The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1825) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed through a series of Weights and Measures Acts and amendments. The imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement but imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.
The Weights and Measures Act of 1824 was initially scheduled to go into effect on 1 May 1825. The Weights and Measures Act of 1825 pushed back the date to 1 January 1826. The 1824 Act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.
Apothecaries' units are mentioned neither in the act of 1824 nor 1825. At the time, apothecaries' weights and measures were regulated "in England, Wales, and Berwick-upon-Tweed" by the London College of Physicians, and in Ireland by the Dublin College of Physicians. In Scotland, apothecaries' units were unofficially regulated by the Edinburgh College of Physicians. The three colleges published, at infrequent intervals, pharmacopoeiae, the London and Dublin editions having the force of law.
Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836, the Edinburgh Pharmacopoeia of 1839, and the Dublin Pharmacopoeia of 1850. The Medical Act of 1858 transferred to The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.
Metric equivalents in this article usually assume the latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 0.914398415 metres.
|Unit||Relative to previous||Feet||Millimetres||Metres||Notes|
|inch (in or ″)||1000 thou||1⁄12||25.4||0.0254||
|foot (ft or ′)||12 inches||1||304.8||0.3048|
|yard (yd)||3 feet||3||914.4||0.9144||
|chain (ch)||22 yards||66||20116.8||20.1168|
|furlong (fur)||10 chains||660||201.168||
|mile (mi)||8 furlongs||5280||1609.344||
|league (lea)||3 miles||15840||4828.032||
|fathom (ftm)||2.02667 yards||6.0761||1852||1.852|
|nautical mile||10 cables||6076.1||1852|
|Gunter's survey units (17th century onwards)|
units of length
|Square feet||Square rods||Square miles||Square metres||Hectares||Notes|
|perch||1 rod × 1 rod||272.25||1||1⁄102400||25.29285264||0.002529285264||
|rood||1 furlong × 1 rod||10890||40||1⁄2560||1011.7141056||0.10117141056||
|acre||1 furlong × 1 chain||43560||160||1⁄640||4046.8564224||0.40468564224||
|Note: All equivalences are exact.|
The Weights and Measures Act 1824 invalidated the various different gallons in use in the British Empire, declaring them to be replaced by the statute gallon (which became known as the Imperial Gallon), a unit close in volume to the ale gallon. The 1824 Act defined as the volume of a gallon to be that of 10 pounds (4.54 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C). The 1824 Act went on to give this volume as 277.274 cubic inches (4.54371 litres). The Weights and Measures Act 1963 refined this definition to be the volume of 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL, which works out to 4.546092 L.[nb 1] The Weights and Measures Act 1985 defined a gallon to be exactly 4.54609 L (approximately 277.4194 cu in).
|Millilitres||Cubic inches||US ounces||US pints|
|fluid ounce (fl oz)||1||1⁄20||28.4130625||1.7339||0.96076||0.060047|
|Note: The millilitre equivalences are exact, but cubic-inch and US measures are correct to 5 significant figures.|
|1⁄2 gill||4.32 cu in (70.8 ml)|
|gill||1⁄4 pint||8.64 cu in (141.6 ml)|
|1⁄2 pint||1⁄2 pint||17.38 cu in (284.8 ml)|
|pint||pint||34.76 cu in (569.6 ml; 0.5696 l)|
|quart||quart||69.32 cu in (1.1360 l)|
|1⁄2 gallon||1⁄4 peck or 1⁄2 gallon||138.64 cu in (2.2719 l)|
|gallon||1⁄2 peck or gallon||277.274 cu in (4.54371 l)|
|2 gallons (peck)||peck||554.548 cu in (9.08741 l)|
|4 gallons (1⁄2 bushel)||1⁄2 bushel||1,109.096 cu in (18.17483 l)|
|8 gallons||bushel||2,218.192 cu in (36.34965 l)|
|64 gallons||quarter||17,745.536 cu in (290.79723 l)|
|Note: The 1824 Act removed the distinction between liquid and dry measure, specifying instead that |
the dry quantities shall be unheaped. The metric equivalences shown are approximate.
British apothecaries' volume measuresEdit
These measurements were in use from 1826, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971. In the US, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.
metric value[note 1]
|minim||♏︎, , m, m., min||(1⁄9600 pint)||59.1938802083 µL|
|fluid scruple||fl ℈, fl s||20 minims (1⁄480 pint)||1.18387760416 mL|
(fluid dram, fluidram)
|ʒ, fl ʒ, fʒ, ƒ 3, fl dr||3 fluid scruples (1⁄160 pint)||3.5516328125 mL|
|fluid ounce||℥, fl ℥, f℥, ƒ ℥, fl oz||8 fluid drachms||28.4130625 mL|
|pint||O, pt||20 fluid ounces||568.26125 mL|
|gallon||C, gal||8 pints||4.54609 L|
Mass and weightEdit
In the 19th and 20th centuries, the UK used three different systems for mass and weight.
- troy weight, used for precious metals;
- avoirdupois weight, used for most other purposes; and
- apothecaries' weight, now virtually unused since the metric system is used for all scientific purposes.
The distinction between mass and weight is not always clearly drawn. Strictly a pound is a unit of mass, but it is commonly referred to as a weight. When a distinction is necessary, the term pound-force may refer to a unit of force rather than mass. The troy pound (373.2417216 g) was made the primary unit of mass by the 1824 Act and its use was abolished in the UK on 1 January 1879, with only the troy ounce (31.1034768 g) and its decimal subdivisions retained. The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
|grain (gr)||1⁄7000||0.06479891||Exactly 64.79891 milligrams.|
|drachm (dr)||1⁄256||1.7718451953125||A dram is 1⁄16 of an ounce|
|ounce (oz)||1⁄16||28.349523125||An ounce is 1⁄16 of a pound|
|pound (lb)||1||453.59237||0.45359237||Defined by The Units of Measurement Regulations 1994|
|stone (st)||14||6350.29318||6.35029318||The plural stone is often used when providing a weight (e.g. "this sack weighs 8 stone"). A person's weight is usually quoted in stone and pounds in English-speaking countries that use the avoirdupois system, with the exception of the United States and Canada, where it is usually quoted in pounds.|
|quarter (qr or qtr)||28||12.70058636||One quarter (literally a quarter of a hundredweight) is equal to two stone or 28 pounds. The term quarter is also used in retail contexts, where it refers to four ounces, i.e. a quarter of a pound. (The 1824 Act defined a quarter as a unit of volume, as above: thus a 'quarter of wheat', 64 gallons, would weigh about 494 lb.).|
|hundredweight (cwt)||112||50.80234544||One imperial hundredweight is equal to eight stone. This is the long hundredweight, 112 pounds, as opposed to the short hundredweight of 100 pounds used in the United States and Canada.|
|ton (t)||2240||1016.0469088||Twenty hundredweight equals a ton (as in the US and Canadian systems). The Imperial hundredweight is 12% greater than the US and Canadian one. The Imperial ton (or long ton) is 2240 pounds, which is much closer to a metric tonne (about 2204.6 pounds), compared to the 12% smaller North American short ton of 2000 pounds (907.185 kg).|
|slug (slug)||32.17404856||14593.90294||14.59390294||The slug, a unit associated with imperial and US customary systems, is a mass that accelerates by 1 ft/s2 when a force of one pound (lbf) is exerted on it.|
The 1824 Act of Parliament defined the yard and pound by reference to the prototype standards, and it also defined the values of certain physical constants, to make provision for re-creation of the standards if they were to be damaged. For the yard, the length of a pendulum beating seconds at the latitude of Greenwich at Mean Sea Level in vacuo was defined as 39.01393 inches. For the pound, the mass of a cubic inch of distilled water at an atmospheric pressure of 30 inches of mercury and a temperature of 62° Fahrenheit was defined as 252.458 grains, with there being 7,000 grains per pound.
Following the destruction of the original prototypes in the 1834 Houses of Parliament fire, it proved impossible to recreate the standards from these definitions, and a new Weights and Measures Act (18 & 19 Victoria. Cap. 72) was passed in 1855 which permitted the recreation of the prototypes from recognized secondary standards.
Relation to other systemsEdit
The imperial system is one of many systems of English units. Most of the units are defined in more than one system, and some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than another. The distinctions between the systems are often not drawn precisely.
One such distinction is that between the Imperial system and older British/English units/systems or newer additions. The term imperial should not be applied to English units that were outlawed by the Weights and Measures Act 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions, such as the slug or poundal.
The US customary system is derived from those English units which were in use at the time of the original settlement of North America from England in the 17th century. And because the United States had achieved independence from Britain prior to the 19th Century, US customary units were unaffected by Britain's introduction of the Imperial system in 1834.
Since the Weights and Measures Act 1985, British law defines base imperial units in terms of their metric equivalent and declares that none may be used "for trade" (except that milk, beer and cider may also be sold by the pint). The metric system is routinely used in business and technology within the United Kingdom, with Imperial units remaining in widespread use amongst the public. All UK roads use the imperial system except for weight limits, and newer height or width restriction signs give metric alongside imperial.
Units of measurement regulations require all measuring devices used in trade or retail to display measurements in metric quantities. Almost all traders in the UK will accept requests from customers specified in imperial units, and scales which display in both unit systems are commonplace in the retail trade. Metric price signs may be accompanied by imperial price signs provided that the imperial signs are no larger and no more prominent than the metric ones.
The United Kingdom completed its official partial transition to the metric system in 1995, with imperial units still legally mandated for certain applications such as draught beer and cider, and road-signs. Therefore, the speedometers on vehicles sold in the UK must be capable of displaying miles per hour. Even though the troy pound was outlawed in the UK in the Weights and Measures Act of 1878, the troy ounce may still be used for the weights of precious stones and metals. The original railways (many built in the Victorian era) are a big user of imperial units, with distances officially measured in miles and yards or miles and chains, and also feet and inches, and speeds are in miles per hour. More recent systems and the London Underground use metric.
Most British people still use imperial units in everyday life for distance (miles, yards, feet and inches) and volume in some cases (especially milk and beer in pints) but rarely for canned or bottled soft drinks or petrol. Though use of kilograms is increasing, many British people also still use imperial units in everyday life for body weight (stones and pounds for adults, pounds and ounces for babies). Government documents aimed at the public may give body weight and height in imperial units as well as in metric. A survey in 2015 found that many people did not know their body weight or height in one system or the other. People under the age of 40 preferred the metric system but people aged 40 and over preferred the imperial system. The height of horses in English-speaking countries, including Australia, Canada, the United Kingdom and the United States is usually measured in hands, standardized to 4 inches (101.6 mm). Fuel consumption for vehicles is commonly stated in miles per gallon (mpg), though official figures always include litres per 100 km equivalents. When sold draught in licensed premises, beer and cider must be sold in pints, half-pints and third-pints. Cow's milk is available in both litre- and pint-based containers in supermarkets and shops. Areas of land associated with farming, forestry and real estate are commonly advertised in acres and square feet but, for contracts and land registration purposes, the units are always hectares and square metres.
Office space and industrial units are usually advertised in square feet. Steel pipe sizes are sold in increments of inches, while copper pipe is sold in increments of millimetres. Road bicycles have their frames measured in centimetres, while off-road bicycles have their frames measured in inches. The size (diagonal) of television and computer monitor screens is always denominated in inches. Food sold by length or width, e.g. pizzas or sandwiches, is generally sold in inches. Clothing is always sized in inches, with the metric equivalent often shown as a small supplementary indicator. Gas is usually measured by the cubic foot or cubic metre, but is billed like electricity by the kilowatt hour.
Pre-packaged products can show both metric and imperial measures, and it is also common to see imperial pack sizes with metric only labels, e.g. a 1 lb (454 g) tin of Lyle's Golden Syrup is always labelled 454 g with no imperial indicator. Similarly most jars of jam and packs of sausages are labelled 454 g with no imperial indicator. But, whatever the label says, the tins or packets are invariably still packaged in Imperial quantities (typically 8 or 16 ounces).
India's conversion to the metric system from the imperial system occurred in stages between 1955 and 1962. The metric system in weights and measures was adopted by the Indian Parliament in December 1956 with the Standards of Weights and Measures Act, which took effect beginning 1 October 1958. For the next five years, both the previous and new system were legal. In April 1962, all other systems were banned.
Today all official measurements are made in the metric system. In common usage some older Indians may still refer to imperial units. Some measurements, such as the heights of mountains, are still recorded in feet. Tyre rim diameters are still measured in inches, as used worldwide. Industries like the construction and the real estate industry still use both the metric and the imperial system though it is more common for sizes of homes to be given in square feet and land in acres.
In Standard Indian English, as in Australian, Singaporean, and British English, metric units such as the litre, metre, and metric tonne utilise the traditional spellings brought over from French, which differ from those used in the United States and the Philippines. The imperial long ton is invariably spelt with one 'n'.
- The Chinese units of measurement of the Qing Empire (no longer in widespread use in China);
- British imperial units; and
- The metric system.
In 1976 the Hong Kong Government started the conversion to the metric system, and as of 2012 measurements for government purposes, such as road signs, are almost always in metric units. All three systems are officially permitted for trade, and in the wider society a mixture of all three systems prevails.
The Chinese system's most commonly used units for length are 里 (lei5), 丈 (zoeng6), 尺 (cek3), 寸 (cyun3), 分 (fan1) in descending scale order. These units are now rarely used in daily life, the imperial and metric systems being preferred. The imperial equivalents are written with the same basic Chinese characters as the Chinese system. In order to distinguish between the units of the two systems, the units can be prefixed with "Ying" (英, jing1) for the Imperial system and "Wa" (華, waa4) for the Chinese system. In writing, derived characters are often used, with an additional 口 (mouth) radical to the left of the original Chinese character, for writing imperial units. The most commonly used units are the mile or "li" (哩, li1), the yard or "ma" (碼, maa5), the foot or "chek" (呎, cek3), and the inch or "tsun" (吋, cyun3).
The traditional measure of flat area is the square foot (方呎, 平方呎, fong1 cek3, ping4 fong1 cek3) of the imperial system, which is still in common use for real estate purposes. The measurement of agricultural plots and fields is traditionally conducted in 畝 (mau5) of the Chinese system.
For the measurement of volume, Hong Kong officially uses the metric system, though the gallon (加侖, gaa1 leon4-2) is also occasionally used.
During the 1970s, the metric system and SI units were introduced in Canada to replace the imperial system. Within the government, efforts to implement the metric system were extensive; almost any agency, institution, or function provided by the government uses SI units exclusively. Imperial units were eliminated from all public road signs and both systems of measurement will still be found on privately owned signs, such as the height warnings at the entrance of a parkade. In the 1980s, momentum to fully convert to the metric system stalled when the government of Brian Mulroney was elected. There was heavy opposition to metrication and as a compromise the government maintains legal definitions for and allows use of imperial units as long as metric units are shown as well. The law requires that measured products (such as fuel and meat) be priced in metric units and an imperial price can be shown if a metric price is present. There tends to be leniency in regards to fruits and vegetables being priced in imperial units only. Environment Canada still offers an imperial unit option beside metric units, even though weather is typically measured and reported in metric units in the Canadian media. Some radio stations near the United States border (such as CIMX and CIDR) primarily use imperial units to report the weather. Railways in Canada also continue to use Imperial units.
Imperial units are still used in ordinary conversation. Today, Canadians typically use a mix of metric and imperial measurements in their daily lives. The use of the metric and imperial systems varies by age. The older generation mostly uses the imperial system, while the younger generation more often uses the metric system. Quebec has implemented metrication more fully. Newborns are measured in SI at hospitals, but the birth weight and length is also announced to family and friends in imperial units. Drivers' licences use SI units, though many English-speaking Canadians give their height and weight in imperial. In livestock auction markets, cattle are sold in dollars per hundredweight (short), whereas hogs are sold in dollars per hundred kilograms. Imperial units still dominate in recipes, construction, house renovation and gardening. Land is now surveyed and registered in metric units whist initial surveys used imperial units. For example, partitioning of farm land on the prairies in the late 19th and early 20th centuries was done in imperial units; this accounts for imperial units of distance and area retaining wide use in the Prairie Provinces. In English-speaking Canada commercial and residential spaces are mostly (but not exclusively) constructed using square feet, while in French-speaking Quebec commercial and residential spaces are constructed in metres and advertised using both square metres and square feet as equivalents. Carpet or flooring tile is purchased by the square foot, but less frequently also in square metres. Motor-vehicle fuel consumption is reported in both litres per 100 km and statute miles per imperial gallon, leading to the erroneous impression that Canadian vehicles are 20% more fuel-efficient than their apparently identical American counterparts for which fuel economy is reported in statute miles per US gallon (neither country specifies which gallon is used). Canadian railways maintain exclusive use of imperial measurements to describe train length (feet), train height (feet), capacity (tons), speed (mph), and trackage (miles).
Imperial units also retain common use in firearms and ammunition. Imperial measures are still used in the description of cartridge types, even when the cartridge is of relatively recent invention (e.g., .204 Ruger, .17 HMR, where the calibre is expressed in decimal fractions of an inch). Ammunition that is already classified in metric is still kept metric (e.g., 9×19mm). In the manufacture of ammunition, bullet and powder weights are expressed in terms of grains for both metric and imperial cartridges.
As in most of the western world, air navigation is based on nautical units, e.g., the nautical mile, which is neither imperial nor metric, though altitude is still measured in imperial feet in keeping with the international standard.
Metrication in Australia has largely ended the official use of imperial units, though for particular measurements, international use of imperial units is still followed. In licensed venues, draught beer and cider is sold in glasses and jugs with sizes based on the imperial fluid ounce, though rounded to the nearest 5 mL.
Newborns are measured in metric at hospitals, but the birth weight and length is also announced to family and friends in imperial units.
Screen sizes, are frequently advertised in inches as well as centimetres.
Property size is frequently advertised in acres, but is mostly as square metres.
Navigation is done in nautical miles, and water-based speed limits are in nautical miles per hour.
In New Zealand, which completed metrication in the 1970s, a study of university students undertaken in 1992 found a continued use of imperial units for birth weight and human height alongside metric units.
In aviation, altitude and airport elevation are measured in feet whilst navigation is done in nautical miles; all other aspects (fuel quantity, aircraft weight, runway length, etc.) use metric units.
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Ireland has officially changed over to the metric system since entering the European Union, with distances on new road signs being metric since 1997 and speed limits being metric since 2005. The imperial system remains in limited use – for sales of beer in pubs (traditionally sold by the pint). All other goods are required by law to be sold in metric units with traditional quantities being retained for goods like butter and sausages, which are sold in 454-gram (1 lb) packaging. The majority of cars sold pre-2005 feature speedometers with miles per hour as the primary unit, but with a kilometres per hour display as well. Often signs such as those for bridge height can display both metric and imperial units. Imperial measurements continue to be used colloquially by the general population especially with height and distance measurements such as feet, inches, and acres as well as for weight with pounds and stones still in common use among people of all ages. Measurements such as yards have fallen out of favour with younger generations. All of Ireland's railways still operate on imperial measurements and property is usually listed in square feet as well as metres also.
Some imperial measurements remain in limited use in Malaysia, the Philippines, Sri Lanka and South Africa. Measurements in feet and inches, especially for a person's height, are frequently encountered in conversation and non-governmental publications.
Prior to metrication, it was a common practice in Malaysia for people to refer to unnamed locations and small settlements along major roads by referring to how many miles the said locations were from the nearest major town. In some cases, these eventually became the official names of the locations; in other cases, such names have been largely or completely superseded by new names. An example of the former is Batu 32 (literally "Mile 32" in Malay), which refers to the area surrounding the intersection between Federal Route 22 (the Tamparuli-Sandakan highway) and Federal Route 13 (the Sandakan-Tawau highway). The area is so named because it is 32 miles west of Sandakan, the nearest major town.
Petrol is still sold by the imperial gallon in Anguilla, Antigua and Barbuda, Belize, Myanmar, the Cayman Islands, Dominica, Grenada, Montserrat, St Kitts and Nevis and St. Vincent and the Grenadines. The United Arab Emirates Cabinet in 2009 issued the Decree No. (270 / 3) specifying that, from 1 January 2010, the new unit sale price for petrol will be the litre and not the gallon. This in line with the UAE Cabinet Decision No. 31 of 2006 on the national system of measurement, which mandates the use of International System of units as a basis for the legal units of measurement in the country. Sierra Leone switched to selling fuel by the litre in May 2011.
In October 2011, the Antigua and Barbuda government announced the re-launch of the Metrication Programme in accordance with the Metrology Act 2007, which established the International System of Units as the legal system of units. The Antigua and Barbuda government has committed to a full conversion from the imperial system by the first quarter of 2015.
|Wikimedia Commons has media related to British Imperial units.|
- 10 pounds = 4535.9237 grams. @ 0.998859 g/ml => 4546.092 ml
- References for the Table of British apothecaries' volume units: Unit column;:C-7 Symbols & abbreviations column;:C-5, C-17–C-18 Relative to previous column;:C-7 Exact metric value column — fluid ounce, pint and gallon, all other values calculated using value for fluid ounce and the Relative to previous column's values.
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- Chaney, Henry James (1897). A Practical Treatise on the Standard Weights and Measures in Use in the British Empire with some account of the metric system. Eyre and Spottiswoode. p. 3. Retrieved 11 September 2016.
- "Weights and Measures Act 1985". legislation.gov.uk. Retrieved 20 January 2020.
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- Great Britain; William David Evans; Anthony Hammond; Thomas Colpitts Granger (1836). A collection of statutes connected with the general administration of the law: arranged according to the order of subjects. W. H. Bond. pp. 306–27. Retrieved 31 December 2011.
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- Gray, Samuel Frederick (1836). A supplement to the Pharmacopœia and treatise on pharmacology in general: including not only the drugs and preparations used by practitioners of medicine, but also most of those employed in the chemical arts : together with a collection of the most useful medical formulæ ... Longman, Rees, Orme, Brown, Green, and Longman. p. 516. Retrieved 29 July 2012.
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- Sears et al. 1928. Phil Trans A, 227:281.
- Jerrard and McNeill, Dictionary of Scientific Units, second edition, Chapman and Hall; cites first appearance in print in Journal of the Institution of Electrical Engineers (G.B.) vol. 1, page 246 (1872)
- The exact figure was 6.08 feet, but 6 feet was in use in practice. The commonly accepted definition of a fathom was always 6 feet. The conflict was inconsequential, as Admiralty nautical charts designated depths shallower than 5 fathoms in feet on older imperial charts. Today, all charts worldwide are metric, except for USA Hydrographic Office charts, which use feet for all depth ranges.
- The nautical mile was not readily expressible in terms of any of the intermediate units, because it was derived from the circumference of the Earth (like the original metre).
- "Appendix C: General Tables of Units of Measurements" (PDF). NIST. Archived from the original (PDF) on 26 November 2006. Retrieved 4 January 2007.
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Two such Gallons shall be a Peck, and Eight such Gallons shall be a Bushel, and Eight such Bushels a Quarter of Corn or other dry Goods, not measured by Heaped Measure.. (The date of coming into effect was 1 May 1825).
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- 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.: US Department of Commerce, Technology Administration, National Institute of Standards and Technology. ISSN 0271-4027. OCLC OCLC 58927093. Retrieved 6 July 2012.
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