Monday, September 29, 2008

More Weight

This is not about Giles Corey, but rather in case your child asks, as mine did this morning, here are some things you might want to know about the unit "pound"and its seemingly disconnected abbreviation.

pound (lb, lbm, or #)

[1] a traditional unit of mass or weight. The Romans used a pound (the libra pondo, "pound of weight") divided into 12 ounces. All the countries of western Europe used similar units, divided into 12 or 16 ounces, until the advent of the metric system. 12-ounce pounds were common in Italy and southern France, but in Spain and northern Europe 16-ounce pounds became the norm. The word libra is used for this unit in Italy, Spain, and Portugal; in France it is called the livre. Further north, the Latin word pondo ("weight") is the origin of the names of the English pound, Dutch pond, Danish pund, German pfund, and Russian funt. In England, two different "pound" units became standard. The unit now in general use in the United States is the avoirdupois pound, so-called from a French phrase avoir du poids, literally "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The avoirdupois pound is divided into 16 ounces. By international agreement, one avoirdupois pound is equal to exactly 453.592 37 grams; this is exactly 175/144 = 1.215 28 troy pounds. See avoirdupois weights for additional information. The traditional symbol lb stands for libra, the Latin word for the unit. The avoirdupois pound is sometimes abbreviated lb av or lb ap to distinguish it from the less common troy pound.The symbol lbm is used in science to distinguish the pound of mass from the pound of force (lbf): see pound force, below.

[2] a second traditional unit of mass or weight. The troy pound, named for the French market town of Troyes, was the unit used in England by apothecaries and jewelers. The troy pound is divided into 12 ounces like the Roman pound. One troy pound is 373.242 grams, or exactly 144/175 = 0.822 858 avoirdupois pounds (13.165 72 avoirdupois ounces). The troy and avoirdupois pounds are connected by the grain: there are 5760 grains in a troy pound and 7000 grains in an avoirdupois pound. See troy weights for additional information. The troy pound should be abbreviated lb. t. to distinguish it from the more common avoirdupois pound.

This comes from How Many? A Dictionary of Units of Measurement via the University of North Carolina at Chapel Hill. Another helpful site is Ask Dr. Math.

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