The International System of Units And its base units PDF generated using the open source mwlib toolkit. See http://code. pediapress.
com/ for more information. PDF generated at: Wed, 08 May 2013 17:06:00 UTC Contents Articles Overview International System of Units 1 1 20 20 30 35 37 39 39 42 51 69 75 78 82 86 90 90 93 Organisations Metre Convention General Conference on Weights and Measures International Bureau of Weights and Measures International Committee for Weights and Measures Base unitsSI base unit Metre Kilogram Second Ampere Kelvin Mole Candela Apprendix SI derived unit Units accepted for use with SI References Article Sources and Contributors Image Sources, Licenses and Contributors 95 98 Article Licenses License 101 1 Overview International System of Units For a topical guide to this subject, see Outline of the metric system. The International System of Units (abbreviated SI from French: Le Systeme international d’unites) is the modern form of the metric system.It comprises a coherent system of units of measurement built around seven base units, 22 named and an indeterminate number of unnamed coherent derived units, and a set of prefixes that act as decimal-based multipliers.
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The standards, published in 1960, are based on the metre-kilogram-second system, rather than the centimetre-gram-second system, which, in turn, had several variants. The SI has been declared to be an evolving system; thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.SI is the world’s most widely used system of measurement, used in both everyday commerce and science.  The seven SI base units and the interdependency of their definitions. Clockwise from top: kelvin (temperature), second (time), metre (length), kilogram (mass), candela (luminous intensity), mole (amount of substance) and ampere (electric current). The system has been nearly globally adopted. Only Burma, Liberia and the United States have not adopted SI units as their official system of weights and measures.
In the United States metric units are not commonly used outside of science, medicine and the government; however, United States customary units are officially defined in terms of SI units. The United Kingdom has officially adopted a partial metrication policy, with no intention of replacing imperial units entirely. 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. 5] History The metric system was first implemented during the French Revolution (1790s) with just the metre and kilogram as standards. In the 1860s British scientists, working through the British Association for the Advancement of Science laid the foundations for a coherent system based on length, mass and time, but the inclusion of electrical units into the system was hampered until 1900 when Giorgi identified the need to define an electrical quantity alongside the original three quantities.Meanwhile, in 1875, the Treaty of the Metre passed custodianship of the prototype kilogram and metre from French to international control. In 1921 the Treaty was extended to include all physics measurements International System of Units and in 1948 an overhaul of the metric system was set in motion which resulted in the publication of the International System of Units in 1960. 2 Uncoordinated developmentThe metric system was developed from 1791 onwards by a group of scientists commissioned by the Assemblee nationale and Louis XVI of France to create a unified and rational system of measures.
 The group, which included Antoine-Laurent Lavoisier (the “father of modern chemistry”) and the mathematicians Pierre-Simon Laplace and Adrien-Marie Legendre, used the principles that had been proposed by the English cleric John Wilkins in 1668 and naming concepts developed from those proposed in 1670 by the French cleric Gabriel Mouton. 12] On 1 August 1793, the National Convention adopted the new decimal metre with a provisional length as well as the other decimal units with preliminary definitions and terms. The law of 7 April 1795 (Loi du 18 germinal, an III) defined the terms gramme and kilogramme which replaced the former terms gravet (correctly milligrave) and grave, and on 22 June 1799, after Pierre Mechain and Jean-Baptiste Delambre completed their survey, the definitive standard metre was deposited in the French National Archives.On 10 December 1799 (a month after Napoleon’s coup d’etat), the law by which metric system was to be definitively adopted in France 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 (10,000 metres) was in widespread use in both France and parts of Germany, while the kilogram (1,000 grams) rather than the myriagram was used for mass. ] In 1832 Carl Friedrich Gauss implicitly defined a coherent system of units when he measured the earth’s magnetic field in absolute units quoted in terms of millimetres, grams, and seconds.
 In the 1860s James Clerk Maxwell and William Thomson (later Lord Kelvin), working through the British Association for the Advancement of Science formulated the concept of a coherent system of units with base units and derived units.The principle of coherence was successfully used to define a number of units of measure based on the centimetre–gram–second (cgs) system of units (cgs) including the erg for energy, the dyne for force, the barye for pressure, dynamic viscosity in poise and the kinematic viscosity in stokes.  Old boundary stone in Pontebba, marking the former border between Austria-Hungary and Italy; the myriametre (10 km), since deprecated, was in common use in Central Europe during the  mid-nineteenth century. Metre ConventionThe desire for international cooperation in metrology led to the signing in 1875 of the Metre Convention, a treaty that established three international organisations to oversee the keeping of metric standards: • General Conference on Weights and Measures (Conference generale des poids et mesures or CGPM) – a meeting every four to six years of delegates from all member states; • International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) – an international metrology centre at Sevres in France; and • International Committee for Weights and Measures (Comite international des poids et mesures or CIPM) – an administrative committee that meets annually at the BIPM. Initially the convention only covered standards for the metre and the kilogram, new prototypes of which were manufactured by the British firm Johnson, Matthey ; Co and accepted by the GCPM in 1889. It was only in 1921 International System of Units that the convention was extended to include all physical units that the CGPM was able to address inconsistencies in the way that the metric system had been used.
 3 Towards SI In the nineteenth century, attempts to produce a coherent set of electrical units was beset with difficulties.At the close of the nineteenth century three different systems of units of measure existed for electrical measurements – a CGS-based system for electrostatic units (also known as the Gaussian system), a CGS-based system for electromechanical units and an MKS-based system (the “International system”) for electrical distribution systems. In 1900 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 either electric current or voltage or electrical resistance.  William Thomson, later Lord Kelvin, who, with James Clerk Maxwell, was one of the most influential figures in the theoretical development of the metric system.In the late nineteenth and early twentieth centuries a number of non-coherent units of measure were developed such as the Pferdestarke or “metric horsepower” for power, the darcy for permeability and the use of “millimetres of mercury” for the measurement of both barometric and blood pressure. All these units incorporate standard gravity in their definitions.
At the end of Second World War, a number of different systems of measurement were in use throughout the world. Some of these systems were metric system variations, whereas others were based on customary systems of measure. It was recognised that additional steps were needed to promote a worldwide measurement system.After representations by the International Union of Pure and Applied Physics (IUPAP) and by the French Government, the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.  James Clerk Maxwell, who, with William Thomson, later Lord Kelvin, was one of the most influential figures in the theoretical development of the metric system.
Based on 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.The six base units that were recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name, Le Systeme international d’unites.  The BIPM has also International System of Units described SI as “the modern metric system”.  The seventh base unit, the mole, was added in 1971 by the 14th CGPM.
 4 SI Brochure and conversion factors The CGPM have published a brochure, the 8th edition of which appeared in 2006, in which the various recommendations that make up SI have been codified.  This brochure leaves some scope for local interpretation, particularly in respect of language.The United States National Institute of Standards and Technology has produced a version of the CGPM document (NIST SP 330) which clarifies local interpretation in respect of 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.
 The writing and maintenance of the CGPM brochure is carried out by one of the consultative 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 its membership is made up of representatives of various other international bodies rather than CIPM or CGPM nominees.  This committee also provides a forum for the bodies concerned to provide input to the CIPM in respect of on-going enhancements to SI. In 2010 the CCU proposed a number of changes to the definitions of the base units used in SI. 31] The CIPM meeting of October 2010 found that the proposal was not complete, and it is expected that the CGPM will consider the full proposal in 2014.
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 ISO/IEC 80000 Quantities and Units.Appendix B of NIST SP 811, a list of conversion factor between SI and customary units, is an extension to the SI Brochure.
 Units and prefixes The International System of Units consists of a set of base units, a set of derived units, some of which have special names and a set decimal-based multipliers that are denoted as prefixes. The term “SI Units” includes all three categories, but the term “coherent SI units” includes only base units and coherent derived units.  Base units Base units are the building blocks of SI – all other units of measure can be derived from the base units. When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units – mass, length and time.Giorgi later identified the need for an electrical base unit – theoretically electrical current, potential difference, electrical resistance, electrical charge or any one of a number of other units could have been used as the base unit with the remaining units being then defined by the laws of physics – the unit of electric current was chosen for SI.
The remaining three base units were added later. International System of Units 5 SI base units Unit name metre Unit symbol m length Quantity name • Definition (Incomplete) Dimension symbol L • Original (1793): 110000000 of the meridian through Paris between the North Pole and the EquatorFG Current (1983): The distance travelled by light in vacuum in 1299792458 second Original (1793): The grave was defined as being the weight [mass] of one cubic decimetre of pure water at its freezing point.FG Current (1889): The mass of the International Prototype Kilogram Original (Medieval): 186400 of a day Current (1967): The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom Original (1881): A tenth of the electromagnetic CGS unit of current. The [CGS] emu unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius creates a field of one oersted at  IEC the centre.
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 2? 10? newton per metre of length Original (1743): The centigrade scale is obtained by assigning 0° to the freezing point of water and 100° to the boiling point of water. Current (1967): The fraction 1273. 16 of the thermodynamic temperature of the triple point of water 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. kilogram  kg mass • M • second s time • • T ampere A electric current • I • kelvin K thermodynamic temperature • ? • mole mol amount of substance • • N International System of Units 6 cd luminous intensity J candela • •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 540? 1012 hertz and that has a radiant intensity in that direction of 1683 watt per steradian. Note The original definitions of the various base units in the above table were made by the following authorities: • • • FG = French Government IEC = International Electrotechnical Commission ICAW = International Committee on Atomic Weights Derived unitsDerived units are formed by powers, products or quotients of the base units and are unlimited in number; Derived units are associated with derived quantities, for example velocity is a quantity that is derived from the base quantities of time and distance which, in SI, has the dimensions metres per second (symbol m/s). The dimensions of derived units can be expressed in terms of the dimensions of the base units. Some derived units have special names, for example the unit of force is the newton. Coherent units (such as those in SI) are derived units that contain no numerical factor other than 1: 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. Named units derived from SI base units Name Symbol Quantity Expressed in terms of other SI units 1 1 Expressed in terms of SI base units m/m m2/m2 s? 1 kg? m? s? 2 N/m2 N? m J/s kg? m? 1? s? 2 kg? m2? s? 2 kg? m2? s? 3 s? A W/A kg? m2? s? 3? A? 1 kg? 1? m? 2? s4? A2 radian steradian hertz newton pascal joule watt coulomb volt rad sr Hz N Pa J W C V angle solid angle frequency force, weight pressure, stress energy, work, heat power, radiant flux electric charge or quantity of electricity voltage (electrical potential difference), electromotive force electric capacitance farad F C/V International System of Units 7 S Wb T H °C lm lx Bq Gy Sv kat electric resistance, impedance, reactance electrical conductance magnetic flux magnetic field strength inductance temperature relative to 273. 15 K luminous flux illuminance radioactivity (decays per unit time) absorbed dose (of ionizing radiation) equivalent dose (of ionizing radiation) catalytic activity J/kg J/kg cd? sr lm/m2 V/A A/V V? s Wb/m2 Wb/A ohm siemens weber tesla henry degree Celsius lumen lux becquerel gray sievert katal kg? m2? s? 3? A? 2 kg? 1? m? 2? s3? A2 kg? m2? s? 2? A? 1 kg? s? 2? A? 1 kg? m2? s? 2? A? 2 K cd m? 2? cd s? 1 m2? s? 2 m2? s? 2 s? 1? mol Notes  1. The radian and steradian, once given special status, are now considered dimensionless derived units. 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. Prefixes A prefix may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten, and beyond a hundred(th) all are integer powers of a thousand. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth; hence there are one thousand millimetres to the metre and one thousand metres to the kilometre.
The prefixes are never combined, and multiples of the kilogram are named as if the gram was the base unit. Thus a millionth of a metre is a micrometre, not a millimillimetre, and a millionth of a kilogram is a milligram, not a icrokilogram.  Standard prefixes for the SI units of measure Multiples Name Prefix deca- hecto- kiloda h 102 k 103 megaM 106 giga- teraG 109 T 1012 petaP 1015 exaE 1018 zetta- yottaZ 1021 Y 1024 Factor 100 101 Fractions Name Prefix deci- centi- milli- micro- nano- pico- femto- atto- zepto- yoctod c 10? 2 m 10? 3 ? 10? 6 n 10? 9 p f a z y 10? 24 Factor 100 10? 1 10? 12 10? 15 10? 18 10? 21 Non-SI units accepted for use with SI 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 a number of such non-SI units accepted for use with SI and published them in the SI Brochure thereby ensuring that their use is consistent across the globe. International System of Units These units have been grouped as follows: • Non-SI units accepted for use with the SI (Table 6): Certain units of time, angles and legacy non-SI metric units have a long history of consistent use.
Most of mankind has used the 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 it was being measured. The radian, being 12? f a revolution has mathematical niceties, 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 and tonne • 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 physical values need to be measured. : electronvolt, dalton/unified atomic mass unit, astronomical unit, speed of light, Planck constant and electron mass • 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.
ar, millimetre of mercury, angstrom, nautical mile, barn, knot, neper and [deci]bel • 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 catalolgued are: erg, dyne, poise, stokes, stilb, phot, gal, maxwell, gauss and ? rsted. 8 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.Writing unit symbols and the values of quantities Before 1948, the writing of metric quantities was haphazard. 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 “? ” for “micrometre” (or “micron”), “? ” for “microlitre”, and “? ” for “microgram” started to use the symbols “? m”, “? L” and “? g”, 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 “? ” be used universally as a prefix for 10? 6. 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.  When, in 1960, the International System of International System of Units Units was introduced, the rules that were put in place in 1948 were adapted for use with the new system.
Since then the rules, apart from some minor modifications, have remained in place. 9 Writing the unit names The CGPM rules state that the 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, just like 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 uses the spelling deca-, metre, and litre whereas American English uses the spelling deka-, meter, and liter, respectively.  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.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). Representation of SI units in Chinese and Japanese In Japanese and Chinese there exist logograms and the rules for the writing unit names adapted to suit the languages. Japanese A set of characters representing various metric units was created in Japan in the late 19th century. Characters exist for three base units: the metre (? ), litre (? ) and gram (? ). These were combined with a set of six prefix characters—kilo- (? ), hecto- (? ), deca(? ), deci- (? ), centi- (? ) and milli- (? —to form an additional eighteen single-character units. The seven length units (kilometre to millimetre), for example, are ? , ? , ? , ? , ? , ? and ?.
These characters, however, are not in common use today instead units are written out in katakana, a Japanese syllabary. Chinese The basic units are ? mi “metre”, ? sheng “litre”, ? ke “gram”, and ? miao “second”. Some sample prefixes are ? fen “deci”, ? li “centi”, ? hao “milli”, and ? wei “micro”. These are not combined into a single character, so for example centimetres are simply ? ? limi.  The symbols for the metric units are internationally recognised Latin characters or, in respect of “? ” or “? , Greek characters. Chinese expressway distances road sign in eastern Beijing. Although the primary text is in Chinese, the distances use the internationally recognised numerals and symbols. Writing 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 as such the SI brochure has specific rules in respect of writing them.
 The guideline produced by 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. International System of Units General rules General rules for writing SI units and quantities apply to text that is either hand-written or produced using an automated process: • 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-break space; for example “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 ms. 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. For example, 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 teslas, a measure of magnetic field strength and “t” the symbol for tonnes, a measure of mass.Since 1979 the litre may exceptionally be written using either an upper case “L” or a lower case “l”, a decision prompted by the similarity of the lower case letter “l” to the numeral “1”, especially with certain type-faces or English-style handwriting. The American National Institute of Standards and Technology recommends that within the United States “L” be used rather than “l”. 10 Acceleration due to gravity.
Note the lower case letters (neither “metres” nor “seconds” were named after people), the space between the value and the units and the superscript “2” to denote “squared” • Symbols of units do not have a plural form; e. g. , “25 kg”, not “25 kgs”.
Upper-case and lower-case prefixes are not interchangeable—the quantities “1 mW” and “1 MW” represent two different quantities, the former is the typical power requirement of a hearing aid and the latter typical power requirement of a suburban train. • 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 Latin America and in continental European languages.  • Spaces should be used as a thousands separator (1000000) in contrast to commas or periods (1,000,000 or 1.
000. 00) to reduce confusion resulting from the variation between these forms in different countries. • Any line-break inside a number, inside a compound unit, or between number and unit should be avoided. Where this is not possible, line breaks should coincide with thousands separators. • Since the value of “billion” and “trillion” can vary from language to language, the dimensionless terms “ppb” (parts per billion) and “ppt” (parts per trillion) should be avoided. However, no alternative is suggested in the SI Brochure. International System of Units 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. • 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”, “7.
3? 102 m2″, “22 K”. This rule explicitly includes the per cent sign (%) and the symbol for degrees of temperature (°C).  Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ? and ? ), which are placed immediately after the number with no intervening space. 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. Unicode includes these in its CJK Compatibility (http://www.
unicode. org/charts/PDF/U3300. pdf) and Letter like Symbols (http://www. unicode. org/charts/PDF/U2100. pdf) subranges for back compatibility, without necessarily recommending future usage.
These are summarised in Unicode symbols. The cursive ? , a letter-like symbol, has been used in a number of countries in addition to China and Japan as a symbol for the litre, but this is not currently recommended by any standards body. In print, the space used as a thousands separator (commonly called a thin space) is typically narrower than that used between words. 11 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 practical realisation (French: Mise en pratique) of the base units is given in an electronic appendix to the SI brochure.
18] 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. “ In the current (2012) 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 5? 10? 8 and at least one of these values should be better than 2? 10? 8. Both the Watt balance and the Avogadro project should be included in the experiments and any differences between these be reconciled. A silicon sphere for the Avogadro project used for measuring the Avogadro constant to a relative  uncertainty of 2? 10? 8 or less. • When determining the kelvin, the relative uncertainty of Boltzmann constant derived from two fundamentally different methods such as acoustic gas thermometry and dielectric constant gas thermometry be better than one part in 10? 6 and that these values be corroborated by other measurements.  International System of Units 12 Post 1960 changes 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 .
.. “. 23] 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) • 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.  • 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 ist of named derived units  • In order to distinguish between “absorbed dose” and “dose equivalent”, the 16th CGPM added the sievert (symbol Sv) to the list of named derived units as the unit of dose equivalent.
 • 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.  • The 21st CGPM (1999) added the katal (symbol kat) for catalytic activity to the list of named derived units.  • In its original form (1960), the SI defined prefixes for values ranging from the pica (symbol p) having a value of 10? 12 to the tera (symbol T) having a value of 1012.The list was extended at the 12th CGPM (1964), the 15th CGPM (1975) and at the 19th CGPM (1991) 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, A sphygmomanometer – the traditional device that measures blood pressure using mercury in a technical and commercial literature, and will continue to be used manometer. Pressures are recorded in “millimetres of for many years to come.In addition, certain other units are so mercury” – a non-SI unit 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 are 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 12? f 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. International System of Units 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. 32] 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; the becquerel, sievert and gray were adopted in later years.
Similarly, the millimetre of mercury (symbol mmHg) was retained for measuring blood pressure.  13 Worldwide adoption of SI 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 manufactured in 1979 with a lead plug and assayer’s mark that is similar to, but which predates OIML recommendation R52.
] The change to SI had little effect 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 had no formal role in the enforcement of such changes although another inter-governmental organisation, the International Organization of Legal Metrology (OIML) had a formal role in providing a forum for the harmonisation of national legislation in respect of metrology. The formal adoption of SI varied from country to country: when those countries that had not yet adopted the metric system did so, they adopted SI directly. There was no “standard” way in which ountries where the metric system was in use migrated to using SI. In 1960, the world’s largest economy was that of the United States, followed by the United Kingdom, West Germany, France, Japan, China and India.  Of these the principal non-metric countries were the United States and the United Kingdom. France and Germany had been using the metric system for about a century, China had been using the metric system for 35 years while India and Japan had adopted the metric system within the preceding five years. Other non-metric countries were those where the United Kingdom or the United States had considerable influence.
 International System of Units 14 United Kingdom and the former EmpireEven though the use of metric units were legalised for trade in 1864, the United Kingdom had signed the Metre Convention in 1884 and Parliament had defined the yard and the pound in terms of the metre and the kilogram in 1897, the United Kingdom 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 Commonwealth.  When the SI standard was published in 1960, the only major Commonwealth country to have adopted the metric system was India. In 1965, after numerous 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, on behalf of the Government, agreed to support a ten-year metrication programme. The government agreed to a voluntary policy requiring minimal legislation and costs being to be borne where they fell.
SI would be used from the outset.  The rest of the Commonwealth, South Africa and Ireland followed within a few years; in some counties such as South Africa and Australia metrication was mandatory rather than voluntary.  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 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.  Although SI or units approved for use alongside SI are used in most areas where units of measure are regulated imperial units are widely encountered in unregulated areas such as the press and everyday speech. The situation in Ireland, apart from road signs which were metricated in the early 2000s, is similar to that in the United Kingdom.
 United StatesEven though Congress legalised the use of the metric system in 1866, signed the Metre Convention in 1875 and under the Mendenhall Order in 1893 defined the pound and the yard in terms of the kilogram and metre respectively, the United States continued to use customary units – when the SI standard was published in 1960, the United States was one of the largest countries that did not use the metric system, though as a result of their Spanish heritage, metric units were used widely in Puerto Rico.  On February 10, 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 the United States Congress authorised the U.
S. Metric Study the first volume of which was delivered in 1970. 52] This was followed in 1975 by Congress passing the Metric Conversion Act of 1975.
Voluntary metrication was to be coordinated by the United States Metric Board (USMB), but public response included resistance, apathy, and sometimes ridicule.  The board was disbanded in 1982. The 1988 Omnibus Foreign Trade and Competitiveness Act, an act that removed international trade barriers amended the Metric Conversion Act of 1975 and 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. 54] 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 U. S.
military uses of the metric system widely, partly because of the need to work with armed services from other nations.  International System of Units Although overall responsibility for labeling requirements of consumer goods lies with Congress and are 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 UPLA 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 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 U. S. 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 UPLA was amended to permit metric-only labelling and automatically became law in those states that accept UPLA “as is”. By 1 January 2009, 48 out f 50 states permit metric-only labelling, either through UPLA or through their own legislation.  As of February 2013 (http:/ / en. wikipedia.
org/ w/ index. php? title=International_System_of_Units; action=edit) the use of metric (and therefore SI) units in the United States follows no coherent pattern. Dual-unit labelling on consumer goods is mandatory.
Some consumer goods such as soft drinks are sold in metric quantities, others such as milk are sold in customary units. The engineering industry is equally split. The automotive industry is largely metric, but aircraft such as the Boeing 787 Dreamliner were designed using customary units.
15 European UnionIn 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 program 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 GCPM 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. 61] 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 Pferdestarke (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 the use of colloquial units such as the pond, pfund, livre and the like, thereby effectively prohibiting their use as well. International System of Units 16When the directive was revisited during 1977, some of the older units that were being reviewed were retained as being permitted where they were still in use, but others were phased out while the directive was aligned 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.  IndiaIndia was one of the last countries to start a metrication program before the the trader is using hexagonal weights advent of SI. When it became independent in 1947, both imperial and native – a shape that is consistent with  units of measure were in use.
Her metrication program started in 1956 with the metric weights. passing of the Standards of Weights and Measures Act. Part of the act fixed the value of the steer (a legacy unit of mass) to 0.
9331 kg exactly; elsewhere the Act declared that from 1960 all non-metric units of measure were to be illegal.  Four years after the Indian Government announced its metrication program, SI was published. The result was that initial metrication program was a conversion to the cgs system of units.Fifty years later, many of the country’s schoolbooks still use cgs or imperial units 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 and lakhs is widespread and is often found alongside or in place of the western numbering system.  Banana market stall in Kerala, India – International System of Units 17 New SI” When the metre was redefined in 1960, the kilogram was the only SI base unit that relied on a specific artefact. Moreover, after the 1996–1998 recalibration, a clear divergence between the various prototype kilograms was observed. At its 23rd meeting (2007), the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artifacts that were then in use. 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. 31] The proposals that the CCU put forward were: • In addition to the speed of light, four constants of nature – Planck’s constant, an elementary charge, Boltzmann constant and Avogadro’s number – be defined to have exact values. Relations between proposed SI units definitions (in colour) and seven physical constants (in grey) with fixed numerical values in the proposed system.
• 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 tightened up The CIPM meeting of October 2010 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”. 70] The CIPM did however sponsor a resolution at the 24th CGPM in which the changes were agreed in principle and which were expected to be finalised at the CGPM’s next meeting in 2014.  Notes  SI Brochure, op cit, p 103; NIST, op cit, p 3  SI brochure, op cit, p 122; NIST, op cit, p 14  SI Brochure, op cit, p 123–129; NIST, op cit, p 7–11  This grouping reflects the 8th Edition of the SU Brochure (2006  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 Academie francaise in the case of French or Council for German Orthography () in the case of German  Except where specifically noted, these rules are common to both the SI brochure and the NIST brochure.
19] SI brochure, op cit, p 111  SI Brochure – pg 156  pg 221 – McGreevy  SI Brochure – pg 157  SI Brochure – pg 158  SI Brochure – pg 159  SI Brochure – pg 165  SI Brochure – pg 152  SI Brochure – pg 164  The CGPM have defined the metre in terms of the speed of light, so the speed of light has an exact value.  SI Brochure, pg 152  SI Brochure, pg 127  These countries included the Commonwealth countries (excluding India), Ireland, Burma, Liberia, Iraq, Kuwait, Saudi Arabia, Nepal and Ethiopia International System of Units  In the context of this article, the word “Empire” excludes the United States.  High-profile exceptions include road signs in the United Kingdom, (Irish road signs were converted to metric units during the first decade of the 21st century) the sale of draught beer and the sale of milk in returnable containers  USMA (http:/ / lamar. colostate.
edu/ ~hillger/ products/ measured. tml)  Angular units: degree, minute and second, Units of time:day, minute  The units that made use of the acceleration due to gravity in their definitions included the kilogram-force/kilopond, torr, technical atmosphere, manometric units of pressure such as metres of water and millimetres of mercury 18 References • [SI Brochure] International Bureau of Weights and Measures (2006), The International System of Units (SI) (http://www. bipm. org/utils/common/pdf/si_brochure_8_en.
pdf) (8th ed. ), ISBN 92-822-2213-6 • [NIST 330] Thompson, Ambler; Taylor, Barry N. (2008). The International System of Units (SI) (Special publication 330) (http://physics. nist. gov/Pubs/SP330/sp330.
pdf). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 2008-06-18. [NIST] Thompson, Ambler; Taylor, Barry N. (2008). Guide for the Use of the International System of Units (SI) (Special publication 811) (http://physics. nist.
gov/cuu/pdf/sp811. pdf). Gaithersburg, MD:: National Institute of Standards and Technology.
Further reading • 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. (http://old. iupac. org/ publications/books/gbook/green_book_2ed.
pdf) • BW Petley (2004). Symbols, Units, Nomenclature, ; Fundamental Physical Constants. IUPAP-39. (http://www. physics. ohio-state. du/~jossem/IUPAP/PhysicsNowText-A4-1. pdf) • Unit Systems in Electromagnetism (http://info.
ee. surrey. ac. uk/Workshop/advice/coils/unit_systems/#rms) • MW Keller et al. (http://qdev. boulder.
nist. gov/817. 03/pubs/downloads/set/Watt_Triangle_sub1. pdf) Metrology Triangle Using a Watt Balance, a Calculable Capacitor, and a Single-Electron Tunneling Device • “The Current SI Seen From the Perspective of the Proposed New SI” (http://nvlpubs.
nist. gov/nistpubs/jres/ 116/6/V116. N06.
A01. pdf). Barry N. Taylor. Journal of Research of the National Institute of Standards and Technology, Vol. 116, No.
6, Pgs. 797–807, Nov–Dec 2011. External linksOfficial • BIPM Bureau International des Poids et Mesures (SI maintenance agency) (http://www. bipm. org/en/si/) (home page) • BIPM brochure (http://www. bipm.
org/en/si/si_brochure/) (SI reference) • ISO 80000-1:2009 Quantities and units – Part 1: General (http://www. iso. org/iso/iso_catalogue/ catalogue_ics/catalogue_detail_ics. htm? csnumber=30669) • NIST Official Publications (http://physics. nist. gov/cuu/Units/bibliography.
html) • NIST Special Publication 330, 2008 Edition: The International System of Units (SI) (http://physics. nist. gov/ Pubs/SP330/sp330. pdf) • NIST Special Publication 811, 2008 Edition: Guide for the Use of the International System of Units (http:// www. nist. gov/pml/pubs/sp811/index. fm) • NIST Special Pub 814: Interpretation of the SI for the United States and Federal Government Metric Conversion Policy (http://ts. nist.
gov/WeightsAndMeasures/Metric/pub814. cfm) • Rules for SAE Use of SI (Metric) Units (http://www. sae. org/standardsdev/tsb/tsb003. pdf) International System of Units • International System of Units (http://www.
dmoz. org/Science/Reference/Units_of_Measurement//) at the Open Directory Project • EngNet Metric Conversion Chart (http://www. engnetglobal. com/tips/convert.
aspx) Online Categorised Metric Conversion Calculator • U. S. Metric Association. 2008.
A Practical Guide to the International System of Units (http://lamar. olostate. edu/~hillger/pdf/Practical_Guide_to_the_SI. pdf) History • LaTeX SIunits package manual (ftp://cam.
ctan. org/tex-archive/macros/latex/contrib/SIunits/SIunits. pdf) gives a historical background to the SI system. Research • The metrological triangle (http://www. npl.
co. uk/server. php? show=ConWebDoc. 1835) • Recommendation of ICWM 1 (CI-2005) (http://www. bipm. org/cc/CIPM/Allowed/94/ CIPM-Recom1CI-2005-EN. pdf) Pro-metric advocacy groups • The UK Metric Association (http://www.
ukma. org. uk/) • The US Metric Association (http://www. metric. org/) Pro-customary measures pressure groups • Pro-customary measures groups (http://www. dmoz. rg/Society/Issues/Government_Operations/ Anti-Metrication//) at the Open Directory Project 19 20 Organisations Metre Convention For a topical guide to this subject, see Introduction to the metric system.
Metre Convention (French: Convention du Metre, also known as the Treaty of the Metre) is an international treaty, signed in Paris on 20 May 1875 by representatives of seventeen nations which set up an institute for the purpose of coordinating international metrology and for coordinating the development of the metric system. The treaty also Metre Convention signatories: Member States Associates set up associated organisations to oversee the running of the institute.Initially it was only concerned with the units of mass and length, but in 1921, at the 6th meeting of the General Conference on Weights and Measures (CGPM), it was revised and its mandate extended to cover all physical measurements. In 1960, at the 11th meetings of the CGPM, the system of units it had established was overhauled and relaunched as the “International System of Units” (SI). The Convention created three main organizations: • The General Conference on Weights and Measures (Conference generale des poids et mesures or CGPM) – a meeting every four to six years of delegates from all member states. • The International Committee for Weights and Measures (Comite international des poids et mesures or CIPM) – an advisory body to the CGPM consisting of eighteen prominent metrologists from eighteen different countries.
The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) – an organisation based at Sevres, France that has custody of the International Prototype Kilogram, provides metrology services for the GCPM and CIPM, houses the secretariat for these organisations and hosts their formal meetings. Membership of the convention is restricted to countries who have diplomatic relations with France, but in 1999 the category of associate membership was introduced for those nations that wished to partake in the calibration and measurement Mutual Recognition Agreement (MRA) program without taking part in the activities of the BIPM. Metre Convention 21 BackgroundIn England in 1215, clause 25 of the Magna Carta set out the standards of measure that were to be applied throughout the realm prefixed. The wording of the clause emphasised that “There is to be a single measure …
throughout our realm”.  Five centuries later, when in 1707 England and Scotland were united into a single kingdom, the Scots agreed to use the same units of measure that were already established in England.  During the eighteenth century, in order to facilitate trade, Peter the Great, Czar of Russia adopted the English system of measure.  Abuse of units of measures were one of the causes of the French Revolution and its reform was one of the items on the agenda of National Assembly.Talleyrand, an influential leader of the Assembly invited British and American participation in the establishment of a new system, but in the event, the Assembly went it alone and introduced the metre and the kilogram which were to form the basis of the metric system, manufacturing prototypes which, in 1799, were lodged with Archives.
 Between 1850 and 1870, a number of countries adopted the metric system as their system of measure including Spain, many South American republics and many of the Italian and German states (the Netherlands had adopted the system in 1817). In 1863, the International Postal Union used grams to express permitted weights of letters. Woodcut dated 1800 illustrating the new decimal units which became the legal norm across all France on 4 November 1800 In the 1860s, inspections of the prototype metre revealed wear and tear at the measuring faces of the bar and also that the bar was wont to flex slightly when in use. ] In view of the doubts being cast on the reproducibility of the metre and the kilogram and the threat that a rival standard might be set up, Napoleon III invited scientists from all the world’s nations to attend a conference in Paris. In July 1870, two weeks before the conference was due to start, the Franco-Prussian War broke out. Although the delegates did meet (without a German delegation), it was agreed that the conference should be recalled once all the delegates (including the German delegation) were present.
France was defeated in the war, Napoleon went into exile and Germany and Italy, now unified nations, adopted the metric system as their national system of units, but with the prototype copy of the kilogram and metre under the control of the Third French Republic.In 1872 the new republican government reissued the invitations and in 1875 scientists from thirty European and American countries met in Paris.  The 1875 Conference The principal tasks facing the delegates at the 1875 conference was the replacement of the existing metre and kilogram artefacts that were held by the French Government and the setting up of an organisation to administer the maintenance of standards around the globe. The conference did not concern itself with other units of measure. The conference had undertones of Franco-German political manoeuvring, particularly since the French had been humiliated by the Prussians during the war a few years previously.Although France lost control of the metric system, they ensured that it passed to international rather than German control and that the international headquarters Metre Convention were based in Paris.
 22 Reference standards The conference was called to discuss the maintenance of international standards based on the existing French standards rather than using French standards which, at that time, were 70 years old and which, through wear and tear, might not be exactly the same as when they adopted in 1799. Prior to the 1870 conference, French politicians had feared that the world community might reject the existing metre as it was 0. 03% (0. 3 mm) shorter than its design length ordering a new meridional measurement.
They were eventually reassured Historical International Prototype Metre bar, made of an when the German-born Swiss delegate said “no serious alloy of platinum and iridium, that was the standard from scientist would in our day and age contemplate a metre 1889 to 1960. deduced from the size of the earth”. When the conference was reconvened in 1875, it was proposed that new prototype metre and kilogram standards be manufactured to reproduce the values of the existing artefacts as closely as possible.  Although the new standard metre had the same value as the old metre, it had an “X” cross-section rather than a rectangular cross-section as this reduced the flexing when taking measurements.Moreover then new bar, rather than being exactly one metre in length was a little longer than one metre and had lines engraved on them that were exactly one metre apart.  The London firm Johnson Matthey delivered 30 prototype metres and 40 prototype kilograms.
At the first meeting of the CGPM in 1889 bar No. 6 and cylinder No. X were chosen by lot as the international prototypes. The remainder were either kept as BIPM working copies or distributed by lot to member states as national prototypes.  The metre was retained as the international standard until 1960 when the metre was redefined in terms of the wavelength of the orange-red line of krypton-86. As of 2012 the prototype kilogram was still in use, though it is expected to be replaced by a new definition within the next few years. OrganisationsThe Convention created three main organizations to facilitate the standardisation of weights and measures around the world. The first, the CGPM provided a forum for representative of member states, the second, the CIPM was an advisory committee of metrologists of high standing and the third, the BIPM was an institute that provided appropriate secretarial and laboratory facilities in support of the GCPM and CIPM.
 General Conference on Weights and Measures The General Conference on Weights and Measures (Conference generale des poids et mesures or CGPM) is the principal decision-making body put on place by the convention. It is made up of delegates from member states and [non-voting] observers from associate states.The conference usually meets every four to six years to receive and discuss a report from the CIPM and to endorse new developments in the SI on the advice of the CIPM though at the 2011 meeting, it agreed to meet again in 2014 rather than 2015 to discuss the maturity of the new SI proposals.  It is also responsible for new appointments to the CIPM and decides on major issues concerning the development and financing of the BIPM.
It normally meets every four to six years, The body comprises delegates from all member states.  Metre Convention International Committee for Weights and Measures The International Committee for Weights and Measures (Comite international des poids et mesures or CIPM) is made up of eighteen (originally fourteen) individuals from a member state of high scientific standing, nominated by the CGPM to advise the CGPM on administrative and technical matters.It is responsible for the running of ten consultative committees (CCs), each of which investigates different aspects of metrology – one CC discusses the measurement of temperature, another the measurement of mass and so on.
The CIPM meets annually at Sevres to discuss annual reports from the various CCs, to submit an annual report to the governments of member states in respect of the administration and finances of the BIPM and to advise the CGPM on technical matters as and when necessary. Each member of the CIPM is from a different member state – with France, in recognition of its work in setting up the Convention, always having one seat on the CIPM. 11] International Bureau of Weights and Measures The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) is an organisation based at Sevres, France that has custody of the International Prototype Kilogram, provides metrology services for the GCPM and CIPM, houses the secretariat for these organisations and hosts their formal meetings. It also has custody of the former International Prototype Metre which was retired in 1960.
Over the years the various international prototype metres and kilograms were returned to BIPM headquarters for recalibration services. Initially it has a staff of 9, falling to 4 once the initial batch of prtotypes had been distributed; in 2012 it has a staff of over 70 people and a budget of over €10 million.  The director of the BIPM is ex-officio a member of the CIPM and a member of all consultative committees. 23 Seal of the BIPM Headquarters, language and protocolThe original treaty was written in French and the authoritative language of all official documents is French.
Communication between the BIPM and member states is, in the case of France, via the French Foreign Minister and in the case of all other members, via the members’ amabassador to France.  The French government offered the treaty members the Pavillon de Breteuil in Sevres to house the BIPM. The pavillon was originally built in 1675 on the estate of the Chateau de Saint-Cloud and was home Pavillon de Breteuil to, amongst others, Emperor Napoleon III. The chateau was all but destroyed during the Franco-Prussian War (1870-1) and the pavillon badly damaged. 16] The pavillon has been fully restored and, as headquarters of an intergovernmental organization has extraterritorial rights (similar to those enjoyed by embassies).  Metre Convention 24 Post 1875 developments The science of metrology has increased vastly since 1875.
In particular the treay was amended in 1921 with the result that many other international organisations have a forum within the CIPM to ensure harmonisation of measurement standards across many disciplines. In addition, what was originally conceived as standards for the purposes of trade has now been extended to cover large number of aspects of human activity including medicine, science, engineering and technology. Extensions to the treaty (1921) and development of SIThe metre convention was originally drawn up with the main purpose of providing a standards of length and mass only.
Standards relating to other quantities were under the control of other bodies – time was measured by astronomers,electrical units by a series of ad-hoc international conferences, and other physical standards and concepts were maintained or defined by international bodies such as International Congress of Applied Chemistry or the International Union of Pure and Applied Physics. In 1901 Giorgi published a proposal for building a coherent set of units based on four base units – the metre, kilogram, second and one electrical unit (ampere, volt or ohm).In 1921 the Convention was extended to permit the promotion of standards relating to any physical quantity which greatly increased the scope of the CIPM’s remit and implicitly giving it freedom to exploit Giortgi’s proposals.
The 8th CGPM (1933) resolved to work with other international bodies to agree standards for electrical units that could be related back to the international prototypes.  This agreed in principal by the International Electrotechnical Commission at its congress in Brussels in 1935 subject to the choice of the fourth unit being agreed with, amongst others, the appropriate consultative committee of the CIPM.  In 1948, three years after the end of the Second World War and fifteen years after the 8th CGPM, the 9th CGPM was convened.In response to formal requests made by the International Union of Pure and Applied Physics and by the French Government to establish a practical system of units of measure, the CGPM requested the CIPM to prepare recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention.  At the same time the CGPM formally adopted a recommendation for the writing and printing of unit symbols and of numbers.  The recommendation also catalogued the recommended symbols for the most important MKS and CGS units of measure and for the first time the CGPM made recommendations concerning derived units.The CIPM’s draft proposal, which was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units, was put to the 10th CGPM in 1954. In the proposal the CIPM recommended that the ampere be the base unit from which electromechanical would be derived. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units.  The full system and name “Systeme International d’Unites” were adopted at the 11th CGPM.  During the years that followed the definitions of the base units and particularly the mise en pratique to realise these definitions have been refined.The formal definition of International System of Units (SI) along with the associated resolutions passed by the CGPM and the CIPM are published by the BIPM on the Internet and in brochure form at regular intervals. The eighth edition of the brochure Le Systeme International d’Unites – The International System of Units was published in 2006.  Metre Convention 25 MRA program During the 1940s, the United States government recognised the benefits of its suppliers keeping quality control records in respect of manufactured goods that would provide traceability of the process. This process was formalised by the British Government and in 1979 as the quality control standard BS 5750. In 1987 BS 5750 was adopted by ISO as the basis for ISO 9000. 27] ISO 9000 is a general purpose quality control standard which works in conjunction industry-specific standards: for example ISO 15195:2003 which gives the specific requirements for reference measurement laboratories in laboratory medicine.  International trade is hampered by one country not recognising the quality controls in place in other countries – often due to standards being different or being incompatible with each other. At the 20th CGPM (1995), it was recognised that although ad-hoc recognition of instrument calibration between cooperating countries had been taking place for a hundred years, a need had arisen for a more comprehensive agreement.Consequently the CIPM was mandated to investigate the setting up of a Mutual Recognition Agreement in respect of instrument calibration. Any such agreemnt would require the keeping of records that could demonstrate the tracability of calibrations back to the base standards. Such records would be recorded within an ISO 9000 framework. Four years later, in 1999 the text of the CIPM-MRA was agreed at the 21st CGPM.  The CIPM-MRA scheme is to catalogue the capabilities of National Measurement Institutes (NMIs) such as NIST in the United States or the National Physical Laboratory in the United Kingdom whose calibration procedures have been peer-assessed.The essential points of CIPM-MRA are: • The agreement is only open to countries that have signed the Metre Convetnion, either as full or as associate members. • A country may have more than one NMI, though only one NMI is chosen as the signatory organisation. • The measurement capabilituies of NMI’s will be peer-reviewed at regular intervals and each NMI will recognise the measurement capabilities of other NMIs. • The BIPM maintains a publically available database of the measurement capabilities of each NMI. • NMI’s Subsequent to launch of the CIPM MRA and in response to a European Community directive on in vitro medical devices, the Joint Committee forTraceability in Laboratory Medicine (JCTLM) was created in 2002 through a Declaration of Cooperation between the International Committee of Weights and Mesures (CIPM), the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), and the International Laboratory Accreditation Cooperation (ILAC).  The joint committee provides a forum for the harmonisation of standards of the various participants. Coordination of International Atomic Time With the advent of the atomic clock it has been possible to define and measure International Atomic Time with sufficient precision that variations in the Earth’s rotation can be detected.The International Earth Rotation Service monitors these changes relative to the starts and at regular intervals and proposes leap seconds as and when these are needed. Currently there are over 200 atomic clocks in over 50 national laboratories around the world and the BIPM, in terms of the mandate given to it under the Metre Convention coordinates the various atomic clocks.  Metre Convention 26 New SI Since 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype metre, the only unit of measure that has been dependent on a particular artifact has been the kilogram. Over the years, small drifts which could be as high as 20? 10? kilograms per annum in the mass of the international prototype kilogram have been detected.  At the 21st meeting of the GCPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artifact. Independently of this drift having been identified, the Avogadro project and development of the Watt balance promised methods of indirectly measuring mass with a very high precision. These projects provided tools that would enable alternative means of redefining the kilogram.  Relations between proposed SI units definitions (in color) and with seven fundamental constants of nature (in grey) with fixed numerical values in the proposed system.A report published in 2007 by the Consultative Committee for Thermometry to the CIPM noted that their current definition of temperature has proved to be unsatisfactory for temperatures below 20 K and for temperatures above 1300 K. The committee was of the view that the Boltzmann constant provided a better basis for temperature measurement than did the triple point of water, as it overcame these difficulties.  At its 23rd meeting (2007), the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artifacts that were then in use. The following year this was endorsed by the International Union of Pure and Applied Physics (IUPAP). 37] In 2011 CIPM presented a resolution for consideration at the 24th CGPM to agree in principle the new definitions that had been proposed the previous year by the Consultative Committee for Units, but not to implement them until the details have been finalised.  This resolution was accepted by the conference, and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014 when it is hoped to implement these proposals.  Membership The CGPM recognises two classes of membership – full membership for those states that wish to participate in the activities of the BIPM and associate membership for those countries or economies that only wish to participate in the MRA program. Associate members have observer status at the CGPM.Since all formal liaison between the convention organisations and national governments is handled by the member state’s ambassador to France, it is implicit that member states must have diplomatic relations with France, though during both world wars, nations that were are war with France retained their membership of the CGPM.  The opening session of each CGPM is chaired by the French foreign minister and subsequent sessions by the President de l’Academie des Sciences de Paris.  On 20 May 1875 representatives from seventeen of countries that attended the Conference of the Metre in 1875, signed the Convention of the Metre.  InApril 1884 HJ Chaney, Warden of Standards in London unofficially contacted the BIPM inquiring whether the BIPM would calibrate some metre standards that had been manufactured in the United Kingdom. Broch, director of the BIPM replied that he was not authorised to perform any such calibrations for non-member states. On 17 September 1884, the British Government signed the convention on behalf Metre Convention of the United Kingdom.  This number grew to 21 in 1900, 32 in 1950, and 49 in 2001. As of 22 October 2012, the General Conference membership was made up of 54 Member States, 37 Associate States and Economies and four international organisations as follows (with year of partnership between brackets): 27 Member StatesArgentina (1877) Australia (1947) Austria (1875) Belgium (1875) Brazil (1921) Bulgaria (1911) Canada (1907) Chile (1908) China (1977) Colombia (2012) Croatia (2008) Czech Republic (1922) Denmark (1875) Dominican Republic (1954) Egypt (1962) Finland (1923) France (1875) Germany (1875) Greece (2001) Hungary (1925) India (1957) Indonesia (1960) Iran (1975) Ireland (1925) Israel (1985) Italy (1875) Japan (1885) Kazakhstan (2008) Kenya (2010) Malaysia (2001) Mexico (1890) Netherlands (1929) New Zealand (1991) Norway (1875) Pakistan (1973) Poland (1925) Portugal (1876) Romania (1884) Russia (1875) Saudi Arabia (2011) Serbia (2001) Singapore (1994)Metre Convention Slovakia (1922) South Africa (1964) South Korea (1959) Spain (1875) Sweden (1875) Switzerland (1875) Thailand (1912) Tunisia (2012) Turkey (1875) United Kingdom (1884) United States (1878) Uruguay (1908) Venezuela (1879) Notes  Act of Union 1707, Article 3  Convention of the Metre (1875), Appendix 1 (Regulation), Article 8  Convention of the Metre (1875), Appendix 1 (Regulation), Article 6  Convention of the Metre, Appendix 1 (Regulations), Articles 16 and 19  As of 2012, the only “economy” that was an associate member was CARICOM (Caribbean Community) – its membership comprising Antigua and Barbuda, Saint Kitts and Nevis, Barbados, Saint Lucia, Belize, Saint Vincent and the Grenadines, Dominica, Suriname, Grenada, Trinidad and Tobago and Guyana. Jamaica, although also a member of CARICOM, is an associate of the CGPM in its own right. 41] In the case of France, the French Foreign Minister  Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States and Venezuela.  http:/ / en. wikipedia. org/ w/ index. php? title=Metre_Convention; action=edit  Joined originally as Austria-Hungary  Joined originally as part of Czechoslovakia  Joined originally as part of Sweden and Norway  Joined originally as the Russian Empire  Joined originally as part of Czechoslovakia  Joined originally as part of Sweden and Norway 28 Associates At its 21st meeting (October 1999), the CGPM created the category of “associate” for those states not yet members of the BIPM and for economic unions. 1] Albania (2007) Bangladesh (2010) Belarus (2003) Bolivia (2008) Bosnia and Herzegovina (2011) Botswana (2012) Caribbean Community (2005) Chinese Taipei (2002) Costa Rica (2004) Cuba (2000) Ecuador (2000) Malta (2001) Moldova (2007) Estonia (2005) Mauritius (2010) Seychelles (2010) Georgia (2008) Montenegro Slovenia (2003) Ghana (2009) (2011) Sri Lanka (2007) Hong Kong Namibia (2012) Syria (2012) (2000) Oman (2012) Ukraine (2002) Jamaica (2003) Panama (2003) Vietnam (2003) Latvia (2001) Paraguay (2009) Zambia (2010) Lithuania (2001) Peru (2009) Zimbabwe (2010) Macedonia (2006) Philippines (2002) On 26 October 2012, Colombia announced that she would be signing the Metre Convention.    Metre Convention 29 International Organisations The following international organisations have signed the CIPM MRA: • • • • International Atomic Energy Agency (IAEA), Vienna, Austria (1999) Institute for Reference Materials and Measurements (IRMM), Geel, Belgium (1999) World Meteorological Organization (WMO), Geneva, Switzerland (2010) European Space Agency (ESA), Paris, France (2012) Notes  BIPM official site: Procedure for a State or Economy to become an Associate of the General Conference (http:/ / www. bipm. rg/ en/ convention/ member_states/ join_associate. html)  Spanish http:/ / www. elcolombiano. com/ BancoConocimiento/ C/ convencion_del_metro_colombia_se_adhirio/ convencion_del_metro_colombia_se_adhirio. asp? CodSeccion=211)  Spanish http:/ / www. lagrannoticia. com/ 29-noticias/ el-personaje/ 9459-colombia-adhiere-a-la-convencion-del-metro  Spanish http:/ / www. paisabook. com/ index. php/ chismes/ noticias/ 2484-colombia-se-adhirio-a-la-convencion-del-metro-para-regular-las-mediciones-con-estandares-internacionales References Offical texts • Text of the current version of the Convention (in French) (http://www. bipm. org/utils/en/pdf/ metre_convention. df) • English translation of the original text of the Convention (http://lamar. colostate. edu/~hillger/laws/ metric-convention. html) • Text of the CIPM-MRA agreement (http://www. bipm. org/en/cipm-mra/mra_online. html) Citations General Conference on Weights and Measures 30 General Conference on Weights and Measures For a topical guide to this subject, see Outline of the metric system. The General Conference on Weights and Measures (French: Conference generale des poids et mesures – CGPM) is the senior of the three Inter-governmental organizations established in 1875 under the terms of the Metre Convention (French: Convention du Metre) to represent the interests of member states.The treaty, which also set up two further bodies, the International Committee for Weights and Measures (French: Comite international des poids et mesures- CIPM) and the International Bureau of Weights and Measures (French: Bureau international des poids et mesures – BIPM,) was drawn up to coordinate international metrology and to coordinate the development of the metric system. The conference meets in Sevres (south-west of Paris) every four to six years. Initially it was only concerned with the kilogram and the metre, but in 1921 the scope of the treaty was extended to accommodate all physical measurements and hence all aspects of the metric system. In 1960 the 11th CGPM approved the Systeme International d’Unites, usually known as “SI”. Establishment On 20 May 1875 an international treaty known as the Convention du Metre (Metre Convention) was signed by 17 states.This treaty established the following organisations to conduct international activities relating to a uniform system for measurements: • Conference generale des poids et mesures (CGPM), an intergovernmental conference of official delegates of member nations and the supreme authority for all actions; • Comite international des poids et mesures (CIPM), consisting of selected scientists and metrologists, which prepares and executes the decisions of the CGPM and is responsible for the supervision of the International Bureau of Weights and Measures; • Bureau international des poids et mesures (BIPM), a permanent laboratory and world centre of scientific metrology, the activities of which include the establishment of the basic standards and scales of the principal physical quantities and maintenance of the international prototype standards. The CGPM acts on behalf of the governments of its members. In so doing, it appoints members to the CIPM, receives reports from the CIPM which it passes on to the governments and national laboratories on member states, examines and where appropriate approves proposals from the CIPM in respect of changes to the International System of Units (SI), approves the budget for the BIPM (over €10 million in 2012) and it decides all major issues concerning the organization and development of the BIPM.  Membership criteriaThe CGPM recognises two classes of membership – full membership for those states that wish to participate in the activities of the BIPM and associate membership for those countries or economies that only wish to participate in the MRA program. Associate members have observer status at the CGPM. Since all formal liaison between the convention organisations and national governments is handled by the member state’s ambassador to France, it is implicit that member states must have diplomatic relations with France, though during both world wars, nations that were at war with France retained their membership of the CGPM.  The opening session of each CGPM is chaired by the French foreign minister and subsequent sessions by the President de l’Academie des Sciences de Paris. 9] Of the twenty countries that attended the Conference of the Metre in 1875, representatives of seventeen signed the convention on 20 May 1875.  In April 1884 HJ Chaney, Warden of Standards in London unofficially contacted the BIPM inquiring whether the BIPM would calibrate some metre standards that had been manufactured in the United General Conference on Weights and Measures Kingdom. Broch, director of the BIPM replied that he was not authorised to perform any such calibrations for non-member states. On 17 September 1884, the British Government signed the convention on behalf of the United Kingdom.  This number grew to 21 in 1900, 32 in 1950, and 49 in 2001.As of 10 August 2012, there are 56 Member States and 37 Associate States and Economies of the General Conference (with year of partnership between brackets): Member States Argentina (1877) Australia (1947) Austria (1875) Belgium (1875) Brazil (1921) Bulgaria (1911) Canada (1907) Chile (1908) China (1977) Colombia (2012) Croatia (2008) Czech Republic (1922) Denmark (1875) Dominican Republic (1954) Egypt (1962) Finland (1923) France (1875) Germany (1875) Greece (2001) Hungary (1925) India (1957) Indonesia (1960) Iran (1975) Ireland (1925) Israel (1985) Italy (1875) Japan (1885) Kazakhstan (2008) Kenya (2010) Malaysia (2001) Mexico (1890) Netherlands (1929) New Zealand (1991) Norway (1875) Pakistan (1973) Poland (1925) Portugal (1876) Romania (1884) Russia (1875) Saudi Arabia (2011) 31General Conference on Weights and Measures Serbia (2001) Singapore (1994) Slovakia (1922) South Africa (1964) South Korea (1959) Spain (1875) Sweden (1875) Switzerland (1875) Thailand (1912) Tunisia (2012) Turkey (1875) United Kingdom (1884) United States (1878) Uruguay (1908) Venezuela (1879) Notes  As of 2012, the only “economy” that was an associate member was CARICOM (Caribbean Community) – its membership comprising Antigua and Barbuda, Saint Kitts and Nevis, Barbados, Saint Lucia, Belize, Saint Vincent and the Grenadines, Dominica, Suriname, Grenada, Trinidad and Tobago and Guyana. Jamaica, although also a member of CARICOM, is an associate of the CGPM in its own right. 6] In the case of France, the French Foreign Minister  Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States and Venezuela.  http:/ / en. wikipedia. org/ w/ index. php? title=General_Conference_on_Weights_and_Measures& action=edit  Joined originally as Austria-Hungary  Joined originally as part of Czechoslovakia  Joined originally as part of Sweden and Norway  Joined originally as the Russian Empire  Joined originally as part of Czechoslovakia  Joined originally as part of Sweden and Norway 32 Associates At its 21st meeting (October 1999), the CGPM created the category of “associate” for those states not yet members of the BIPM and for economic unions. 1] Albania (2007) Bangladesh (2010) Belarus (2003) Bolivia (2008) Bosnia and Herzegovina (2011) Botswana (2012) Caribbean Community (2005) Chinese Taipei (2002) Costa Rica (2004) Cuba (2000) Ecuador (2000) Malta (2001) Moldova (2007) Estonia (2005) Mauritius (2010) Seychelles (2010) Georgia (2008) Montenegro Slovenia (2003) Ghana (2009) (2011) Sri Lanka (2007) Hong Kong Namibia (2012) Syria (2012) (2000) Oman (2012) Ukraine (2002) Jamaica (2003) Panama (2003) Vietnam (2003) Latvia (2001) Paraguay (2009) Zambia (2010) Lithuania (2001) Peru (2009) Zimbabwe (2010) Macedonia (2006) Philippines (2002) CGPM meetings General Conference on Weights and Measures 33 1st (http:/ / www. bipm. rg/ jsp/ en/ ListCGPMResolution. jsp? CGPM=1) (1889) The International Prototype Kilogram (IPK), a cylinder made of platinum-iridium and the International Prototype Metre, an X-cross-section bar also made from platinum-iridium were selected by lot from batches manufactured by the British firm Johnson Matthey. Working copies of both artifacts were also selected by lot and other copies distributed to member nations, again by lot. The prototypes and working copies were deposited at the International Bureau of Weights and Measures (Bureau international des poids et mesures), Sevres, France. No resolutions were passed by the 2nd CGPM. 2nd (http:/ / www. bipm. rg/ jsp/ en/ ListCGPMResolution. jsp? CGPM=2) (1897) 3rd (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=3) (1901) 4th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=4) (1907) 5th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=5) (1913) 6th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=6) (1921) 7th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=7) (1927) 8th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=8) (1933) 9th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=9) (1948) The litre was redefined as volume of 1 kg of water.Clarified that kilograms are units of mass, “standard weight” defined, standard acceleration of gravity defined endorsing use of grams force and making them well-defined. The carat was defined as 200 mg. The International Temperature Scale was proposed. The Metre Convention revised. The Consultative Committee for Electricity (CCE) created. The need for absolute electrical unit identified. The ampere, bar, coulomb, farad, henry, joule, newton, ohm, volt, watt, weber were defined. The degree Celsius was selected from three names in use as the name of the unit of temperature. The symbol l (lowercase L) was adopted as symbol for litre. Both the comma and dot on a line are accepted as decimal marker symbols. Symbols for the stere and second changed (http:/ / www. bipm. rg/ jsp/ en/ ViewCGPMResolution. jsp? CGPM=9& RES=7). The universal return to the Long Scale numbering system was proposed but not adopted. The kelvin, standard atmosphere defined. Work on the International System of Units (metre, kilogram, second, ampere, kelvin, candela) began. 10th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=10) (1954) 11th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=11) (1960) 12th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=12) (1964) 13th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=13) (1967) 14th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. sp? CGPM=14) (1971) The metre was redefined in terms of wavelengths of light. The Units hertz, lumen, lux, tesla were adopted. The new MKSA-based metric system given the official symbol SI for Systeme International d’Unites and launched as the “modernized metric system”. The prefixes pico-, nano-, micro-, mega-, giga- and tera- were confirmed. The original definition of litre = 1 dm3 restored. The prefixes atto- and femto- were adopted. The second was redefined as duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at a temperature of 0 K.The Degree Kelvin renamed kelvin and the candela redefined. A new SI base unit, the mole defined. The names pascal and siemens as units of pressure and electrical conductivity were approved. General Conference on Weights and Measures 34 15th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=15) (1975) 16th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=16) (1979) 17th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=17) (1983) 18th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=18) (1987) 19th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=19) (1991) 20th (http:/ / www. ipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=20) (1995) 21st (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=21) (1999) 22nd (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=22) (2003) 23rd (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=23) (2007) 24th (http:/ / www. bipm. org/ jsp/ en/ ListCGPMResolution. jsp? CGPM=24) (2011) The prefixes peta- and exa- were adopted. The units gray and becquerel were adopted as radiological units within SI. The candela and sievert were defined. Both l and L provisionally allowed as symbols for litre. The metre was redefined in terms of the speed of light.