Pound mass: Encyclopedia II - Pound mass - Measurement systems

Pound mass - Measurement systems

In the Imperial system (often referred to as the pound-inch system, or the English system in the United States) there are two basic pounds defined, and also an obsolete definition of one variant of the pound.

Pound mass - Avoirdupois or international

Main article: avoirdupois

The avoirdupois pound was invented by London merchants in 1303.

The pound (avoirdupois) or international pound, abbreviation "lb" or sometimes # in the United States, is the mass unit defined as exactly 0.45359237 kilogram (or 453.59237 grams). This definition has been in effect since a 1959 agreement among the national standards laboratories of the United States, Canada, the United Kingdom, South Africa, Australia, and New Zealand. [1] It is part of the avoirdupois system of mass units.

In the United States, the pound has been officially defined as a unit of mass and defined in relation to the kilogram since 1893, but its value in relation to the kilogram was altered slightly in 1894, and again to its current value in 1959 (which only differs from the 1894 definition by approximately one part in 10 million).

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 very slightly different value (in relation to the kilogram) than it does now, of approximately 0.453592338 kg. (This was a measured quantity, with the independently maintained artifact still serving as the official standard 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.

There are 16 ounces in a pound (avoirdupois), an ounce being equal to 28.349523125 grams. This pound is equal to exactly 7000 grains, where a grain is exactly 0.06479891 gram. This relationship between avoirdupois pounds and troy grains has held true since the avoirdupois pound was redefined in terms of the troy units in the reign of Henry VIII, abandoning independent standards which had been measured as about 7002 grains troy. Since then, the grain has often been considered as a part of the avoirdupois system as well, even though it does not fit very well in that system.

Pound mass - Troy

Main article: Troy weight

A troy pound, named for the French market town of Troyes in France where English merchants traded at least as early as the time of Charlemagne (early ninth century), was the mass unit used in England by apothecaries and jewelers. In Canada, Australia, the United Kingdom, and other places the unit is no longer a legal unit for trade, but its ounce subdivision enjoys a specific exemption from their metrication laws. The troy pound is a unit of mass equalling exactly 0.3732417216 kilogram (or 373.2417216 grams). There are 12 troy ounces in a troy pound, a troy ounce being equal to 31.1034768 grams. A troy pound is equal to exactly 5760 grains, making a troy pound equal to exactly 144/175 pound avoirdupois.

The troy pound is now used only for measurements of precious metals such as gold, silver, and platinum, and sometimes gems such as opals. Most weight measurements of precious metals using pounds and ounces use troy pounds and ounces, even though it is not always explicitly stated that this is the case. Some notable exceptions are Encyclopædia Britannica (a U.S. encyclopedia for about a century now) which uses either avoirdupois pounds or troy ounces, likely never both in the same article (which would make an awkward system with 14 7/12 ounces to a pound), and King Tut's sarcophagus lid, which is often stated to have been 242 or 243 pounds (avoirdupois (110 kg); when it is, much less commonly, stated as 296 pounds (110 kg), then the pounds are troy).

A pennyweight was literally the weight of a penny, as adopted by King Henry II (1154–1189). This was a sterling silver penny weighing 1/240 of a troy pound (1.55517384 g).

Pound mass - Metric

Main article: kilogram

In many countries that use the SI or metric system, the pound (or its translation, for example, the German Pfund, the French livre, the Dutch pond, the Spanish and Portuguese libras, or the Chinese jin) is used as an informal term for half of a kilogram, therefore for this case the pound is 500 grams. In many cases, this was an official redefinition back in the 19th century, but its use is generally no longer officially sanctioned. These replaced hundreds of older pounds, for example, one of around 459 to 460 grams in Spain, Portugal, and Latin America; 498.1 g in Norway; and several different ones in what is now Germany. In the case of the Dutch pond, this was officially redefined as 1 kg, with an ounce of 100 g; the former has fallen out of use, and if the pound is used today it is likely the 500 g variety, but the 100 g ounce remains in limited use.

Despite the use of the pound term persisting as a slang term no scale or measuring device exists in metric countries that measures in pounds. All scales are in grams and a pound must be determined by weighing the product in grams. Thus a pound is weighed out as 500 g.

Adapted from the Wikipedia article "Measurement systems", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki