The idea for the metric system – a system of units based on meters and kilograms – came during the French Revolution when two standard benchmarks of platinum per meter and kilogram were built, which were deposited at the French National Archives in Paris in 1799 – later they would be known as Meter of Archives and Kilogram of Archives. The French Academy of Sciences was authorized by the National Assembly to build a new system of units used all over the world, and in 1946 the Meter Convent countries accepted the MKSA system (meters, kilograms, seconds, amps). In 1954, MKSA expanded further including kelvin and spark plug. The system later became known as the International System of Units, SI (Le Système International d’Unités).
The SI system was established in the 1960s by the 11th General Conference of Weights and Measures CGPM: “The International System of Units, SI, is a coherent unit of units approved and recommended by CGPM.”
At the 14th CGPM Conference held in 1971, SI was again expanded with the molar as the base unit for the amount of substance. The SI system now consists of seven base units, which together with the current units constitute a coherent unit of units. In addition, some other units are accepted outside SI system for use with SI units.
Table 1: Basic SI Units
Table 2: Examples of Current SI Units expressed in SI Base Unit
A base unit is a unit of measurement of a base size in a given size system. Determination and implementation of each SI base unit has been modified as a metrology research that reveals the possibility of achieving a more accurate determination and realization of the unit. Example: Determining the meter in 1889 was based on the international prototype of platinum-iridium located in Paris. At 1960 meters it was redefined as 1 650 763.73 wavelength of a special spectrum of crypto-86. By 1983 this designation was inadequate and was decided to redefine the meter as the length of the road covered by light in the vacuum during a time interval of 1/299 792 458 seconds, and presented as the wavelength of helium laser radiation – iodine stabilized neon. These redefinitions have reduced the relative uncertainty of 10-7m to 10-11 m.
Definitions of SI base units Meter is the length of the street covered by light in the vacuum during a time interval of 1/299 792 458 seconds. Kilogram is equal to the mass of the international prototype kilogram. Second is the duration of 9,192,631,770 radiation periods, corresponding to the transition between the two superfin levels of the basic state of the Cezium atom 133. Ampere is the constant stream which if held in two straight conductors of infinite length, without circular interruption, and placed 1 meter along the vacuum, can produce between two conductors a force equal to 2 x 10-7 Newton for every meter length. Kelvin is equal to 1 / 273.16 parts of the thermodynamic temperature of the triple water point. Mol is the amount of matter of a system containing as elemental units as atoms in 0.012 kg of carbon12. When used as moles, elementary sizes should be specified and may be atoms, molecules, junctions, electrons, other particles or specified groups of such particles. Candle is the intensity of the light in a given direction, of a monochromatic (monochromatic) radiation source with a frequency of 540 x 1012 eras and with a radius of 1/683 wav per steradian in this regard.
Table 3: Current SI units with special names and symbols
A derived unit is a measurement unit of a derived size in a given system of these sizes. SI derived units derive from SI base units in accordance with the physical connection between the sizes. Example: From the physical connection between the length of the measured length to the unit m and the size of the time measured in the unit, the measured velocity measured in the m / s unit flows. The derived units are expressed in the base units by multiplying and dividing the mathematical symbols. CGPM has approved specific names and symbols for several derived units, as shown in Table 3. Some base units are used in different sizes, as shown in Table 4. A derived unit can often be expressed with different combinations of 1) base units and 2) units derived from specific names. In practice, there is a preference for unique names of units and combinations between them in order to distinguish between sizes of the same size. Therefore, a measuring instrument should show the unit as well as the size measured by the instrument.
Table 4: Examples of Current Units AS Names and Symbols of which Include SI Units with Special Names and Symbols Units metrology-4
UNITS OUTSIDE SI
Table 5 provides units outside SI which are accepted for use with SI units because they can be used extensively or they may have been used in specific subject areas.
Table 6 gives examples of units outside the SI system that are accepted for use in specific data fields.
Table 7 presents units outside the SI system that are accepted for specific subject fields, and whose values are experimentally determined. Combined uncertainty (coverage factor k = 1) in the last two digits of the number is given in brackets.
Table 5: Units received outside the SI system Units metrology-5
Table 6: Units outside the SI system that are accepted for use in specific subject areas Units metrology-6
Table 7: Units outside SI that are accepted for specific subject areas and whose values are experimentally determined Units metrology-7
* Taken from “Metrology February”, 3rd edition