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# SI base units definedSI base units defined

SI symbols are unique. SI symbols are not abbreviations. In other words, not followed by a period. SI symbols always have a space between the symbol and the numeric value. There are only seven base units. SI base units also form the basis for Imperial and US customary units. SI base units are no longer defined physically, but mathematically.

The definition of the units given below are all from Wikipedia.
SI Base Units
0
Quantity
Unit Name
Symbol
Current definition
Original Defintion
1
length
meter
m
Length of the path travelled by light in a vacuum in 1⁄299,792,458 of a second (17th CGPM)
The meter equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris
2
mass
kilogram
kg
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
The grave (the precursor of the kilogram) was defined as the mass of 1 litre (dm3) of water, which was determined to be 18841 grains.
3
time
second
s
The second is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.
The second was historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each (24 × 60 × 60 = 86400).
4
electric current
ampere
A
The ampere is defined by fixing the elementary charge e to be exactly 1.602176634×10−19 C (coulomb),
One ampere is equal to 6.241509074×1018 electrons worth of charge moving past a point in a second
5
thermodynamic temperature
kelvin
K
The scale is defined by fixing the Boltzmann constant k to be exactly 1.380649×10−23 J⋅K−1
The Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the melting point of water to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius
6
amount of substance
mole
mol
The mole is defined as containing exactly 6.02214076×1023 elementary entities
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.
7
luminous intensity
candela
cd
The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
A common wax candle emits light with a luminous intensity of roughly one candela.
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