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Precious metals, for example, are often weighed in "troy ounces", because just "ounce" would be more likely to be assumed to mean an avoirdupois ounce. For the pound and smaller units, the U.S. customary system and the British imperial system are identical. However, they differ when dealing with units larger than the pound.
Avoirdupois is a system of mass based on a pound of 16 ounces, while Troy weight is the system of mass where 12 troy ounces equals one troy pound. The symbol g 0 is used to denote standard gravity in order to avoid confusion with the (upright) g symbol for gram.
An overview of ranges of mass. To help compare different orders of magnitude, the following lists describe various mass levels between 10 −67 kg and 10 52 kg. The least massive thing listed here is a graviton, and the most massive thing is the observable universe.
An ounce-force is 1 ⁄ 16 of a pound-force, or about 0.2780139 newtons. It is defined as the force exerted by a mass of one avoirdupois ounce under standard gravity (at the surface of the earth, its weight). The "ounce" in "ounce-force" is equivalent to an avoirdupois ounce; ounce-force is a measurement of force using avoirdupois ounces.
The table of imperial avoirdupois mass is the same as the United States table up to one pound, but above that point, the tables differ. The imperial system has a hundredweight, defined as eight stone of 14 lb each, or 112 lb ( 50.802 345 44 kg ), whereas a US hundredweight is 100 lb ( 45.359 237 kg ).
The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system.
C-19 [17] Thus one troy ounce = 480 grains × 0.064 798 91 grams/grain = 31.103 476 80 grams. Since the ounce avoirdupois is defined as 437.5 grains, a troy ounce is exactly 480 ⁄ 437.5 = 192 ⁄ 175 or about 1.09714 ounces avoirdupois or about 9.7% more. The troy ounce for trading precious metals is considered to be sufficiently approximated ...
In the apothecaries' system, which was widely used in the United States until the middle of the 20th century, [15] the dram is the mass of 1 ⁄ 96 pounds apothecaries (lb ap), or 1 ⁄ 8 ounces apothecaries (oz ap or ℥) [5]: C-7 (the pound apothecaries and ounce apothecaries are equal to the troy pound (lb t), and troy ounce (oz t ...