The International System of Units is a convention for writing units of measure. It includes standard units of measure such as meter, and prefixes and abbreviations for multiples of units of measure. The International System of Units is also called the Système international d'unités and is abbreviated SI.
This convention establishes seven base units of measure from which most other units are derived, plus rules for writing quantities, especially very large or very small quantities. Table 2 shows the seven base units. Table 1 shows the convention for writing quantities. The prefixes in table 1 are prepended to the base unit. For example, 3×109 meters is written 3 gigameter. The abbreviation for 3 gigameter is 3Gm.
|Table 1: International System of Units - Multiples|
|length||meter||m||The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.||none|
|mass||kilogram||kg||The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.||The international prototype is a carefully protected bar of metal.|
|time||second||s||The second is 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 cesium 133 atom.||This standard refers to the period of the light wave emitted during a particular state transition of a cesium 133 atom.|
|electric current||ampere||A||The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length.||The ampere is defined by the electro-magnetic force it would generate under certain theoretical conditions.|
|thermodynamic temperature||kelvin||K||The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.||The isotopes of the hydrogen and oxygen making up the water affect the triple point of water, a clarification was added: |
"This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of 2H per mole of 1H, 0.000 379 9 mole of 17O per mole of 16O, and 0.002 005 2 mole of 18O per mole of 16O."[1 pg 20]The triple point of water is a single combination of temperature and pressure at which water can coexist in gaseous, liquid and solid states.
|amount of substance||mole||mol||1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol.” 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.||A mole is a specific quantity of usually very small particles, such as atoms. Later, a clarification was added, |
"In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to."[1 pg 21]
|luminous intensity||candela||cd||The candela is 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 1/683 watt per steradian.||Luminous intensity is a measure of the brightness of a light.|
|Table 2: Base units of measure|
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