Collection of units of measurement and rules relating them to each other
A
system of units of measurement
, also known as a
system of units
or
system of measurement
, is a collection of
units of measurement
and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of
science
and
commerce
. Instances in use include the
International System of Units
or
SI
(the modern form of the
metric system
), the
British imperial system
, and the
United States customary system
.
History
[
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]
In antiquity,
systems of measurement
were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in
hands
and
knuckles
. The unifying characteristic is that there was some definition based on some standard. Eventually
cubits
and
strides
gave way to "customary units" to meet the needs of merchants and scientists.
The preference for a more universal and consistent system only gradually spread with the growth of international trade and science. Changing a measurement system has costs in the near term, which often results in resistance to such a change. The substantial benefit of conversion to a more rational and internationally consistent system of measurement has been recognized and promoted by scientists, engineers, businesses and politicians, and has resulted in most of the world adopting a commonly agreed metric system.
The
French Revolution
gave rise to the
metric system
, and this has spread around the world, replacing most customary units of measure. In most systems,
length
(distance),
mass
, and
time
are
base quantities
.
Later science developments showed that an electromagnetic quantity such as
electric charge
or electric current could be added to extend the set of base quantities.
Gaussian units
have only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such as
power
and
speed
, are derived from the base quantities: for example, speed is distance per unit time. Historically a wide range of units was used for the same type of quantity. In different contexts length was measured in
inches
,
feet
,
yards
,
fathoms
,
rods
,
chains
,
furlongs
,
miles
,
nautical miles
,
stadia
,
leagues
, with conversion factors that were not based on power of ten.
In the metric system and other recent systems, underlying relationships between quantities, as expressed by formulae of physics such as
Newton's laws of motion
, is used to select a small number of base quantities for which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten. For example, where the unit of length is the
metre
; a distance of 1 metre is 1,000 millimetres, or 0.001 kilometres.
Current practice
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Metrication is complete or nearly complete in most countries.
However,
US customary units
remain heavily used in the
United States
and to some degree in
Liberia
. Traditional
Burmese units of measurement
are used in
Burma
, with partial transition to the metric system. U.S. units are used in limited contexts in Canada due to the large volume of trade with the U.S. There is also considerable use of imperial weights and measures, despite
de jure
Canadian conversion to metric.
A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom ? whose
road signage legislation
, for instance, only allows distance signs displaying
imperial units
(miles or yards)
[1]
? or Hong Kong.
[2]
In the United States metric units are virtually always used in science, frequently in the military, and partially in industry. U.S. customary units are primarily used in U.S. households. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than grains, are used for medications.
Some other non-
SI
units are still in international use, such as
nautical miles
and
knots
in aviation and shipping, and
feet
for aircraft altitude.
Metric system
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]
Metric systems
of units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries.
Multiples and submultiples of metric units are related by powers of ten and their names are formed with
prefixes
. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.
In the early metric system there were two base units, the
metre
for length and the
gram
for mass. The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units.
Mesures usuelles
(
French
for
customary measurements
) were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was used in France from 1812 to 1839.
A number of variations on the metric system have been in use. These include
gravitational systems
, the
centimetre?gram?second systems
(cgs) useful in science, the
metre?tonne?second system
(mts) once used in the USSR and the
metre?kilogram?second system
(mks). In some engineering fields, like
computer-aided design
, millimetre?gram?second (mmgs) is also used.
[3]
The current international standard for the metric system is the
International System of Units
(
Systeme international d'unites
or SI). It is a system in which all units can be expressed in terms of seven units. The units that serve as the
SI base units
are the
metre
,
kilogram
,
second
,
ampere
,
kelvin
,
mole
, and
candela
.
Imperial and US customary units
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]
Both
imperial units
and
US customary units
derive from earlier
English units
. Imperial units were mostly used in the former
British Empire
and the
British Commonwealth
, but in all these countries they have been largely supplanted by the metric system. They are still used for some applications in the United Kingdom but have been mostly replaced by the metric system in
commercial
,
scientific
, and
industrial
applications. US customary units, however, are still the main system of measurement in the
United States
. While some steps towards
metrication
have been made (mainly in the late 1960s and early 1970s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development.
While imperial and US customary systems are closely related, there are a number of
differences between them
. Units of length and area (the
inch
,
foot
,
yard
,
mile
, etc.) have been identical since the adoption of the
International Yard and Pound Agreement
; however, the US and, formerly, India retained older definitions for surveying purposes. This gave rise to the US survey foot for instance. The
avoirdupois
units of mass and weight differ for units larger than a
pound
(lb). The imperial system uses a stone of 14 lb, a long
hundredweight
of 112 lb and a long
ton
of 2240 lb. The stone is not used in the US and the hundredweights and tons are short: 100 lb and 2000 lb respectively.
Where these systems most notably differ is in their units of volume. A US
fluid ounce
(fl oz), about 29.6
millilitres
(ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a US
pint
and 20 imp fl oz per imperial pint, the imperial pint is about 20% larger. The same is true of
quarts
,
gallons
, etc.; six US gallons are a little less than five imperial gallons.
The
avoirdupois
system served as the general system of mass and weight. In addition to this there are the
troy
and the
apothecaries' systems
. Troy weight was customarily used for
precious metals
,
black powder
and
gemstones
. The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system. The apothecaries' system was traditionally used in
pharmacology
, but has now been replaced by the metric system; it shared the same pound and ounce as the troy system but with different further subdivisions.
Natural units
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]
Natural units
are
units of measurement
defined in terms of universal
physical constants
in such a manner that selected physical constants take on the numerical value of one when expressed in terms of those units. Natural units are so named because their definition relies on only properties of
nature
and not on any human construct. Varying systems of natural units are possible, depending on the choice of constants used.
Some examples are as follows:
Non-standard units
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Non-standard measurement units
also found in books, newspapers etc., include:
Area
[
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]
- The
American football field
, which has a playing area 100
yards
(91.4 m) long by 160 feet (48.8 m) wide. This is often used by the
American
public media for the sizes of large buildings or parks. It is used both as a unit of length (100 yd or 91.4 m, the length of the playing field excluding goal areas) and as a unit of area (57,600 sq ft or 5,350 m
2
), about 1.32
acres
(0.53
ha
).
- British media also frequently uses the
football pitch
for equivalent purposes, although
soccer
pitches are not of a fixed size, but instead can vary within defined limits (100?130 yd or 91.4?118.9 m long, and 50?100 yd or 45.7?91.4 m wide, giving an area of 5,000 to 13,000 sq yd or 4,181 to 10,870 m
2
). However the
UEFA Champions League
field must be exactly 105 by 68 m (114.83 by 74.37 yd) giving an area of 7,140 m
2
(0.714 ha) or 8,539
sq yd
(1.764
acres
). For example, "
HSS vessels
are aluminium catamarans
about the size of a football pitch
."
[4]
- Larger areas are also expressed as a multiple of the areas of certain American states, or subdivisions of the UK etc.
Energy
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]
Units of currency
[
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]
A unit of measurement that applies to
money
is called a
unit of account
in economics and unit of measure in accounting.
[5]
This is normally a
currency
issued by a
country
or a fraction thereof; for instance, the
US dollar
and US cent (
1
⁄
100
of a dollar), or the
euro
and euro cent.
ISO 4217
is the international standard describing three letter codes (also known as the currency code) to define the names of currencies established by the International Organization for Standardization (ISO).
Historical systems of measurement
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]
Throughout history, many official systems of measurement have been used. While no longer in official use, some of these
customary systems
are occasionally used in day-to-day life, for instance in
cooking
.
Africa
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Asia
[
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]
Europe
[
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]
North America
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]
South America
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Ancient
[
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See also
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Notes and references
[
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]
- ^
"Statutory Instrument 2002 No. 3113 The Traffic Signs Regulations and General Directions 2002"
. Her Majesty's Stationery Office (HMSO). 2002
. Retrieved
18 March
2010
.
- ^
HK Weights and Measures Ordinance
- ^
"Units and Dimension Standard - 2021 - SOLIDWORKS Help"
.
help.solidworks.com
. Retrieved
2024-01-11
.
- ^
Henry, Lesley-Anne (23 June 2007).
"Sad sight of a superferry laid up due to soaring jet fuel bills"
.
The
Belfast Telegraph
. Archived from
the original
on 2008-10-06.
- ^
Financial Accounting Standards Research Initiative: The Unit of Account Issue
[
permanent dead link
]
- ^
M. Ismail Marcinkowski,
Measures and Weights in the Islamic World. An English Translation of Professor Walther Hinz's Handbook "Islamische Maße und Gewichte"
, with a foreword by Professor Bosworth, F.B.A. Kuala Lumpur, ISTAC, 2002,
ISBN
983-9379-27-5
. This work is an annotated translation of a work in German by the late German orientalist Walther Hinz, published in the
Handbuch der Orientalistik
, erste Abteilung, Erganzungsband I, Heft 1, Leiden, The Netherlands: E. J. Brill, 1970.
Bibliography
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]
External links
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]
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Current
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Background
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Historic
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Europe
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Asia
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Africa
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North America
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South America
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Ancient
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List articles
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Other
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