Avoirdupois pound
The avoirdQupois pound was invented by London merchants in 1303. This invention was nearly four hundred years before Sir Isaac Newton defined the word mass to be a property of matter. Originally it was based on independent standards. During the reign of Henry VIII of England, the avoirdupois pound was redefined as 7,000 troy grains.[citation needed] Since then, the grain has often been considered as a part of the avoirdupois system. By 1758, two standard 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.[4]
In the United Kingdom, the avoirdupois pound was defined as a unit of mass by the Weights and Measures Act of 1878, but having a different value (in relation to the kilogram) than it does now, of approximately 0.453592338 kg, which would make the kilogram approximately equal to 2.20462278 pounds. (This was a measured quantity, with the independently maintained artifact still serving as the official standard[citation needed] for this pound.) This old value is sometimes called the imperial pound, and this definition and terminology are obsolete unless referring to the slightly different 1878 definition. In 1883 it was determined that 0.4535924277 kg was a better approximation. With the Weights and measures Act 1889 the United Kingdom legally defined the avoirdupois pound as the rounded value of 0.45359243 kg.[5]
In the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since the Mendenhall Order of 1893. In 1893, the relationship was specified to be 2.20462 pounds per kilogram. In 1894, the relationship was specified to be 2.20462234 pounds per kilogram. This change followed a determination of the British pound.[5]
According to a 1959 NIST publication, the international pound differed from the United States 1894 pound by approximately one part in 10 million.[6] The difference is so insignificant that it can be ignored for almost all practical purposes.[7]