The INTERNATIONAL SYSTEM OF UNITS (abbreviated as SI, from the French _Système internationale (d'unités)_) is the modern form of the metric system , and is the most widely used system of measurement . It comprises a coherent system of units of measurement built on seven base units . The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 24th and 25th General Conferences on Weights and Measures (CGPM) in 2011 and 2014, for example, discussed a proposal to change the definition of the kilogram , linking it to an invariant of nature rather than to the mass of a material artefact, thereby ensuring long-term stability.
The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems and the lack of coordination between the various disciplines that used them. The CGPM, which was established by the Metre Convention of 1875, brought together many international organisations to not only agree on the definitions and standards of the new system but also agree on the rules for writing and presenting measurements in a standardised manner around the world.
International System of Units
* 1 History
* 2 SI Brochure and conversion factors
* 3 Units and prefixes
* 4 Writing unit symbols and the values of quantities
* 4.1 Unit names * 4.2 Unit names as adjectives * 4.3 Chinese and Japanese
* 4.4 Unit symbols and the values of quantities
* 4.4.1 General rules * 4.4.2 Printing SI symbols
* 5 Realisation of units
* 6 Post-1960 changes
* 6.1 Changes to the SI * 6.2 Retention of non-SI units
* 7 Global adoption
* 8 Redefinition of units * 9 See also * 10 Notes * 11 References * 12 Further reading * 13 External links
Stone marking the Austro-Hungarian /Italian border at Pontebba
displaying myriametres , a unit of 10 km used in
The metric system was first implemented during the French Revolution
(1790s) with just the metre and kilogram as standards of length and
mass respectively. In the 1830s
Carl Friedrich Gauss
Meanwhile, in 1875, the Treaty of the
In 1948, an overhaul of the metric system was set in motion which resulted in the development of the "Practical system of units" which, on its publication in 1960, was given the name "The International System of Units". In 1954, the 10th General Conference on Weights and Measures (CGPM) identified electric current as the fourth base quantity in the practical system of units and added two more base quantities—temperature and luminous intensity —making six base quantities in all. The units associated with these quantities were the metre , kilogram , second , ampere , kelvin and candela . In 1971, a seventh base quantity, amount of substance represented by the mole , was added to the definition of SI.
The metric system was developed from 1791 onwards by a committee of
French Academy of Sciences , commissioned by the National Assembly
On 30 March 1791, the Assembly adopted the committee's proposed
principles for the new decimal system of measure and authorised a
The law of 7 April 1795 (_loi du 18 germinal_) defined the terms _gramme _ and _kilogramme _, which replaced the former terms _gravet_ (correctly _milligrave_) and _grave_, and on 22 June 1799 (after Pierre Méchain and Jean-Baptiste Delambre had completed the meridian survey) the definitive standard _mètre des Archives _ and _kilogramme des Archives _ were deposited in the French National Archives . On 10 December 1799 (a month after Napoleon\'s coup d\'état ), the law by which the metric system was to be definitively adopted in France (_loi du 19 frimaire_ ) was passed.
During the first half of the nineteenth century there was little consistency in the choice of preferred multiples of the base units – typically the myriametre (7004100000000000000♠10000 metres) was in widespread use in both France and parts of Germany, while the kilogram (7003100000000000000♠1000 grams) rather than the myriagram was used for mass.
In 1832, the German mathematician
Carl Friedrich Gauss
In the 1860s,
James Clerk Maxwell
CGPM vocabulary FRENCH ENGLISH PAGES
étalons standard 5, 95
prototype prototype 5,95
special names 16,106
mise en pratique _mise en pratique_ 82, 171
Main article: Metre Convention
A French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention . :353–354 Initially the convention only covered standards for the metre and the kilogram. A set of 30 prototypes of the metre and 40 prototypes of the kilogram, in each case made of a 90% platinum -10% iridium alloy, were manufactured by the British firm Johnson, Matthey "> Closeup of the National Prototype Metre, serial number 27, allocated to the United States
The treaty established three international organisations to oversee the keeping of international standards of measurement:
General Conference on Weights and Measures (_Conférence
générale des poids et mesures_ or CGPM) – a meeting every four to
six years of delegates from all member states that receives and
discusses a report from the
CIPM and that endorses new developments in
the SI on the advice of the CIPM.
International Committee for Weights and Measures (_Comité
international des poids et mesures_ or CIPM) – a committee that
meets annually at the
In 1921, the Metre Convention was extended to include all physical units, including the ampere and others defined by the Fourth International Conference of Electricians in Chicago in 1893, thereby enabling the CGPM to address inconsistencies in the way that the metric system had been used. :96
TOWARDS THE SI
Main article: Metrication World map showing metrication, colour-coded by year of conversion: from ca 1800 (green) to 1980 (red). Black indicates the nations that have not adopted the SI-system: Burma, Liberia, and the United States. Canada and the United Kingdom both have fairly common use of previously used units, e.g., speed limits in the UK and human height reporting in Canada.
At the close of the 19th century three different systems of units of measure existed for electrical measurements: a CGS-based system for electrostatic units , also known as the Gaussian or ESU system, a CGS-based system for electromechanical units (EMU) and an MKS-based system ("international system") for electrical distribution systems. Attempts to resolve the electrical units in terms of length, mass, and time using dimensional analysis was beset with difficulties—the dimensions depended on whether one used the ESU or EMU systems. This anomaly was resolved in 1900 when Giovanni Giorgi published a paper in which he advocated using a fourth base unit alongside the existing three base units. The fourth unit could be chosen to be electric current , voltage , or electrical resistance .
In the late 19th and early 20th centuries, a number of non-coherent units of measure based on the gram/kilogram, the centimetre/metre, and the second, such as the _ Pferdestärke _ (metric horsepower) for power , the darcy for permeability and the use of "millimetres of mercury " for the measurement of both barometric and blood pressure were developed or propagated, some of which incorporated standard gravity in their definitions.
At the end of the
On the basis of the findings of this study, the 10th
CGPM in 1954
decided that an international system should be derived from six base
units to provide for the measurement of temperature and optical
radiation in addition to mechanical and electromagnetic quantities.
Six base units were recommended: the metre, kilogram, second, ampere,
INTERNATIONAL SYSTEM OF QUANTITIES
The International System of Quantities (ISQ) is a system based on seven base quantities : length , mass , time , electric current , thermodynamic temperature , amount of substance , and luminous intensity . Other quantities such as area , pressure , and electrical resistance are derived from these base quantities by clear non-contradictory equations. The ISQ defines the quantities that are measured with the SI units. The ISQ is defined in the international standard ISO/IEC 80000 , and was finalised in 2009 with the publication of ISO 80000-1 .
SI BROCHURE AND CONVERSION FACTORS
_ Cover of brochure The International System of Units_
The CGPM publishes a brochure which defines and presents SI. Its official version is in French, in line with the Metre Convention . :102 It leaves some scope for local interpretation, particularly regarding names and terms in different languages, so for example the United States' National Institute of Standards and Technology (NIST) has produced a version of the CGPM document (NIST SP 330) which clarifies local interpretation for English-language publications that use American English and another document (NIST SP 811) that gives general guidance for the use of SI in the United States and conversion factors between SI and customary units.
The writing and maintenance of the CGPM brochure is carried out by one of the committees of the International Committee for Weights and Measures (CIPM), the Consultative Committee for Units (CCU). The CIPM nominates the chairman of this committee, but the committee includes representatives of various other international bodies rather than CIPM or CGPM nominees. This committee thus provides a forum for the bodies concerned to provide input to the CIPM in respect of ongoing enhancements to SI.
The definitions of the terms "quantity", "unit", "dimension" etc. that are used in the _SI Brochure_ are those given in the International vocabulary of metrology , a publication produced by the Joint Committee for Guides in Metrology (JCGM), a working group consisting of eight international standards organisations under the chairmanship of the director of the BIPM. The quantities and equations that define the SI units are now referred to as the _International System of Quantities_ (ISQ), and are set out in the International Standard _ ISO/IEC 80000 Quantities and Units _.
UNITS AND PREFIXES
International System of Units
SI base units
SI base units
SI base units
METRE m length
* ORIGINAL (1793): 1/7007100000000000000♠10000000 of the meridian through Paris between the North Pole and the Equator.FG * INTERIM (1960): 7006165076373000000♠1650763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom . * CURRENT (1983): The distance travelled by light in a vacuum in 1/7008299792458000000♠299792458 second.
KILOGRAM kg mass
* ORIGINAL (1793): The GRAVE was defined as being the weight of one cubic decimetre of pure water at its freezing point.FG * CURRENT (1889): The mass of the International Prototype Kilogram (Le Grand K).
SECOND s time
* ORIGINAL (Medieval): 1/7004864000000000000♠86400 of a day. * INTERIM (1956): 1/7007315569259747000♠31556925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time . * CURRENT (1967): The duration of 7009919263177000000♠9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
AMPERE A electric current
* ORIGINAL (1881): A tenth of the electromagnetic CGS unit of current. The electromagnetic unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius, that creates a field of one oersted at the centre. IEC * CURRENT (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 6993200000000000000♠2×10−7 newtons per metre of length.
KELVIN K thermodynamic temperature
* ORIGINAL (1743): The CENTIGRADE SCALE is obtained by assigning 0 °C to the freezing point of water and 100 °C to the boiling point of water. * INTERIM (1954): The triple point of water (0.01 °C) defined to be exactly 273.16 K. * CURRENT (1967): 1/273.16 of the thermodynamic temperature of the triple point of water
MOLE mol amount of substance
* ORIGINAL (1900): The molecular weight of a substance in mass grams.ICAW * CURRENT (1967): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12 .
CANDELA cd luminous intensity
* ORIGINAL (1946): The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre. * CURRENT (1979): The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000♠5.4×1014 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian .
* ^ Interim definitions are given here only when there has been a _significant_ difference in the definition. * ^ Despite the prefix "kilo-", the kilogram is the base unit of mass. The kilogram, not the gram, is used in the definitions of derived units. Nonetheless, units of mass are _named_ as if the gram were the base unit. * ^ In 1954 the unit of thermodynamic temperature was known as the "_degree Kelvin_" (symbol °K; "Kelvin" spelt with an upper-case "K"). It was renamed the "_kelvin_" (symbol "K"; "kelvin" spelt with a lower case "k") in 1967. * ^ When the mole is used, the elementary entities must be specified and may be atoms , molecules , ions , electrons , other particles, or specified groups of such particles.
The original definitions of the various base units in the above table were made by the following authorities:
Main article: Derived units
The derived units in the SI are formed by powers, products or quotients of the base units and are unlimited in number. :103 :3 Derived units are associated with derived quantities, for example velocity is a quantity that is derived from the base quantities of time and length, so in SI the derived unit is metres per second (symbol m/s). The dimensions of derived units can be expressed in terms of the dimensions of the base units.
Coherent units are derived units that contain no numerical factor other than 1—quantities such as standard gravity and density of water are absent from their definitions. In the example above, _one_ newton is the force required to accelerate a mass of _one_ kilogram by _one_ metre per second squared . Since the SI units of mass and acceleration are kg and m·s−2 respectively and _F_ ∝ _m_ × _a_, the units of force (and hence of newtons) is formed by multiplication to give kg·m·s−2. Since the newton is part of a coherent set of units, the constant of proportionality is 1.
For the sake of convenience, some derived units have special names and symbols. Such units may themselves be used in combination with the names and symbols for base units and for other derived units to express the units of other derived quantities. For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa)—and the pascal can be defined as one newton per square metre (N/m2).
RADIAN rad angle
STERADIAN sr solid angle
HERTZ Hz frequency
NEWTON N force , weight
PASCAL Pa pressure , stress N/m2 kg·m−1·s−2
JOULE J energy , work , heat N·m kg·m2·s−2
WATT W power , radiant flux J/s kg·m2·s−3
COULOMB C electric charge or quantity of electricity
VOLT V voltage (electrical potential difference ), electromotive force W/A kg·m2·s−3·A−1
FARAD F capacitance C/V kg−1·m−2·s4·A2
OHM Ω electric resistance , impedance , reactance V/A kg·m2·s−3·A−2
SIEMENS S electrical conductance A/V kg−1·m−2·s3·A2
WEBER Wb magnetic flux V·s kg·m2·s−2·A−1
TESLA T magnetic flux density Wb/m2 kg·s−2·A−1
HENRY H inductance Wb/A kg·m2·s−2·A−2
DEGREE CELSIUS °C temperature relative to 273.15 K
LUMEN lm luminous flux cd·sr cd
LUX lx illuminance lm/m2 m−2·cd
BECQUEREL Bq radioactivity (decays per unit time)
GRAY Gy absorbed dose (of ionizing radiation ) J/kg m2·s−2
SIEVERT Sv equivalent dose (of ionizing radiation ) J/kg m2·s−2
KATAL kat catalytic activity
NOTES 1. The radian and steradian , once given special status, are now considered dimensionless derived units. :3 2. The ordering of this table is such that any derived unit is based only on base units or derived units that precede it in the table.
Main article: Metric prefix
Prefixes are added to unit names to produce multiple and sub-multiples of the original unit. All multiples are integer powers of ten, and above a hundred or below a hundredth all are integer powers of a thousand. For example, _kilo-_ denotes a multiple of a thousand and _milli-_ denotes a multiple of a thousandth, so there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, so for example a millionth of a metre is a _micrometre_, not a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a _milligram_, not a microkilogram. :122 :14
* v * t * e
Standard prefixes for the SI units of measure MULTIPLES PREFIX NAME
DECA HECTO KILO MEGA GIGA TERA PETA EXA ZETTA YOTTA
da h k M G T P E Z Y
FACTOR 100 101 102 103 106 109 1012 1015 1018 1021 1024
FRACTIONS PREFIX NAME
DECI CENTI MILLI MICRO NANO PICO FEMTO ATTO ZEPTO YOCTO
d c m μ n p f a z y
FACTOR 100 10−1 10−2 10−3 10−6 10−9 10−12 10−15 10−18 10−21 10−24
NON-SI UNITS ACCEPTED FOR USE WITH SI
Main article: non-SI units accepted for use with SI
The SI is capable of measurement of any physical quantity, but many non-SI units still appear in the scientific, technical, and commercial literature, and will continue to be used for many years. Some units are so deeply embedded in history and many cultures that they will continue to be used for the foreseeable future. The CIPM recognized and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI , which are grouped as follows: :123–129 :7–11 The litre is classed as a non-SI unit accepted for use with the SI. Being one thousandth of a cubic metre, the litre is not a coherent unit of measure with respect to SI.
* NON-SI UNITS ACCEPTED FOR USE WITH THE SI (Table 6):
Certain units of time, angle, and legacy non-SI metric units have a long history of consistent use. Most societies have used the solar day and its non-decimal subdivisions as a basis of time and, unlike the foot or the pound , these were the same regardless of where they were being measured. The radian , being 1/2π of a revolution, has mathematical advantages but it is cumbersome for navigation, and, as with time, the units used in navigation have a large degree of consistency around the world. The tonne , litre , and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are minute , hour , day , degree of arc , minute of arc , second of arc , hectare , litre , tonne , astronomical unit and bel
* NON-SI UNITS WHOSE VALUES IN SI UNITS MUST BE OBTAINED EXPERIMENTALLY (Table 7).
Physicists often use units of measure that are based on natural phenomena, particularly when the quantities associated with these phenomena are many orders of magnitude greater than or less than the equivalent SI unit. The most common ones have been catalogued in the SI Brochure together with consistent symbols and accepted values, but with the caveat that their values in SI units need to be measured. electronvolt (symbol eV), and dalton/unified atomic mass unit (Da or u)
* OTHER NON-SI UNITS (Table 8):
A number of non-SI units that had never been formally sanctioned by the CGPM have continued to be used across the globe in many spheres including health care and navigation . As with the units of measure in Tables 6 and 7, these have been catalogued by the CIPM in the SI Brochure to ensure consistent usage, but with the recommendation that authors who use them should define them wherever they are used. bar , millimetre of mercury , ångström , nautical mile , barn , knot and neper
* NON-SI UNITS ASSOCIATED WITH THE CGS AND THE CGS-GAUSSIAN SYSTEM OF UNITS (Table 9)
The SI manual also catalogues a number of legacy units of measure that are used in specific fields such as geodesy and geophysics or are found in the literature, particularly in classical and relativistic electrodynamics where they have certain advantages: The units that are catalogued are: erg , dyne , poise , stokes , stilb , phot , gal , maxwell , gauss , and oersted .
WRITING UNIT SYMBOLS AND THE VALUES OF QUANTITIES
Before 1948, the use and representation of metric quantities was inconsistent and ambiguous. In 1879, the CIPM published recommendations for writing the symbols for length, area, volume and mass, but it was outside its domain to publish recommendations for other quantities. Beginning in about 1900, physicists who had been using the symbol "μ" for "micrometre" (or "micron"), "λ" for "microlitre", and "γ" for "microgram" started to use the symbols "μm", "μL" and "μg", but it was only in 1935, a decade after the revision of the Metre Convention that the CIPM formally adopted this proposal and recommended that the symbol "μ" be used universally as a prefix for 6994100000000000000♠10−6.
In 1948, the ninth CGPM approved the first formal recommendation for the writing of symbols in the metric system when the basis of the rules as they are now known was laid down. These rules were subsequently extended by International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) and now cover unit symbols and names, prefix symbols and names, how quantity symbols should be written and used and how the values of quantities should be expressed. :104,130 Both ISO and the IEC have published rules for the presentation of SI units that are generally compatible with those published in the SI Brochure. As of August 2013 ISO and IEC were in the process of merging their standards for quantities and units into a single set of compatible documents identified as the ISO/IEC 80000 Standard. The rules covering printing of quantities and units are part of ISO 80000-1:2009.
Names of units follow the grammatical rules associated with common nouns : in English and in French they start with a lowercase letter (e.g., newton, hertz, pascal), even when the symbol for the unit begins with a capital letter. This also applies to "degrees Celsius", since "degree" is the unit. In German, however, the names of units, as with all German nouns, start with capital letters. The spelling of unit names is a matter for the guardians of the language concerned – the official British and American spellings for certain SI units differ – British English , as well as Australian, Canadian and New Zealand English, uses the spelling _deca-_, _metre_, and _litre_ whereas American English uses the spelling _deka-_, _meter_, and _liter_, respectively. :3
Likewise, the plural forms of units follow the grammar of the language concerned: in English, the normal rules of English grammar are used, e.g. "henries" is the plural of "henry ". :31 However, the units lux , hertz , and siemens have irregular plurals in that they remain the same in both their singular and plural form. Plural forms can be even more complicated in other languages. For example, in Polish the plural form depends on the actual quantity: 0 metrów, 1 metr, 2 metry, 3 metry, 4 metry, 5 metrów, 6 metrów, 7 metrów, 8 metrów, etc. For fractions it is metrów, e.g., 0,67 metrów, 2,45 metrów.
In English, when unit names are combined to denote multiplication of the units concerned, they are separated with a hyphen or a space (e.g. newton-metre or newton metre). The plural is formed by converting the last unit name to the plural form (e.g. ten newton-metres).
UNIT NAMES AS ADJECTIVES
A space between the number and the unit symbol (i.e., no hyphen) is specified when the combination is used as an adjective, e.g. "a 25 kg sphere". :133 However, in English a hyphen would be used as normal in this context if the unit name is spelt out, e.g. "a 25-kilogram sphere". :133
CHINESE AND JAPANESE
Chinese expressway distances road sign in eastern
Chinese uses traditional logograms for writing the unit names, while in Japanese unit names are written in the phonetic katakana script; in both cases, symbols are also written using the internationally recognised Latin and Greek characters . Chinese
The basic Chinese units are metre (米 _mǐ_), litre (升 _shēng_), gram (克 _kè_), and second (秒 _miǎo_), while others include watt (瓦 _wǎ_). Prefixes include _deci-_ (分 _fēn_), _centi-_ (厘 _lí_), _milli-_ (毫 _háo_), _micro-_ (微 _wēi_), _kilo-_ (千 _qiān_), and _mega-_ (兆 _zhào_). These are combined to form disyllabic characters, such as 厘米 _límǐ_ "centimetre" or 千瓦 _qiānwǎ_ "kilowatt". In the 19th century, various compound characters were also used, similar to Japanese, either imported or formed on the same principles, such as 瓩 for 千瓦 _qiānwǎ_ (kilowatt) or 糎 for 厘米. These are generally not used today, but are occasionally found in older or technical writing.
Some units have different names in
A set of characters representing various metric units was created in Japan in the late 19th century . Characters, which are the same in Chinese, exist for three base units: the metre (米), litre (升) and gram (瓦). These were combined with a set of six prefix characters – _kilo-_ (千), _hecto-_ (百), _deca-_ (十), _deci-_ (分), _centi-_ (厘) and _milli-_ (毛) – to form an additional 18 single-character units. The seven length units (kilometre to millimetre), for example, are 粁, 粨, 籵, 米, 粉, 糎 and 粍. These characters, however, are not in common use today; instead, units are written in katakana , the Japanese syllabary used for foreign borrowings, such as キロメートル (kiromētoru) for kilometre, but are also written in standard prefixes such as "km" for kilometre. A few Sino-Japanese words for these units remain in use in Japanese, most significantly "平米" (_heibei_) for "square metre", but otherwise borrowed pronunciations are used.
These characters are examples of the rare phenomenon of single-character loan words – a foreign word represented by a single Japanese character – and form the plurality of such words. Similar characters were also coined for other units, such as British units, though these also have fallen out of use; see Single character gairaigo: Metric units and Single character gairaigo: Other units for a full list.
UNIT SYMBOLS AND THE VALUES OF QUANTITIES
Although the writing of unit names is language-specific, the writing of unit symbols and the values of quantities is consistent across all languages and therefore the SI Brochure has specific rules in respect of writing them. :130–135 The guideline produced by the National Institute of Standards and Technology (NIST) clarifies language-specific areas in respect of American English that were left open by the SI Brochure, but is otherwise identical to the SI Brochure.
General rules for writing SI units and quantities apply to text that is either handwritten or produced using an automated process:
* The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, 7002730000000000000♠7.3×102 m2, 22 K. This rule explicitly includes the percent sign (%) :134 and the symbol for degrees of temperature (°C). : 133 Exceptions are the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″), which are placed immediately after the number with no intervening space. * Symbols are mathematical entities, not abbreviations, and as such do not have an appended period/full stop (.), unless the rules of grammar demand one for another reason, such as denoting the end of a sentence. * A prefix is part of the unit, and its symbol is prepended to the unit symbol without a separator (e.g., k in km, M in MPa, G in GHz). Compound prefixes are not allowed. * Symbols for derived units formed by multiplication are joined with a centre dot (·) or a non-breaking space; e.g., N·m or N m. * Symbols for derived units formed by division are joined with a solidus (/), or given as a negative exponent . E.g., the "metre per second" can be written m/s, m s−1, m·s−1, or m/s. Only one solidus should be used; e.g., kg/(m·s2) and kg·m−1·s−2 are acceptable, but kg/m/s2 is ambiguous and unacceptable.
* The first letter of symbols for units derived from the name of a
person is written in upper case ; otherwise, they are written in lower
case . E.g., the unit of pressure is named after
Blaise Pascal , so
its symbol is written "Pa", but the symbol for mole is written "mol".
Thus, "T" is the symbol for tesla , a measure of magnetic field
strength , and "t" the symbol for tonne , a measure of mass . Since
1979, the litre may exceptionally be written using either an uppercase
"L" or a lowercase "l", a decision prompted by the similarity of the
lowercase letter "l" to the numeral "1", especially with certain
typefaces or English-style handwriting. The American NIST recommends
that within the United States "L" be used rather than "l".
* Symbols of units do not have a plural form; e.g., 25 kg, not 25
* Uppercase and lowercase prefixes are not interchangeable. E.g.,
the quantities 1 mW and 1 MW represent two different quantities; the
former is the typical power requirement of a hearing aid (1 milliwatt
or 0.001 watts), and the latter the typical power requirement of a
suburban train (1 megawatt or 7006100000000000000♠1000000 watts).
* The 10th resolution of
CGPM in 2003 declared that "the symbol for
the decimal marker shall be either the point on the line or the comma
on the line." In practice, the decimal point is used in
English-speaking countries and most of Asia, and the comma in most of
Printing SI Symbols
Further rules are specified in respect of production of text using printing presses , word processors , typewriters and the like.
* Symbols are written in upright (Roman ) type (m for metres, s for
seconds), so as to differentiate from the italic type used for
quantities (_m_ for mass, _s_ for displacement). By consensus of
international standards bodies, this rule is applied independent of
the font used for surrounding text.
* In Chinese , Japanese , and
Korean language computing (CJK ), some
of the commonly used units, prefix–unit combinations, or
unit–exponent combinations have been allocated predefined single
characters taking up a full square.
REALISATION OF UNITS
Metrologists carefully distinguish between the definition of a unit and its realisation. The definition of each base unit of the SI is drawn up so that it is unique and provides a sound theoretical basis on which the most accurate and reproducible measurements can be made. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. A description of the _mise en pratique_ of the base units is given in an electronic appendix to the SI Brochure. :168–169
The published _mise en pratique_ is not the only way in which a base unit can be determined: the SI Brochure states that "any method consistent with the laws of physics could be used to realise any SI unit." :111 In the current (2016) exercise to overhaul the definitions of the base units , various consultative committees of the CIPM have required that more than one _mise en pratique_ shall be developed for determining the value of each unit. In particular:
* At least three separate experiments be carried out yielding values
having a relative standard uncertainty in the determination of the
kilogram of no more than 6992500000000000000♠5×10−8 and at least
one of these values should be better than
6992200000000000000♠2×10−8. Both the
The preamble to the Metre Convention read "_Desiring the international uniformity and precision in standards of weight and measure, have resolved to conclude a convention ..."_. Changing technology has led to an evolution of the definitions and standards that has followed two principal strands – changes to SI itself and clarification of how to use units of measure that are not part of SI, but are still nevertheless used on a worldwide basis.
CHANGES TO THE SI
Since 1960 the CGPM has made a number of changes to SI. These include:
* The 13th CGPM (1967) renamed the "degree Kelvin" (symbol °K) to the "kelvin" (symbol K) :156 * The 14th CGPM (1971) added the mole (symbol mol) to the list of base units. * The 14th GCPM (1971) added the pascal (symbol Pa) for pressure and the siemens (symbol S) for electrical conductance to the list of named derived units. :156 * The 15th CGPM (1975) added the becquerel (symbol Bq) for "activity referred to a radionuclide " and the gray (symbol Gy) for ionizing radiation to the list of named derived units :156 * In order to distinguish between "absorbed dose " and "dose equivalent ", the 16th CGPM (1979) added the sievert (symbol Sv) to the list of named derived units as the unit of dose equivalent. :158 * The 16th CGPM (1979) clarified that in a break with convention either the letter "L" or the letter "l" may be used as a symbol for the litre , in order to avoid the risk of confusion between the lower-case letter and the number 1. :159
* The 21st CGPM (1999) added the katal (symbol kat) for catalytic activity to the list of named derived units. :165 * In its original form (1960), the SI defined prefixes for values ranging from pico- (symbol p) having a value of 10−12 to tera- (symbol T) having a value of 1012. The list was extended at the 12th CGPM (1964), :152 at the 15th CGPM (1975) :158 and at the 19th CGPM (1991) :164 to give the current range of prefixes.
In addition, advantage was taken of developments in technology to redefine many of the base units enabling the use of higher precision techniques.
RETENTION OF NON-SI UNITS
Although, in theory, SI can be used for any physical measurement, it is recognised that some non-SI units still appear in the scientific, technical and commercial literature, and will continue to be used for many years to come. In addition, certain other units are so deeply embedded in the history and culture of the human race that they will continue to be used for the foreseeable future. The CIPM has catalogued such units and included them in the SI Brochure so that they can be used consistently.
The first such group comprises the units of time and of angles and certain legacy non-SI metric units. Most of mankind has used the day and its subdivisions as a basis of time with the result that the second, minute, hour and day, unlike the foot or the pound , were the same regardless of where it was being measured. The second has been catalogued as an SI unit, its multiples as units of measure that may be used alongside the SI. The measurement of angles has likewise had a long history of consistent use – the radian , being 1/2π of a revolution, has mathematical niceties, but it is cumbersome for navigation, hence the retention of the degree, minute and second of arc. The tonne , litre and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols.
Physicists often use units of measure that are based on natural phenomena such as the speed of light, the mass of a proton (approximately one dalton ), the charge of an electron and the like. These too have been catalogued in the SI Brochure with consistent symbols, but with the caveat that their physical values need to be measured.
In the interests of standardising health-related units of measure used in the nuclear industry, the 12th CGPM (1964) accepted the continued use of the curie (symbol Ci) as a non-SI unit of activity for radionuclides; : 152 the becquerel, sievert and gray were adopted in later years. Similarly, the millimetre of mercury (symbol mmHg) was retained for measuring blood pressure. : 127
Top view of the 2 kg weight (with a credit card to show the weight's size) Bottom view of the 2 kg showing the lead plug and assayer's stamp. A commercial-quality hexagonal 2 kg weight with a lead plug and assayer's mark and year of manufacture ("79" representing "1979") that is similar to, but which predates OIML recommendation R52.
SI has become the world's most widely used system of measurement , used in both everyday commerce and science . The change to SI had little effect on everyday life in countries that used the metric system – the metre, kilogram, litre and second remained unchanged as did the way in which they were used – most of the changes only affected measurements in the workplace. The CGPM has a role of recommending changes, but no formal role in the enforcement of such changes—another inter-governmental organisation, the International Organization of Legal Metrology (OIML) provides a forum for harmonisation of national standards and legislation in respect of metrology.
Both the degree and rate of adoption of SI varied from country to
country—countries that had not adopted the metric system by 1960 and
subsequently adopted SI did so directly as part of their metrication
programs while others migrated from the CGS system of units to SI. In
1960, the world's largest economy was that of the United States,
followed by the Soviet Union (although an accurate estimate of 1960
GDP for this country is not available), West Germany, France, Japan,
UNITED KINGDOM AND THE FORMER BRITISH EMPIRE
Even though the use of metric units was legalised for trade in the UK
in 1864, the UK had signed the
Metre Convention in 1884 and the UK
Parliament had defined the yard and the pound in terms of the metre
and the kilogram in 1897, the UK continued to use the imperial system
of measure and to export the imperial system of units to the Empire
. In 1932, the system of
Imperial Preference was set up at the Ottawa
Conference . Although Ireland left the Commonwealth in 1948 and South
Africa in 1961, both continued their close economic ties with the
Metrication logo of the
Board of Trade
When the SI standard was published in 1960, the only major
Commonwealth country to have adopted the metric system was India. In
1863, the first reading of a bill that would have made the metric
system compulsory passed its first reading in the House of Commons by
110 votes to 75. The bill, however, failed to make the statute book
because of lack of parliamentary time. :136 In 1965, after this and
similar false starts the then
Federation of British Industry informed
the British Government that its members favoured the adoption of the
metric system. The rationale behind the request was that 80% of
British exports were to countries that used the metric system or that
were considering changing to the metric system. The
Board of Trade
By 1980 all apart from the United Kingdom, Canada and Ireland had effectively completed their programs. In the United Kingdom the breakdown of voluntary metrication in the mid-1970s :§1.8 coincided with the United Kingdom's obligations as part of the EEC to adopt the metric system, resulting in legislation to force metrication in certain areas and the Eurosceptic movement adopting an anti-metrication stance and the United Kingdom seeking a number of derogations from the relevant EEC directives. Once the metrication of most consumer goods was completed in 2000, aspects of British life, especially in government, commerce and industry used SI. :§1.6 otherwise, the situation in Ireland is similar to that in the United Kingdom.
Canada has adopted it for most purposes, but imperial units are still legally permitted and remain in common use throughout a few sectors of Canadian society, particularly in the buildings, trades and railways sectors.
Main article: European units of measurement directives
In 1960, all the largest industrialised nations that had an established history of using the metric system were members of the European Economic Community (EEC).
In 1972, in order to harmonise units of measure as part of a programme to facilitate trade between member states, the EEC issued directive 71/354/EEC . This directive catalogued units of measure that could be used for "economic, public health, public safety and administrative purposes" and also provided instructions for a transition from the existing units of measure that were in use. The directive replicated the CGPM SI recommendations and in addition pre-empted some of the additions whose use had been recommended by the CIPM in 1969, but had not been ratified by the CGPM. The directive also catalogued units of measure whose status would be reviewed by the end of 1977 (mainly coherent CGS units of measure) and also catalogued units of measure that were to be phased out by the end of 1977, including the use of obsolete names for the sale of timber such as the stere , the use of units of force and pressure that made use of the acceleration due to gravity, the use of non-coherent units of power such as the Pferdestärke (PS) , the use of the calorie as a measure of energy and the stilb as a measure of luminance . The directive was silent in respect of units that were specific to one or two countries including the _pond_, _pfund_, _livre_ (Dutch, German and French synonyms for 500 g), thereby effectively prohibiting their use as well.
When the directive was revisited during 1977, some of the older units that were being reviewed (such as millimetre of mercury for blood pressure ) were retained but others were phased out, thereby broadly aligning the allowable units with SI. The directive was however overhauled to accommodate British and Irish interests in retaining the imperial system in certain circumstances. It was reissued as directive 80/181/EEC . During subsequent revisions, the directive has reflected changes in the definition of SI. The directive also formalised the use of _supplementary units_, which in 1979 were permitted for a period of ten years. The cut-off date for the use of supplementary units was extended a number of times and in 2009 was extended indefinitely.
Four years after the Indian Government announced its metrication programme, SI was published. The result was that the initial metrication programme was a conversion to the CGS system of units and the subsequent adoption of SI has been haphazard. Originally the Indian Government had planned to replace all units of measure with metric units by 1960. In 1976 a new Weights and Measures Act replaced the 1956 Act which, amongst other things, required that all weighing devices be approved before being released onto the market place. However, in 2012, it was reported that traditional units were still encountered in small manufacturing establishments and in the marketplace alongside CGS, SI and imperial measures, particularly in the poorer areas.
The use of the Indian numbering system of crores (7007100000000000000♠10000000) and lakhs (7005100000000000000♠100000), which do not map onto the SI system of prefixes, is widespread and is often found alongside or in place of the western numbering system.
Metrication in the United States Logo of the
Metrication Council, an organisation set up by
American National Standards Institute
Even though Congress set up a framework for the use of the metric
system in the nineteenth century, the United States continues to use
US customary units , based on English measure passed by parliament
under the reign of Queen Anne in 1706, for most purposes apart from
science and medicine. In
On 10 February 1964, the National Bureau of Standards (now the National Institute of Standards and Technology ) issued a statement that it was to use SI except where this would have an obvious detrimental effect. In 1968 Congress authorised the U.S. Metric Study the emphasis of which was to examine the feasibility of adopting SI. The first volume was delivered in 1970. The study recommended that the United States adopt the International System of units, and in 1975 Congress passed the Metric Conversion Act of 1975 , which established a national policy of coordinating and planning for the increased use of the metric measurement system in the United States. Metrication was voluntary and to be coordinated by the United States Metric Board (USMB).
Efforts during the Ford and Carter administrations to force
metrication were seized on by many newspaper editorialists as being
dictatorial. :365 Public response included resistance, apathy, and
sometimes ridicule. The underlying reasons for this response include
a relative uniformity of weights and measures (though, notably, US
liquid measure differed by about 20% from British Imperial measure,
which was adopted throughout the
The 1988 Omnibus Foreign Trade and Competitiveness Act removed international trade barriers and amended the Metric Conversion Act of 1975, designating the metric system as "the Preferred system of weights and measures for United States trade and commerce". The legislation stated that the federal government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement. Exceptions were made for the highway and construction industries; the Department of Transportation planned to require metric units by 2000, but this plan was cancelled by the 1998 highway bill TEA21 . However, the US military uses the metric system widely, partly because of the need to work with armed services from other nations. Although overall responsibility for labelling requirements of consumer goods lies with Congress and is therefore covered by federal law , details of labelling requirements for certain commodities are controlled by state law or by other authorities such as the Food and Drug Administration , Environmental Protection Agency and Alcohol and Tobacco Tax and Trade Bureau . The federal Fair Packaging and Labeling Act (FPLA), originally passed in 1964, was amended in 1992 to require consumer goods directly under its jurisdiction to be labelled in both customary and metric units. Some industries are engaged in efforts to amend this law to allow manufacturers to use only metric labelling. The National Conference on Weights and Measures has developed the Uniform Packaging and Labeling Regulations (UPLR) which provides a standard approach to those sections of packaging law that are under state control. Acceptance of the UPLR varies from state to state – fourteen states accept it by merely citing it in their legislation.
During the first decade of the 21st century, the EU directive 80/181/EEC had required that dual unit labelling of goods sold within the EU cease by the end of 2009. This was backed up by requests from other nations including Japan and New Zealand to permit metric-only labelling as an aid to trade with those countries. Opinion in the United States was split – a bill to permit metric-only labelling at the federal level was to have been introduced in 2005 but significant opposition from the Food Marketing Institute , representing US grocers, has delayed the introduction of the bill. During a routine decennial review of the directive in 2008, the EU postponed the sunset clause for dual units indefinitely.
Meanwhile, in 1999 the UPLR was amended to permit metric-only
labelling and automatically became law in those states that accept
UPLR "as is". By 1 January 2009, 48 out of 50 states permit
metric-only labelling, either through UPLR or through their own
legislation. As of February 2013 the use of metric (and therefore
SI) units in the United States does not follow any pattern. Dual-unit
labelling on consumer goods is mandatory. Some consumer goods such as
soft drinks are sold in metric quantities, but milk is sold in
customary units. The engineering industry is equally split. The
automotive industry is largely metric, but aircraft such as the
Boeing 787 Dreamliner
REDEFINITION OF UNITS
Dependencies of proposed SI unit definitions (in colour) and seven physical constants (in grey) with fixed numerical values. Unlike the current (2014) definition, the base units are derived from one or more constants of nature. Main article: Proposed redefinition of SI base units
After the metre was redefined in 1960, the kilogram remained the only
SI base unit
At its 23rd meeting, held in 2007, the CGPM recommended that the CIPM should continue to investigate methods to provide exact fixed values for physical constants of nature that could then be used in the definitions of units of measure in place of the IPK, thus enabling the transition from explicit unit definitions to explicit constant definitions.
At a meeting of the CCU held in Reading, United Kingdom , in September 2010, a resolution and draft changes to the SI Brochure that were to be presented to the next meeting of the CIPM in October 2010 were agreed to in principle. The proposals that the CCU put forward were:
* In addition to the speed of light, four constants of nature – the Planck constant , an elementary charge , the Boltzmann constant and the Avogadro number – be defined to have exact values. * The International prototype kilogram be retired * The current definitions of the kilogram, ampere , kelvin and mole be revised. * The wording of the definitions of all the base units be both tightened up and changed to reflect the change in emphasis from explicit unit to explicit constant definitions.
The CIPM meeting of October 2010 reviewed progress towards establishing fixed values for the constants but found that "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. For this reason the CIPM does not propose a revision of the SI at the present time".
At the 24th CGPM meeting, held in October 2011, the CIPM sponsored a resolution in which the requisite definition changes were agreed to in principle and in which the conditions required to be met before the redefinitions could be implemented were restated.
By November 2014 the conditions set out at the 23rd meeting of the CGPM for the unit redefinitions had still not been met, and the 25th meeting of the CGPM, held in November 2014, adopted a similar resolution encouraging further work towards establishing fixed values for the fundamental constants.
The redefinitions are expected to be adopted at the 26th CGPM in the fall of 2018. The CODATA task group on fundamental constants has announced special submission deadlines for data to compute the values that will be announced at this event.
History of measurement
List of international common standards
List of scientific units named after people
Metre–tonne–second system of units
Names of large numbers
Names of small numbers
Order of magnitude
STANDARDS AND CONVENTIONS
* ^ The differences between "weight" and "mass" were only formally qualified in 1901. * ^ The 8th edition of the SI Brochure (2008) notes that the term "_mise en pratique_" had not been fully defined. * ^ The text "_Des comparaisons périodiques des étalons nationaux avec les prototypes internationaux_" (English: the periodic comparisons of national standards with the international prototypes) in article 6.3 of the Metre Convention distinguishes between the words "standard" (OED: "The legal magnitude of a unit of measure or weight") and "prototype" (OED: "an original on which something is modelled"). * ^ _Pferd_ is German for "horse" and _stärke_ is German for "strength" or "power". The Pferdestärke is the power needed to raise 75 kg against gravity at the rate of one metre per second. (1 PS = 0.985 HP).
* ^ These bodies include:
International Organization for Standardization (ISO)
National Institute of Standards and Technology (NIST) (United
* National Physical Laboratory (NPL) (British)
International Astronomical Union (IAU)
International Union of Pure and Applied Chemistry (IUPAC)
International Union of Pure and Applied Physics (IUPAP)
International Commission on Illumination
* ^ This grouping and the reference to Tables 6, 7, 8, and 9
reflects the 2014 revision of the 8th Edition of the SI Brochure
* ^ The
CGPM have defined the metre in terms of the speed of light,
so the speed of light has an exact value.
* ^ For example, the
Académie française in the case of French or
Council for German Orthography (German : _Rat für deutsche
Rechtschreibung_) in the case of German
* ^ _A_ _B_ Except where specifically noted, these rules are common
to both the SI Brochure and the NIST brochure.
* ^ This term is a translation of the official text of the SI
* ^ The
CGPM has defined the metre in terms of the speed of light,
so the speed of light has an exact value.
* ^ These countries included the Commonwealth countries (excluding
* ^ "Convocation of the General Conference on Weights and Measures
(25th meeting)" (PDF). International Bureau of Weights and Measures.
p. 32. Retrieved 27 May 2014.
* ^ _A_ _B_ "Amtliche Maßeinheiten in Europa 1842" (in German).
Retrieved 26 March 2011Text version of Malaisé's book
* ^ Ferdinand Malaisé (1842). _Theoretisch-practischer Unterricht
im Rechnen_ (in German). München. pp. 307–322. Retrieved 7 January
* ^ "The name "kilogram"". International Bureau of Weights and
Measures . Retrieved 25 July 2006.
* ^ _A_ _B_ _C_ _D_ Alder, Ken (2002). _The Measure of all
Things—The Seven-Year-Odyssey that Transformed the World_. London:
Abacus. ISBN 0-349-11507-9 .
* ^ Quinn, Terry (2012). _From artefacts to atoms: the
* ^ Wilkins, John (1668). "VII". _An Essay towards a Real Character
and a Philosophical Language_. The Royal Society. pp. 190–194.
"Reproduction (33 MB)" (PDF). Retrieved 6 March 2011. ;
"Transcription (126 kB)" (PDF). Retrieved 6 March 2011. * ^
"Mouton, Gabriel". _Complete Dictionary of Scientific Biography_.
encyclopedia.com . 2008. Retrieved 30 December 2012.
* ^ O\'Connor, John J. ; Robertson, Edmund F. (January 2004),
"Gabriel Mouton", _
MacTutor History of Mathematics archive _,
University of St Andrews
* ^ "DYKT: Did You Know That". Lamar.colostate.edu. 5 June 2008.
Retrieved 21 January 2013.
* ^ "Guide for Identification and Development of Metric Standards"
(PDF). Office of the Under Secretary of Defense Acquisition,
Technology & Logistics. December 2003: 3. SD-10. Retrieved 22 January
* ^ _A_ _B_ _C_ Gentry, Elizabeth J. "Voluntary Metric Labeling"
(PDF). National Institute of Standards and Technology. pp. 3–6.
Retrieved 28 January 2013.
* ^ "Forum on Permissible Metric-Only Labeling". NIST. 4 October
2006. Archived from the original on 10 October 2006. Retrieved 22
* ^ Frysinger, James R. (2010). "The Uniform Packaging and Labeling
Regulation (UPLR)". Metric Methods. Retrieved 28 January 2013.
* ^ Greenslade, Joe (August 2012). "Why Specify ISO Standards For
Metric Fasteners?". _American Fastener Journal_. Scotsdale, Arizona:
48–55. ISSN 1064-3834 . Retrieved 16 August 2013. The decision by
General Motors, FORD, and Chrysler to adopt the metric system of
measurement in design impacted all industrialized countries in the
world. The car manufacturers wanted to be able to source products
anywhere in the world and have the components be compatible regardless
of where the parts were made, purchased or assembled.
* ^ Reisman, Lisa (16 August 2011). "Not So Fast, Comac: C919 is
DOA, But Boeing and Airbus Duopoly Dead Anyway". _MetalMiner_. Azul
Partners. Retrieved 16 August 2013.
* ^ "Redefining the kilogram". UK National Physical Laboratory.
Retrieved 30 November 2014.
* ^ Newell, David B. "A more fundamental International System of
Units" (PDF). Physics Today. Retrieved 30 November 2014.
* ^ Mills, Ian (September–October 2011). "Part
II—Explicit-Constant Definitions for the
International Union of Pure and Applied Chemistry (1993).
Quantities, Units and Symbols in Physical Chemistry _, 2nd edition,
Oxford: Blackwell Science. ISBN 0-632-03583-8 . Electronic version.
* Unit Systems in Electromagnetism
* MW Keller _et al._
Metrology Triangle Using a
_ Wikimedia Commons has media related to INTERNATIONAL SYSTEM OF UNITS _.
* ISO 80000-1:2009 _Quantities and units – Part 1: General_
* NIST Official Publications