Imperial units: Difference between revisions

Revision as of 06:36, 10 June 2004

Revision as of 08:42, 11 June 2004

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The '''Imperial units''' are an irregularly standardized [[system of units]] that have been used in the [[United Kingdom]], its former colonies, and the [[The Commonwealth|Commonwealth]] countries. Today it is only widely used in the [[U.S. customary units|United States]], having been replaced elsewhere by the [[SI]] (metric) system. The United Kingdom completed its transition to SI units in [[1995]], though a few Imperial units are still in use. For example, beer may still be sold in pints, and most roadsigns are still in yards and miles.

The Imperial system is also called the '''English system''' or the '''British system'''.

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== Measures of length ==

After [[1959]], the U.S. and the British [[inch]] were defined identically (25.4  mm) for scientific work and were identical in commercial usage (however, the U.S. retained the slightly different ''survey inch'' for specialized [[surveying]] purposes). The tables of length, such as 12 inches = 1 foot, 3 feet = 1 yard, and 1760 yards = 1 international [[mile]], were the same in both countries, though some of the intermediate units such as the chain (22 yards) and the furlong (220 yards) were more used in Britain than in the U.S.

== Measures of volume==

The present British gallon (4.55  l) and bushel (36.4  l)--known as the "Imperial gallon" and "Imperial bushel"--are, respectively, about 20 percent and 3 percent larger than the United States fluid gallon (3.79  l) and bushel (35.2  l). The Imperial gallon was defined as the volume of 10 avoirdupois pounds of water under specified conditions, and the Imperial bushel was defined as 8 Imperial gallons.

Also, the subdivision of the Imperial gallon as presented in the table of British apothecaries' fluid measure differed in two important respects from the corresponding United States subdivision, in that the Imperial gallon was divided into 160 fluid ounces (whereas the United States gallon is divided into 128 fluid ounces), and a "fluid scruple" is included.

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*1 ''gallon'' = 4 quarts = 4.55 l

*1 ''peck'' = 2 gallons = 9.09 l

*1 ''kenning'' = 2 pecks = 18.2  l

*1 ''bushel'' = 8 gallons (4 pecks or 2 kennings) = 36.4 l

*1 ''quarter'' = 8 bushels = 2.91  hl

The full table of British apothecaries' measure is as follows:

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*1 ''gallon'' = 8 pints = 160 fluid ounces

The origins of these differences lie in the variety of systems that were in use in Britain at the time of the establishment of the first colonies in North America. The American colonists adopted the English wine gallon of 231 cubic inches, and used it for all fluid purposes. The English of that period used this wine gallon, but they also had another gallon, the ale gallon of 282 cubic inches. In 1824, the British abandoned these two gallons when they adopted the British Imperial gallon, which they defined as the volume of 10 pounds of water, at a temperature of 62  °F, which, by calculation, is equivalent to about 277.42 cubic inches (4,546  cm³) - much closer to the ale gallon than the wine gallon. At the same time, they redefined the bushel as 8 gallons.

As noted above, in the customary British system the units of dry measure are the same as those of liquid measure. In the United States these two are not the same, the gallon and its subdivisions are used in the measurement of liquids; the bushel, with its subdivisions, is used in the measurement of certain dry commodities. The U.S. gallon (3.79  l) is divided into four liquid [[quart]]s (946  ml each) and the U.S. bushel (4.40  l) into 32 dry quarts or pecks (8.81  l each). All the units of capacity or volume mentioned thus far are larger in the customary British system than in the U.S. system. But the British fluid ounce is smaller than the U.S. fluid ounce, because the British quart is divided into 40 fluid ounces whereas the U.S. quart is divided into 32 fluid ounces.

From this we see that in the customary British system an avoirdupois ounce of water at 62 °F has a volume of one fluid ounce, because 10 pounds is equivalent to 160 avoirdupois ounces, and 1 gallon is equivalent to 4 quarts, or 160 fluid ounces. This convenient relation does not exist in the U.S. system because a U.S. gallon of water at 62  °F weighs about 8 1/3 pounds, or 133 1/3 avoirdupois ounces, and the U.S. gallon is equivalent to 4 × 32, or 128 fluid ounces.

* 1 U.S. fluid ounce = 1.041 British fluid ounces = 29.6  ml

* 1 British fluid ounce = 0.961 U.S. fluid ounce = 28.4  ml

* 1 U.S. gallon = 0.833 British Imperial gallon = 3.79  l

* 1 British Imperial gallon = 1.201 U.S. gallons = 4.55  l

In the apothecary system of liquid measure the British add a unit, the fluid scruple, equal to one third of a fluid drachm (spelled dram in the United States) between their minim and their fluid drachm.

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A discussion of differences between countries is complicated by the fact that both Britain and the U.S. have made some use of three different weight systems, [[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.

Among other differences between the customary British and the United States measurement systems, we should note that the use of the ''troy pound'' (373  g) was abolished in Britain on January 6, [[1879]], with only the ''troy ounce'' (31.1  g) and its subdivisions retained, whereas the troy pound (of 12 troy ounces) is still legal in the United States, although it is not now greatly used. Another important difference is the universal use in Britain, for body weight, of the [[stone (weight)|stone]] of 14 pounds (6.35  kg), this being a unit now unused in the United States, although its influence was shown in the practice until World War II of selling flour by the barrel of 196 pounds (14 stone).

In all the systems, the fundamental unit is the [[pound]], and all other units are defined as fractions or multiples of it. The Avoirdupois pound, the Troy pound, and the apothecaries' pound are identical in Britain and the United States. The tables of British troy mass, and apothecaries' mass are the same as the corresponding United States tables, except for the British spelling "drachm" in the table of apothecaries' mass. The table of British Avoirdupois mass is the same as the United States table up to 1 pound, above that point the table differs:

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*1 ounce = 1?16 pound = 28.3 g

*1 pound = 7000 grains = 454 g

*1 stone = 14 pounds = 6.35  kg

*1 quarter = 2 stones = 28 pounds = 12.7  kg

*1 hundredweight = 4 quarters = 112 pounds = 50.8  kg

*1 ton = 20 hundredweight = 2240 pounds = 1016  kg

Note that the British [[ton]] is 2240 pounds (known in the US as a "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  kg).

British law now defines each Imperial unit entirely in terms of the metric equivalent. See the 'Units of Measurement Regulations 1995'