthington and the English Engineering System (EE). Both systems depend on the gravitational acceleration, and use the pound-force as the unit of force but use different approaches when applying Newton's laws of motion. In the BG system, force, rather than mass has a base unit while the slug is a derived unit of inertia (rather than mass). On the other hand, the EE system uses a different approach and introduces the acceleration due to gravity (g) into its equations.  Both these approaches led to slight variations in the meaning of the pound-force (and also of the kilogram-force) in different parts of the world.  Various countries published standard values that should be used for g, and in 1901 the CGPM published a standard value for g that should be used in the "International Service of Weights and Measures", namely 32.174049 ft/s² (9.80665 m/s²), which is equal to the value of g at 45° latitude.
  Newton's second law in these systems becomes:
  BG: Force (lbf) = inertia (slugs) × acceleration (ft/s²)
  EE: Force (lbf) = mass (lb) × acceleration (ft/s²) ÷ g
  AE: Force (poundals) = mass (lb) × acceleration (ft/s²)
  AE is ignored in many engineering courses and textbooks while some, such as Darby only uses EE (alongside SI), having described the BG and EE systems as "archaic".
caption: Old Bess, a 30 HP beam engine built by James Watt, now preserved in the Science Museum, London.  Watt defined the term "horsepower"
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