Imperial unit
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The Imperial units or the Imperial system is a collection of units, first defined in the Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries (though most Commonwealth countries are officially metric), but excluding the then already independent United States. Systems of Imperial units are sometimes referred to as foot-pound-second, after the base units of length, mass and time.
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[edit] Relation to other systems
The distinction between this imperial system and the U.S. customary units (also called standard or English units there) or older British/English units/systems and newer additions is often not drawn precisely. Most length units are shared among the Imperial and U.S. systems, albeit partially and temporally defined slightly differently. Capacity measures differ the most due to the introduction of the Imperial gallon and the unification of wet and dry measures. The avoirdupois system only applies to weights; it has a long designation and a short designation for the hundredweight and ton. The term imperial should not be applied to English units that were outlawed in Weights and Measures Act of 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions such as the slug or poundal.
Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other.
[edit] Measures of length
After the 1 July 1959 deadline, agreed upon in 1958, the U.S. and the British yard were defined identically (0.9144 m) to the international yard. Metric equivalents in this article usually assume this latest official definition. Before this date, the most precise measurement of the Imperial Standard Yard was 0.914398416 m (Sears et al. 1928. Phil Trans A 227:281).
| Unit | Relative value to yard | Metric value | Notes |
|---|---|---|---|
| thou | 1⁄36000 | 25.4 μm | |
| inch | 1⁄36 | 2.54 cm | |
| foot | 1⁄3 | 30.48 cm | |
| yard | 1 | 91.44 cm | Defined as exactly 0.9144 metres since 1956. |
| furlong | 220 | ~201.2 m | |
| mile | 1760 | ~1609 m | |
| league | 5280 | ~4828 m | No longer an official unit in any nation. |
| Maritime units | |||
| fathom | 2 2⁄75 | ~1.853 m | The British Admiralty used a fathom of 1⁄1000 of a nautical mile (i.e. 6.08 feet) until 1970, when the international nautical mile was adopted. This is larger than the commonly accepted definition of a fathom of 6 feet. |
| cable | 202 2⁄3 | ~185.3 m | One tenth of a nautical mile. |
| nautical mile | 2026 2⁄3 | ~1853 m | Used to measure distances at sea. This value refers to the British nautical ( Admiralty) mile; the modern international mile has a slightly smaller value. |
| Gunter's survey units (17th century onwards) | |||
| link | 22⁄100 | ~20.12 cm | |
| pole | 5 1⁄2 | ~5.029 m | The pole is also called rod or perch. |
| chain | 22 | ~20.12 m | |
Until the adoption of the international definition of 1852 metres in 1970, the British nautical mile was defined as 6080 feet. It 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).
[edit] Measures of area
| 1 rood | = 1 furlong × 1 rod[1] | = 40 square rods | = 1⁄2560 square mile | = 10,890 sq ft | = ~0.1012 ha | = ~1012 m² |
|---|---|---|---|---|---|---|
| 1 acre | = 1 furlong × 1 chain | = 160 square rods | = 1⁄640 square mile | = 43,560 sq ft | = ~0.4047 ha | = ~4047 m² |
[edit] Measures of volume
In 1824, Britain adopted a close approximation to the ale gallon known as the Imperial gallon. The Imperial gallon was based on the volume of 10 lb of distilled water weighed in air with brass weights with the barometer standing at 30 inHg and at a temperature of 62 °F. In 1963, this definition was refined as the space occupied by 10 lb of distilled water of density 0.998 859 g/ml weighed in air of density 0.001 217 g/ml against weights of density 8.136 g/ml. This works out to exactly 4.545 964 591 L, or 277.420 cu in. The Weights and Measures Act of 1985 finally switched to a gallon of exactly 4.546 09 L (approximately 277.4 cu in) [2].
| Unit | Relative value to pint | Metric value | U.S. value | Notes |
|---|---|---|---|---|
| fluid ounce (floz) | 1⁄20 | ~28.41 ml | ~0.9608 fl oz | |
| gill | 1⁄4 | ~142.1 ml | ~1.201 gills | |
| pint (pt) | 1 | ~568.2 ml | ~1.201 pt | Still the usual serving size for beer, cider and milk in the UK. |
| quart (qt) | 2 | ~1.136 L | ~1.201 qt | |
| gallon (gal) | 8 | ~4.546 L | ~1.201 gal | Exactly 4.546 09 litres. |
For a comparison to the U.S. customary system see the article on Comparison of the Imperial and U.S. customary systems.
[edit] Measures of weight and mass
Britain has made some use of three different weight systems in the 19th and 20th century, 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 1824 Act made the Troy pound the primary unit of weight.
The use of the troy pound (373.241 721 6 g) was abolished in Britain on January 6, 1879, making the Avoirdupois pound the primary unit of weight.with only the troy ounce (31.103 476 8 g) and its decimal subdivisions retained. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
| Table of mass units | ||||
|---|---|---|---|---|
| Unit | Relative value to pound | Metric value | U.S. value | Notes |
| grain | 1⁄7000 | ~64.8 mg | ||
| drachm | 1⁄256 | ~1.771 g | Known as a 'teenth' in some UK subcultures. | |
| ounce (oz) | 1⁄16 | ~28.35 g | ||
| pound (lb) | 1 | ~453.6 g | Exactly 453.592 37 grammes. | |
| stone (st) | 14 | ~6.35 kg | A person's weight is often quoted in stones or pounds in English speaking countries. | |
| quarter | 28 | ~12.7 kg | A "quarter" was also commonly used to refer to a quarter of a pound in a retail context. | |
| hundredweight (cwt) | 112 | ~50.8 kg | 100 lb | |
| ton (t) | 2240 | ~1016 kg | 2000 lb | 20 hundredweights in both systems, US hundredweight being lighter. |
Note that the British ton is 2240 pounds (the long ton), which is very close to a metric tonne, whereas the ton generally used in the United States is the "short ton" of 2000 pounds (907.184 74 kg), both are 20 hundredweights.
- Further information: Comparison of the Imperial and US customary systems
[edit] Current use of Imperial units
The primary user of traditional 'English' units is the United States and to a lesser degree in Burma and Liberia. In some of the U.S.-influenced Caribbean countries (like Antigua and Saint Lucia), the U.S. customary units, which are similar to Imperial units based upon older English units and in part share definitions, are still in common use. The metric system (SI) has mostly replaced traditional units in other countries.
[edit] United Kingdom
British law now defines each Imperial unit entirely in terms of the metric equivalent.
The Units of Measurement Regulations 1995 require that all measuring devices used in trade or retail be capable of measuring and displaying metric quantities. This has now been proved in court against the so called 'Metric Martyrs', a small group of market traders who insisted on trading only in Imperial units. Contrary to the impression given by some press reports, these regulations have never placed any obstacle in the way of using Imperial units alongside metric units. 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 retail trade. Metric price signs may currently be accompanied by Imperial price signs, (known as supplementary indicators) provided that the Imperial signs are no larger and no more prominent than the official metric ones. The EU's deadline of December 31st, 2009 to enforce metric only labels and ban any supplementary indicators (Imperial measurements) on goods after the deadline has been abolished. On May 9th, 2007 the European Commission agreed to allow supplementary indications alongside the statutory metric indications beyond 2009. [3]
The United Kingdom completed its legal transition to SI units in 1995, but a few such units are still in official use: draught beer must still be sold in pints, roadsign distances must be in yards and miles, and speed limits are in miles per hour, therefore interfaces in cars must be capable of displaying miles per hour. Even though the troy pound was outlawed in Great Britain in the Weights and Measures Act of 1878, the troy ounce still may be used for the weight of precious stones and metals. The railways are also 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, although many modern metro and tram systems are entirely metric, and London Underground uses both metric (for distances) and Imperial (for speeds). Metric is also used for the Channel Tunnel, and also the Channel Tunnel Rail Link high speed line, Adjacent to Ashford International railway station and Dollands Moor International Freight Terminal, speeds are given in both Metric and Imperial units.
The use of SI units is mandated by law for the retail sale of food and other commodities, but most British people still use Imperial units in colloquial discussion of distance (miles) and speed (miles per hour). Milk is available in both litre and pint containers, with appropriate conversions as well. Many people still measure their weight in stones and pounds, and height in feet and inches — but these must be converted to metric if recorded officially, for example on passports. For example petrol is sometimes quoted as being so much per gallon, despite having been sold exclusively in litres for two decades. Likewise, fuel consumption for cars is still usually in miles per gallon, though official figures always include litre per 100 km equivalents. Fahrenheit equivalents are sometimes given after Celsius in weather forecasts, especially for high temperatures (e.g. temperatures will reach as high as 90 degrees tomorrow).
[edit] Canada
In Canada, the government's efforts to implement the metric system were more extensive: almost any agency, institution, or function provided by the government will use SI units exclusively. In the 1970s, the metric systems and SI units were introduced. Imperial units were eliminated from all road signs, although both systems of measurement will still be found on privately owned signs (such as the height warnings at the entrance of a multi-story parking facility). Temperature is typically measured and reported in degrees Celsius. However some radio stations near the United States border (such as CIMX and CIDR), mainly or only use imperial units to report the weather. In addition, Environment Canada still has reports with an Imperial units option besides the metric ones. The law requires that measured products (such as fuel and meat) be priced in metric units, although there is leniency in regards to fruits and vegetables.
Nevertheless, traditional units persist in ordinary conversation. Few Canadians would use SI units to describe their weight and height, although driver's licences use SI units. In livestock auction markets, cattle are sold in dollars per hundredweight (short), whereas hogs are sold in dollars per hundred kilograms. Land is surveyed and registered in metric units, but imperial units still dominate in recipes, construction, house renovation and gardening talk.
One area where Imperial units are still in common use is in firearms and ammunition. For example, Imperial measures are still used in the description of cartridge types, even where the cartridge is of relatively recent invention (e.g. 0.204 Ruger, 0.17 HMR, where the caliber is expressed in decimal fractions of an inch). However, ammunition which is classified in metric already is still kept metric (e.g. 9 mm, 7.62 mm). In the manufacture of ammunition, bullet and powder weights are still expressed in terms of Imperial grains.