Pound (mass)

See also: U.S. customary units

(PD) Photo: National Institute of Standards and Technology (NIST)
The U.S. copy of the International Prototype Kilogram.

The pound or pound-mass (abbreviation: lb, lbm and sometimes #) is a measurement unit of mass used in the United States customary, Imperial, and other systems of measurement. The word pound is derived from the Latin phrase libra pondo for 'a pound weight' which is also the origin of the abbreviation lb. Many different definitions have been used, the most common today being the International avoirdupois pound of exactly 0.45359237 kilogram.

The kilogram is equal to the mass of the platinum-iridium International Prototype Kilogram (IPK) kept by the Bureau International des Poids et Mesures (BIPM) in Sèvres, near Paris. Copies of the IPK are maintained in several countries and periodically checked against the master copy kept by BIPM. The adjacent photograph is of the United States copy of the IPK, maintained by NIST in Gaithersburg, Virginia.

The common usage in the United States of the word pound to mean weight reflects the confusion between mass and weight resulting from the near uniformity of gravity on Earth. This accounts for the distinguishing terms pound-mass and pound-force. The pound-force is the force of 0.45359237 kilogram subject to a standard gravity of 9.80665 m/s², or approximately 4.448 Newtons.

History

The avoirdupois pound was defined by London merchants in 1303. By 1758, two standard artifact weights for the avoirdupois pound existed and, when measured in troy grains, they were found to be of 7,002 grains and 6,999 grains.

In England, the avoirdupois pound was defined as a unit of mass by the Weights and Measures Act of 1878, but having a different value than it does now, of approximately 0.453592338 kilogram. This was a measured quantity, with the independently maintained artifact still serving as the official standard for that value, and was referred to as the Imperial pound. That value and the term Imperial pound are now essentially obsolete. In 1883, it was determined that 0.4535924277 kg was more accurate. With the Weights and Measures Act 1889, England legally defined the avoirdupois pound as the rounded value of 0.45359243 kg.

In the United States, the avoirdupois pound had been officially defined in terms of the kilogram since the Mendenhall Order of 1893 specified it to be 2.20462 per kilogram. In 1894, following the 1889 determination in the England of the Imperial pound as being 0.45359243 kilograms, the United States changed the relationship to specify that there were 2.20462234 pounds per kilogram.

The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since July 1959, the International avoirdupois pound has been defined as exactly 0.45359237 kilogram.

Troy pound

The troy pound takes its name from the French market town of Troyes in France where English merchants traded in the early ninth century. The system of troy weights was used in England by apothecaries and jewellers.

The troy pound is no longer in general use. In Canada, Australia, the United Kingdom, and other places the troy pound is no longer a legal unit for trade. In the United Kingdom, the use of the troy pound was abolished in 1878. The Weight and Measures Act of 1985, which reaffirmed the metrication of the United Kingdom, also made the troy pound no longer legally recognized in the United Kingdom.