The pound is the base unit of the United States customary system for measuring weight and mass. The pound avoirdupois is used in most applications; a different pound (the troy pound) was used for precious metals, and another, equal to the troy pound (the apothecaries pound) but both are obsolete today.
The official definition, since 1866, in the United States, has been in terms of the kilogram, which would strictly speaking define the pound as a unit of mass. In fact, it is customary to define the pound as a unit of weight, equal to the force with which the Earth attracts a one-pound mass (at some standard point on the Earth's surface). (See also Mass versus weight.) The CGPM (also known as the "General Conference on Weights and Measures") fixed the value of standard gravity at precisely 9.80665 m/s2 (≈32.17405 ft/s2) so that disciplines such as metrology would have a standard value for converting units of defined mass into defined forces and pressures.
History of the pound definitionEdit
Until 1866, the pound was defined as the weight of a particular prototype pound, intended to match a British original.
In 1866, an Act of Congress defined the pound as 1 kg = 0.4535970244035199 kg.
In 1895 it was revised to a figure, then thought to relate more accurately the value of the US standards to their metric equivalents, of 0.45359224277 kg.
The same international agreement that unified length units in the United States and Commonwealth of Nations defined the pound as 0.45359237 kg, coming into force in the United States on July 1, 1959 and in the United Kingdom in 1963. This value can be designated the International avoirdupois pound.