Grain (unit)

Grain
The small golden disc close to the 5 cm marker is a piece of pure gold weighing one troy grain. Shown for comparison is a tape measure and coins of major world currencies.
General information
Unit systemTroy weight, avoirdupois weight, apothecaries' weight
Unit ofMass
Symbolgr
Conversions
1 gr in ...... is equal to ...
   Troy   15760 troy pound
   Avoirdupois   17000 pound
   Apothecaries'   15760 apothecaries' pound
   SI units   64.79891 mg

A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single ideal seed of a cereal. From the Bronze Age into the Renaissance, the average masses of wheat and barley grains were part of the legal definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas.[1]: 27 [2] Another source states that it was defined such that 252.458 units would balance 1 cubic inch (16 cm3) of distilled water at an ambient air-water pressure and temperature of 30 inches of mercury (100 kPa) and 62 °F (17 °C) respectively.[3] Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally.[4]

The grain was the legal foundation of traditional English weight systems,[5] and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' systems of mass.[6]: C-6  The unit was based on the weight of a single grain of barley which was equal to about +43 the weight of a single grain of wheat.[5][7]: 95  The fundamental unit of the pre-1527 English weight system, known as Tower weights, was based on the wheat grain.[8] The tower "wheat" grain was defined as exactly +4564 (≈+34) of the troy "barley" grain.[1]: 74 

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 64.79891 milligrams.[6]: C-19 [9] One gram is thus approximately equivalent to 15.43236 grains.[6]: C-13  The unit formerly used by jewellers to measure pearls, diamonds, and other precious stones, called the jeweller's grain or pearl grain, is equal to 14 carat (50 mg; 0.77 gr).[5] The grain was also the name of a traditional French unit equal to 53.115 mg.[5]

In both British Imperial units and United States customary units, there are precisely 7,000 grains per avoirdupois pound, and 5,760 grains per troy pound or apothecaries' pound.[6]: C-6–C-7  It is obsolete in the United Kingdom and, like most other non-SI units, it has no basis in law and cannot be used in commerce.[10]

  1. ^ a b McDonald, Daniel McLean; Scarre, Christopher (1992). The origins of metrology: collected papers of Dr. Daniel McLean McDonald. McDonald Institute for Archaeological Research. ISBN 9780951942000. Retrieved 15 September 2012.
  2. ^ R. D. Connor (1987). The weights and measures of England. H.M.S.O. p. 124. ISBN 978-0-11-290435-9. Retrieved 2 February 2012.
  3. ^ Universal Dictionary of Weights and Measures. Baltimore. 1850. Retrieved 2016-09-23.{{cite book}}: CS1 maint: location missing publisher (link)
  4. ^ Oldberg, Oscar (1885). A Manual of Weights, Measures, and Specific Gravity. Chicago: author [C. J. Johnson, printer]. p. 87. Retrieved 2016-09-24.
  5. ^ a b c d Rowlett, Russ (13 September 2001). "G". How Many? A Dictionary of Units of Measurement. Chapel Hill, North Carolina: University of North Carolina at Chapel Hill. grain (gr) [1–3]. Retrieved 4 July 2012.
  6. ^ a b c d 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 OCLC 58927093. Retrieved 30 June 2012.
  7. ^ Cite error: The named reference Ridgeway1889 was invoked but never defined (see the help page).
  8. ^ Zupko, Ronald Edward (1977). British weights & measures: a history from antiquity to the sixteenth century. University of Wisconsin Press. p. 11. ISBN 9780299073404. Retrieved 18 December 2011.
  9. ^ Judson, Lewis V. (March 1976) [October 1963]. "8. Refinement of values for the yard and pound". Weights and Measures Standards of the United States: A brief history (PDF). NBS Special Publication. Vol. 447. Washington, D.C.: U.S. Department of Commerce, National Bureau of Standards. p. 20. OCLC 610190761. Archived from the original (PDF) on 3 June 2011. Retrieved 30 June 2012.
  10. ^ "Weights and measures: the law". UK Government. Retrieved 2021-11-11.