# International System of Units

SISI unitsSI unitmetricmetric unitsmetric systemInternational System of Units (SI)International SystemSI systemSI system of units
[[File:SI base units.svg|thumb|The SI base unitswikipedia
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### Metric system

metricmetric unitsmetric unit
The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement.
It is now known as the International System of Units (SI).

### SI base unit

base unitbase unitsbase
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived.

### Unit of measurement

unitunits of measurementunits
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
Now there is a global standard, the International System of Units (SI), the modern form of the metric system.

### Metre

metermmetres
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in some metric systems, including the International System of Units (SI).

### Mole (unit)

molemolmoles
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The mole is the unit of measurement for amount of substance in the International System of Units (SI).

### SI derived unit

derived unitderived unitsJ/kg
The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.
SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI).

### Ampere

AmAamp
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The ampere (symbol: A), often shortened to "amp", is the base unit of electric current in the International System of Units (SI).

### Candela

cdcandelasmegacandela
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The candela ( or ; symbol: cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction.

### General Conference on Weights and Measures

CGPMCGPM conferenceCGPM (General Conference on Weights and Measures)
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 standardise the rules for writing and presenting measurements.
The General Conference receives the report of the CIPM on work accomplished; it discusses and examines the arrangements required to ensure the propagation and improvement of the International System of Units (SI); it endorses the results of new fundamental metrological determinations and various scientific resolutions of international scope; and it decides all major issues concerning the organization and development of the BIPM, including the dotation of the BIPM.

### Metre Convention

Convention of the MetreTreaty of the Metre[61]
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 standardise the rules for writing and presenting measurements.
In 1960, at the 11th meetings of the CGPM, the system of units it had established was named the International System of Units, with the abbreviation SI.

### Speed of light

clight speedvelocity of light
The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, and one physical artefact.
In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second.

### Kelvin

KkelvinsmK
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI).

### Electrical resistance and conductance

resistanceelectrical resistanceconductance
Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, and thus are not independent; for example, electrical conductance is the inverse of electrical resistance, with the consequence that the siemens is the inverse of the ohm, and similarly, the ohm and siemens can be replaced with a ratio of an ampere and a volt, because those quantities bear a defined relationship to each other.
The SI unit of electrical resistance is the ohm, while electrical conductance is measured in siemens (S).

### Second

ssecmegasecond
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: 1 second is defined to be exactly "the 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).

### Katal

kat
The most recent derived unit, the katal, was defined in 1999.
The katal (symbol: kat) is the unit of catalytic activity in the International System of Units (abbreviated SI).

### Coherence (units of measurement)

coherentcoherent systemcoherent system of units
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The International System of Units (1960) was designed around the principle of coherence.

### Acceleration

decelerationacceleratem/s 2
For example, 1 N = 1 kg × 1 m/s 2 says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared, as related through the principle of coherence to the equation relating the corresponding quantities:
The SI unit for acceleration is metre per second squared (m s −2 ).

### Metric prefix

prefixunit prefixdecimal
It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
The SI prefixes are standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991.

### Metrication

Since then, the SI has been adopted by all countries except the United States, Liberia and Myanmar.
However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.

### Mass

inertial massgravitational massweight
The first letter of symbols for units derived from the name of a person is written in upper case; otherwise, they are written in lower case. E.g., the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa", but the symbol for mole is written "mol". Thus, "T" is the symbol for tesla, a measure of magnetic field strength, and "t" the symbol for tonne, a measure of mass. Since 1979, the litre may exceptionally be written using either an uppercase "L" or a lowercase "l", a decision prompted by the similarity of the lowercase letter "l" to the numeral "1", especially with certain typefaces or English-style handwriting. The American NIST recommends that within the United States "L" be used rather than "l".
The basic SI unit of mass is the kilogram (kg).

### Triple point

triple point of waterhigh pressureTriple Point cell
The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, and one physical artefact.
The triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI).

### Giovanni Giorgi

GiorgiGiorgi SystemGiorgi, Giovanni
Giorgi later identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units (for temperature, amount of substance and luminous intensity) were added later.
Giovanni Giorgi (27 November 1871 – 19 August 1950) was an Italian physicist and electrical engineer who proposed the Giorgi system of measurement, the precursor to the International System of Units (SI).

### Time

temporaldurationintervals
Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.

### Newton (unit)

kNnewtonN
For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa)—and the pascal can be defined as one newton per square metre (N/m 2 ).
The newton (symbol: N) is the International System of Units (SI) derived unit of force.

### Force

forcesattractiveforce vector
For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa)—and the pascal can be defined as one newton per square metre (N/m 2 ).
It is measured in the SI unit of newtons and represented by the symbol F.