Imperial units
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The system of imperial units or the imperial
system is the system of units first defined in the British Weights and Measures Act of
1824, which was later refined and reduced. 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; however some Imperial units
are still used in the United Kingdom and Canada. This system developed from what was first
known as English units. Implementation
The Weights and Measures Act of 1824 was initially scheduled to go into effect on 1 May 1825.
However, 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 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
The Crown the right to publish the official pharmacopoeia and to regulate apothecaries’
weights and measures. Units
Length Metric equivalents in this article usually
assume the latest official definition. Before this date, the most precise measurement of
the Imperial Standard Yard was 0.914398416 metres.
Area Volume
In 1824, the various different gallons in use in the British Empire were replaced by
the imperial gallon, a unit close in volume to the ale gallon. It was originally defined
as the volume of 10 pounds of distilled water weighed in air with brass weights with the
barometer standing at 30 inches of mercury at a temperature of 62 °F. In 1963, the
gallon was redefined as 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 4.546096 L or 277.4198 cu in. The Weights and Measures Act of 1985 switched
to a gallon of exactly 4.54609 L. British apothecaries’ volume measures
These measurements were in use from 1824, when the new imperial gallon was defined,
but were officially abolished in the United Kingdom on 1 January 1971. In the USA, though
no longer recommended, the apothecaries’ system is still used occasionally in medicine, especially
in prescriptions for older medications. Mass and weight
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 troy pound was made the primary unit of mass by the 1824 Act; however, its use was
abolished in the UK on 1 January 1879, with only the troy ounce and its decimal subdivisions
retained. The Weights and Measures Act 1855 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. Natural equivalents
Although the 1824 act defined the yard and pound by reference to the prototype standards,
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 39.013 93
inches, and, 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. Relation to other systems The imperial system is one of many systems
of English units. 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. The distinctions between these systems are often not drawn
precisely. One such distinction is that between these
systems and older British/English units/systems or newer additions. The term imperial should
not be applied to English units that were outlawed in 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 historically derived
from the English units that were in use at the time of settlement. Because the United
States was already independent at the time, these units were unaffected by the introduction
of the imperial system. Current use of imperial units United Kingdom British law now defines each imperial unit
in terms of the metric equivalent. The metric system is in official use within the United
Kingdom for most applications; however, use of Imperial units is still widespread amongst
the public and all UK roads still primarily use the imperial system except for tonnage
on main roads. The Units of Measurement Regulations 1995
require that all measuring devices used in trade or retail shall display measurements
in metric quantities. This has been proven in court against the so-called “Metric Martyrs”,
a small group of market traders who insisted on trading in imperial units only. Contrary
to the impression given by some press reports, these regulations do not currently place 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 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 official metric ones. The EU units of measurement directive
had previously permitted the use of supplementary indicators until 31 December 2009, but a revision
of the directive published on 11 March 2009 permitted their use indefinitely.
The United Kingdom completed its legal partial transition to the metric system in 1995, with
some imperial units still legally mandated for certain applications; draught beer and
cider must be sold in pints, road-sign distances must be in yards and miles, length and width
restrictions must be in feet and inches on road signs, and road speed limits must be
in miles per hour, therefore instruments in vehicles sold in the UK must be capable of
displaying miles per hour. Foreign vehicles, such as all post-2005 Irish vehicles, may
legally have instruments displayed only in kilometres per hour. Even though the troy
pound was outlawed in the UK in the Weights and Measures Act of 1878, the troy ounce still
may be used for the weight of precious stones and metals. The original railways 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, although many modern metro
and tram systems are entirely metric, and London Underground uses both metric and imperial.
Metric is also used for the Channel Tunnel and on High Speed 1. Adjacent to Ashford International
railway station and Dollands Moor Freight Yard, railway speeds are given in both metric
and imperial units. Most British people still use imperial units
in everyday life for distance, body weight and volume. Regardless of how people measure
their weight or height, these must be recorded in metric officially, for example in medical
records. Petrol is occasionally quoted as being so much per gallon. Fuel consumption
for vehicles is often discussed in miles per gallon, though official figures always include
litres per 100 km equivalents. When sold “draught” in licenced premises, beer and cider
is measured out and sold in pints and half-pints. Cow’s milk is available in both litre- and
pint-based containers. Non-metric nuts and bolts etc., are available, but usually only
from specialist suppliers. Areas of land associated with farming, forestry and real estate are
often advertised measured in acres and square feet, but for official government purposes
the unit is always hectares and square metres. Office space and industrial units are often
advertised in square feet, despite carpet and flooring products being sold in square
metres with equivalents in square yards. 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 of television and computer monitor screens is denominated in inches.
Canada In 1973, 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 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 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 be priced in metric units, although an imperial
price can be shown if a metric price is present. However, there tends to be leniency in regards
to fruits and vegetables being priced in imperial units only. Unlike the rest of Canada, metrication
in the Francophone province of Quebec has been more fully implemented and metric measures
are more consistently used in Quebec than elsewhere in Canada, both officially and among
the population. 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. However, some radio stations near the United States
border 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. However, 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. 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. In livestock auction markets, cattle are sold in dollars per hundredweight,
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, although 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. The size of most apartments, condominiums and houses continues to be described
in square feet rather than square metres, and carpet or flooring tile is purchased by
the square foot. 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. Canadian railways maintain
exclusive use of imperial measurements to describe train length, train height, capacity,
speed, and trackage. 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. However, ammunition that is already
classified in metric is still kept metric. 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. Australia and New Zealand In Australia and New Zealand, metrication
is mostly complete, although imperial units remain current in some areas, generally where
international standards remain non-metricated. Ireland 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. All other goods are required by law to be
sold in metric units, although old quantities are retained for some goods like butter and
sausages, which are sold in 454-gram 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. Other countries
Some imperial measurements remain in limited use in India, Malaysia, Sri Lanka and Hong
Kong. Real estate agents continue to use acres and square feet to describe area, rarely in
conjunction with hectares and square metres. Measurements in feet and inches, especially
for a person’s height, are frequently met in conversation and non-governmental publications.
In India, inches, feet, yards and degrees Fahrenheit are often used in conjunction with
their metric counterparts, while area is often still measured in acres though hectares are
used in government documents; the Celsius scale is used for weather readings and forecasts,
but the Fahrenheit scale is often used for body temperatures.
Towns and villages in Malaysia with no proper names had adopted the Malay word batu to indicate
their locations along a main road before the use of metric system. Many of their names
remain unchanged even after the adoption of the metric system for distance in the country.
Petrol is still sold by the imperial gallon in Anguilla, Antigua and Barbuda, Burma, 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. 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.
See also References Appendices B and C of NIST Handbook 44
Thompson, A.; Taylor, Barry N.. “The NIST guide for the use of the international system
of units”. also available as a PDF file. NIST. Retrieved 15 October 2012. 
6 George IV chapter 12, 1825 External links
Fast, simple, easy to use and intuitive unit converter
British Weights And Measures Association Canada Weights and Measures Act 1970-71-72
General table of units of measure – NIST – pdf
How Many? A Dictionary of Units of Measurement Statutory Instrument 1995 No. 1804 Units of
Measurement Regulations 1995

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