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# Metric Units - Base Units

## Seven Basic Metric Measurements

The kilogram is the SI unit of mass.

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The metric system is a system of units of measurement established from its beginnings in 1874 by diplomatic treaty to the more modern General Conference on Weights and Measures - CGPM (Conferérence Générale des Poids et Measures). The modern system is actually called the International System of Units or SI. SI is abbreviated from the French Le Système International d'Unités and grew from the original metric system. Today, most people use the name metric and SI interchangeably with SI being the more correct title.

SI or metric is considered the main system of measurement units used in science today. Each unit is considered to be dimensionally independent from each other. These dimensions are described as the measurements of length, mass, time, electric current, temperature, amount of a substance, and luminous intensity. This list has the current definitions of each of the seven base units.

• Length - Meter ( m )

The meter is the SI unit of length. The meter is defined by the length of the path light travels in a vacuum during 1/299 792 458 of a second.

• Mass - Kilogram ( kg )
The kilogram is the SI unit of mass. It is the mass of the international prototype of the kilogram.
There is a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).
• Time - Second ( s )
The basic unit of time is the second. The second is defined to be the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the cesium-133 atomic ground state.
• Electric Current - Ampere ( A )
The basic unit of electric current is the ampere. The ampere is defined to be that constant current which, if maintained in two infinitely long straight parallel conductors which have negligible circular cross-section, and placed 1 m apart in vacuum, would produce between a force between the conductors equal to 2 x 10-7 newton per meter of length.
• Temperature - Kelvin ( K )
The kelvin is the unit of thermodynamic temperature. It is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
• Amount of a Substance - Mole ( mol )
The mole is defined to be the amount of a substance which contains as many entities as there are atoms in 0.012 kilograms of carbon-12. When the mole unit is used, the entities must be specified. For example, the entities may be atoms, molecules, ions, electrons, cows, houses, or anything else.
• Luminous Intensity - candela ( cd )
The unit of luminous intensity, or light, is the candela. The candela is the luminous intensity, in a given direction, of a source emitting monochromatic radiation of frequency 540 x 1012 hertz with radiant intensity in that direction of 1/683 watt per steradian.
Reading through this list, it is easy to see that these definitions are actually methods to realize the unit. Each realization was created with a unique and sound theoretical base to generate reproducible and accurate results.
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Anne Marie Helmenstine, Ph.D.

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