Measurements Standard Based on International Unit

Measurement Standard Based on International Unit

 The International System of Units (French: Système International d'Unités, SI) is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units. It defines twenty-two named units, and includes many more unnamed coherent derived units. The system also establishes 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 system was published in 1960 as the result of an initiative that started in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 25th General Conference on Weights and Measures (CGPM) in 2014, for example, discussed a proposal to change the definition of the kilogram.

History:

The metric system was first implemented during the French Revolution (1790s) with just the metre and kilogram as standards of length and mass respectively. In the 1830s Carl Friedrich Gauss laid the foundations for a coherent system based on length, mass, and time. In the 1860s a group working under the auspices of the British Association for the Advancement of Science formulated the requirement for a coherent system of units with base units and derived units. The inclusion of electrical units into the system was hampered by the customary use of more than one set of units, until 1900 when Giovanni Giorgi identified the need to define one single electrical quantity as a fourth base quantity alongside the original three base quantities.

Base units:

Main article: SI base units

The SI base units are the building blocks of the system and all other units are derived from them. When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length and time. Giorgi later identified the need for an electrical base unit. Theoretically any one of electric current, potential difference, electrical resistance, electrical charge or a number of other quantities could have provided the base unit, with the remaining units then being defined by the laws of physics. In the event, the unit of electric current was chosen for SI. Another three base units (for temperature, substance and luminous intensity) were added later.

SI base units[33]:23[37][38]
Unit name	Unit symbol	Quantity name	Definition (incomplete)[n 1]	Dimension symbol
metre	m	length	Original (1793): 1/10000000 of the meridian through Paris between the North Pole and the Equator.FG Interim (1960): 1650763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom. Current (1983): The distance travelled by light in vacuum in 1/299792458 second.	L
kilogram[n 2]	kg	mass	Original (1793): The grave was defined as being the weight [mass] of one cubic decimetre of pure water at its freezing point.FG Current (1889): The mass of the international prototype kilogram.	M
second	s	time	Original (Medieval): 1/86400 of a day. Interim (1956): 1/31556925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. Current (1967): The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.	T
ampere	A	electric current	Original (1881): A tenth of the electromagnetic CGS unit of current. The [CGS] electromagnetic unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius creates a field of one oersted at the centre.[39] IEC Current (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a forceequal to 2×10−7 newtons per metre of length.	I
kelvin	K	thermodynamic temperature	Original (1743): The centigrade scale is obtained by assigning 0 °C to the freezing point of water and 100 °C to theboiling point of water. Interim (1954): The triple point of water (0.01 °C) defined to be exactly 273.16 K.[n 3] Current (1967): 1/273.16 of the thermodynamic temperature of the triple point of water	Θ
mole	mol	amount of substance	Original (1900): The molecular weight of a substance in mass grams.ICAW Current (1967): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.[n 4]	N
candela	cd	luminous intensity	Original (1946): The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre. Current (1979): 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.	J

Note

1. Jump up^ Interim definitions are given here only when there has been a significant difference in the definition.
2. Jump up^ Despite the prefix "kilo-", the kilogram is the base unit of mass. The kilogram, not the gram, is used in the definitions of derived units. Nonetheless, units of mass are named as if the gram were the base unit.
3. Jump up^ In 1954 the unit of thermodynamic temperature was known as the "degree Kelvin" (symbol °K; "Kelvin" spelt with an upper-case "K"). It was renamed the "kelvin" (symbol "K"; "kelvin" spelt with a lower case "k") in 1967.
4. Jump up^ When the mole is used, the elementary entities must be specified and may be atoms, molecules, ion,  electrons,  other particles, or specified groups of such particles.

The original definitions of the various base units in the above table were made by the following authorities:

• FG = French Government
• IEC = International Electrotechnical Commission.  
• ICAW = International Committee on Atomic Weight.

All other definitions result from resolutions by either CGPM or the CIPM and are catalogued in the SI Brochure.

Founders:

Metre: John Wilkins             

 

Ampre: André-Marie Ampère  

Kelvin: William Thomson  

                                

 Mole: Amadeo Avogadro   

Named units derived from SI base units:

The derived units in the SI are formed by powers, products or quotients of the base units and are unlimited in number.[22]:103[33]:3 Derived units are associated with derived quantities, for example velocity is a quantity that is derived from the base quantities of time and length, so in SI the derived unit is metres per second (symbol m/s). The dimensions of derived units can be expressed in terms of the dimensions of the base units.

Name	Symbol	Quantity	Expressed in terms of other SI units	Expressed in terms of SI base units
radian	rad	angle		m·m−1
steradian	sr	solid angle		m2·m−2
hertz	Hz	frequency		s−1
newton	N	force, weight		kg·m·s−2
pascal	Pa	pressure, stress	N/m2	kg·m−1·s−2
joule	J	energy, work, heat	N·m	kg·m2·s−2
watt	W	power, radiant flux	J/s	kg·m2·s−3
coulomb	C	electric charge or quantity of electricity		s·A
volt	V	voltage (electrical potential difference), electromotive force	W/A	kg·m2·s−3·A−1
farad	F	electric capacitance	C/V	kg−1·m−2·s4·A2
ohm	Ω	electric resistance, impedance, reactance	V/A	kg·m2·s−3·A−2
siemens	S	electrical conductance	A/V	kg−1·m−2·s3·A2
weber	Wb	magnetic flux	V·s	kg·m2·s−2·A−1
tesla	T	magnetic field strength	Wb/m2	kg·s−2·A−1
henry	H	inductance	Wb/A	kg·m2·s−2·A−2
degree Celsius	°C	temperature relative to 273.15 K		K
lumen	lm	luminous flux	cd·sr	cd
lux	lx	illuminance	lm/m2	m−2·cd
becquerel	Bq	radioactivity (decays per unit time)		s−1
gray	Gy	absorbed dose (of ionizing radiation)	J/kg	m2·s−2
sievert	Sv	equivalent dose (of ionizing radiation)	J/kg	m2·s−2
katal	kat	catalytic activity		mol·s−1
Notes 1. The radian and steradian, once given special status, are now considered dimensionless derived units.[33]:3 2. The ordering of this table is such that any derived unit is based only on base units or derived units that precede it in the table.

Source: https://en.wikipedia.org/wiki/International_System_of_Units