# International System of Units

### 1742

Only later were they moulded into an orthogonal coherent decimal system of measurement. The degree centigrade as a unit of temperature resulted from the scale devised by Swedish astronomer Anders Celsius in 1742.

### 1743

Independently, in 1743, the French physicist Jean-Pierre Christin described a scale with 0 as the freezing point of water and 100 the boiling point.

### 1791

The scale became known as the centi-grade, or 100 gradations of temperature, scale. The metric system was developed from 1791 onwards by a committee of the French Academy of Sciences, commissioned to create a unified and rational system of measures.

### 1792

In July 1792, the committee proposed the names metre, are, litre and grave for the units of length, area, capacity, and mass, respectively.

### 1799

In June 1799, based on the results of the meridian survey, the standard mètre des Archives and kilogramme des Archives were deposited in the French National Archives.

### 1860

The resultant calculations enabled him to assign dimensions based on mass, length and time to the magnetic field. A candlepower as a unit of illuminance was originally defined by an 1860 English law as the light produced by a pure spermaceti candle weighing pound (76 grams) and burning at a specified rate.

The centimetre–gram–second (CGS) system was the dominant metric system in the physical sciences and electrical engineering from the 1860s until at least the 1960s, and is still in use in some fields.

### 1875

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements.

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. Adopted in 1889, use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering.

Campbell. The International System was published in 1960, based on the MKS units, as a result of an initiative that began in 1948. ==Controlling authority== The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention.

It was accepted that ten standard candles were about equal to one Carcel lamp. ===Metre Convention=== A French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention, also called Treaty of the Metre, by 17 nations.

### 1879

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 principle of coherence was successfully used to define a number of units of measure based on the CGS, including the erg for energy, the dyne for force, the barye for pressure, the poise for dynamic viscosity and the stokes for kinematic viscosity. 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.

### 1889

The last artefact used by the SI was the International Prototype Kilogram (IPK), a particular cylinder of platinum-iridium; from 1889 to 2019, the kilogram was by definition equal to the mass of the IPK.

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. Adopted in 1889, use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering.

In 1921, the Metre Convention was extended to include all physical units, including the ampere and others thereby enabling the CGPM to address inconsistencies in the way that the metric system had been used. A set of 30 prototypes of the metre and 40 prototypes of the kilogram, in each case made of a 90% platinum-10% iridium alloy, were manufactured by British metallurgy specialty firm(who?) and accepted by the CGPM in 1889.

### 1901

Because of this, the standards of the CGS system were gradually replaced with metric standards incorporated from the MKS system. In 1901, Giovanni Giorgi proposed to the (AEI) that this system, extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system. This system was strongly promoted by electrical engineer George A.

This anomaly was resolved in 1901 when Giovanni Giorgi published a paper in which he advocated using a fourth base unit alongside the existing three base units.

### 1921

In 1921, the Metre Convention was extended to include all physical units, including the ampere and others thereby enabling the CGPM to address inconsistencies in the way that the metric system had been used. A set of 30 prototypes of the metre and 40 prototypes of the kilogram, in each case made of a 90% platinum-10% iridium alloy, were manufactured by British metallurgy specialty firm(who?) and accepted by the CGPM in 1889.

### 1933

The metre–tonne–second (mts) system, used in the Soviet Union from 1933 to 1955, had such SI-unrecognised units as the sthène, pièze, etc.

### 1948

The system was published in 1960 as a result of an initiative that began in 1948, so it is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS. == Introduction == The International System of Units, or SI, is a decimal and metric system of units established in 1960 and periodically updated since then.

Campbell. The International System was published in 1960, based on the MKS units, as a result of an initiative that began in 1948. ==Controlling authority== The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention.

### 1954

Based on this study, the 10th CGPM in 1954 defined an international system derived from six base units including units of temperature and optical radiation in addition to those for the MKS system mass, length, and time units and Giorgi's current unit.

### 1955

The metre–tonne–second (mts) system, used in the Soviet Union from 1933 to 1955, had such SI-unrecognised units as the sthène, pièze, etc.

### 1960

The system was published in 1960 as a result of an initiative that began in 1948, so it is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS. == Introduction == The International System of Units, or SI, is a decimal and metric system of units established in 1960 and periodically updated since then.

This was the biggest change in the SI system since it was first formally defined and established in 1960, and it resulted in the definitions described above. In the past, there were also various other approaches to the definitions of some of the SI units.

Campbell. The International System was published in 1960, based on the MKS units, as a result of an initiative that began in 1948. ==Controlling authority== The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention.

Changing technology has led to an evolution of the definitions and standards that has followed two principal strands – changes to SI itself, and clarification of how to use units of measure that are not part of SI but are still nevertheless used on a worldwide basis. Since 1960 the CGPM has made a number of changes to the SI to meet the needs of specific fields, notably chemistry and radiometry.

These rules were subsequently extended and now cover unit symbols and names, prefix symbols and names, how quantity symbols should be written and used, and how the values of quantities should be expressed. ===Birth of the SI=== In 1960, the 11th CGPM synthesised the results of the 12-year study into a set of 16 resolutions.

The centimetre–gram–second (CGS) system was the dominant metric system in the physical sciences and electrical engineering from the 1860s until at least the 1960s, and is still in use in some fields.

### 1979

Since 1979, the litre may exceptionally be written using either an uppercase "L" or a lowercase "l", a decision prompted by the similarity of the lowercase letter "l" to the numeral "1", especially with certain typefaces or English-style handwriting.

### 2009

The ISQ is formalised, in part, in the international standard ISO/IEC 80000, which was completed in 2009 with the publication of ISO 80000-1, and has largely been revised in 2019–2020 with the remainder being under review. ==Realisation of units== Metrologists carefully distinguish between the definition of a unit and its realisation.

### 2011

797–807, Nov–Dec 2011. B.

### 2014

During extraordinary verifications carried out in 2014 preparatory to redefinition of metric standards, continuing divergence was not confirmed.

### 2018

The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. Prior to the definitions adopted in 2018, the SI was defined through seven base units from which the derived units were constructed as products of powers of the base units.

### 2019

The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. Since 2019, the magnitudes of all SI units have been defined by declaring exact numerical values for seven defining constants when expressed in terms of their SI units.

Prior to 2019, , , , and were not defined a priori but were rather very precisely measured quantities.

In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units.

In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. As far as realisations, what are believed to be the current best practical realisations of units are described in the so-called 'mises en pratique', which are also published by the BIPM.

The last artefact used by the SI was the International Prototype Kilogram (IPK), a particular cylinder of platinum-iridium; from 1889 to 2019, the kilogram was by definition equal to the mass of the IPK.

Concerns regarding its stability on the one hand, and progress in precise measurements of the Planck constant and the Avogadro constant on the other, led to a revision of the definition of the base units, put into effect on 20 May 2019.

The ISQ is formalised, in part, in the international standard ISO/IEC 80000, which was completed in 2009 with the publication of ISO 80000-1, and has largely been revised in 2019–2020 with the remainder being under review. ==Realisation of units== Metrologists carefully distinguish between the definition of a unit and its realisation.

The change was adopted by the European Union through Directive (EU) 2019/1258. ==History== ===The improvisation of units=== The units and unit magnitudes of the metric system which became the SI were improvised piecemeal from everyday physical quantities starting in the mid-18th century.

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