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A grain is a unit of measurement of mass equal to milligrams. It is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever having used actual seeds or cereal grains. Rather, expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate. The grain was the legal foundation of traditional English weight systems, and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' systems of mass.〔 The unit was based on the weight of a single grain of barley, considered equivalent to grains of wheat.〔〔 The fundamental unit of the pre-1527 English weight system known as Tower weights, was a different sort of grain known as the "wheat grain". The Tower wheat grain was defined as exactly of a troy grain.〔 Since the implementation of the international yard and pound agreement of 1 July 1959, the grain or troy grain (Symbol: gr) measure has been defined in terms of units of mass in the International System of Units as precisely .〔National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, Steve; Crown, Linda et al. eds. ("Appendix C – General Tables of Units of Measurement" ) (PDF). (''Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices'' ). NIST Handbook. 44 (2012 ed.). Washington, D.C.: U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. ISSN (0271-4027 ). OCLC . Retrieved 30 June 2012.〕 is approximately .〔 The unit formerly used by jewellers to measure pearls, diamonds, or other precious stones, called the jeweller's grain or pearl grain, is equal to of a carat, or (~ ).〔 The grain was also the name of a traditional French unit equal to .〔 In both British Imperial and U.S. customary units, there are precisely 7,000 grains per avoirdupois pound, and 5,760 grains per troy pound or apothecaries pound.〔 == Current usage == The grain is commonly used to measure the mass of bullets and propellants. The term also refers to a single particle of gunpowder, the size of which varies according to requirements.〔 Earlier version first published in ''New English Dictionary'', 1900.〕 In archery, the grain is the standard unit used to weigh arrows. In dentistry, gold foil, used as a material to restore teeth, is measured in grains. In North America, the hardness of water is often measured in grains per US gallon (gpg) of calcium carbonate equivalents.〔Vaclavik, Vickie A.; Christian, Elizabeth W. (2008). ("Chapter 2: Water — Water Hardness and Treatments" ). (''Essentials of Food Science'' ) (3rd ed.). Heldman, Dennis R. ed. Food Science Text Series. New York; London: Springer. p. 29. ISBN 978-0-387-69939-4. OCLC . Retrieved 4 July 2012.〕 Otherwise, water hardness is measured in the metric unit parts per million (ppm), equivalent to mg/L.〔〔 One grain per US gallon is approximately .〔 Soft water contains 1–4 gpg of calcium carbonate equivalents, while hard water contains 11–20 gpg.〔 Though no longer recommended, grains are still used occasionally in medicine as part of the apothecaries' system, especially in prescriptions for older medicines such as aspirin or phenobarbital. For example, the dosage of a standard tablet of aspirin is sometimes given as .〔 In that example the grain is approximated to , though the grain can also be approximated to , depending on the medication and manufacturer.〔〔 The apothecaries system has its own system of notation, in which the unit's symbol or abbreviation is followed by the quantity in lower case Roman numerals.〔 For amounts less than one, the quantity is written as a fraction, or for one half, ss (or variations such as ss., ṡṡ, or s̅s̅).〔〔〔 Therefore a prescription for tablets containing 325 mg of aspirin and 30 mg of codeine can be written "ASA gr. v c̄ cod. gr. ss tablets" (using the medical abbreviations ASA for aspirin,〔 c̄ for "with",〔〔 and cod. for codeine).〔〔 The apothecaries' system has gradually been replaced by the metric system, and the use of the grain in prescriptions is now rare.〔 Particulate emission levels, used to monitor and regulate pollution, are commonly measured in grains per cubic foot. This is the same unit commonly used to measure the amount of moisture in the air, also known as the absolute humidity. The SI unit used to measure particulate emissions and absolute humidity is mg/.〔〔 One grain per cubic foot is approximately . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Grain (unit)」の詳細全文を読む スポンサード リンク
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