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International System of Units
The International System of Units is the standard modern form of the metric system. The name of this system can be shortened or abbreviated to SI, from the French name Système International d'unités.
The International System of Units is a system of measurement based on 7 base units: the metre (length), kilogram (mass), second (time), ampere (electric current), Kelvin (temperature), mole (quantity), and candela (brightness). These base units can be used in combination with each other. This creates SI derived units, which can be used to describe other quantities, such as volume, energy, pressure, and velocity.
The system is used almost globally. Only Myanmar, Liberia, and the United States do not use SI as their official system of measurement.^{[1]} In these countries, though, SI is commonly used in science and medicine.
Contents
History and use
The metric system was created in France after the French Revolution in 1789. The original system only had two standard units, the kilogram and the metre. The metric system became popular amongst scientists.
In the 1860s, James Clerk Maxwell and William Thomson (later known as Lord Kelvin) suggested a system with three base units  length, mass, and time. Other units would be derived from those three base units. Later, this suggestion would be used to create the centimetregramsecond system of units (CGS), which used the centimetre as the base unit for length, the gram as the base unit for mass, and the second as the base unit for time. It also added the dyne as the base unit for force and the erg as the base unit for energy.
As scientists studied electricity and magnetism, they realized other base units were needed to describe these subjects. By the middle of the 20th century, many different versions of the metric system were being used. This was very confusing.
In 1954, the 9th General Conference on Weights and Measures (CGPM) created the first version of the International System of Units. The six base units that they used were the metre, kilogram, second, ampere, Kelvin, and candela.^{[2]} The seventh base unit, the mole, was added in 1971.^{[3]}
SI is now used almost everywhere in the world, except in the United States, Liberia and Myanmar, where the older imperial units are still widely used. Other countries, most of them historically related to the British Empire, are slowly replacing the old imperial system with the metric system or using both systems at the same time.
Units of measurement
Base units
The SI base units are measurements used by scientists and other people around the world. All the other units can be written by combining these seven base units in different ways. These other units are called "derived units".
Unit name  Unit symbol 
Quantity measured  General definition  

metre  m  length 
 
kilogram ^{[note 1]}  kg  mass 
 
second  s  time 
 
ampere  A  electric current 
 
kelvin  K  temperature 
 
mole  mol  amount of substance  
candela  cd  luminous intensity 
 

Derived units
Derived units are created by combining the base units. The base units can be divided, multiplied, or raised to powers. Some derived units have special names. Usually these were created to make calculations simpler.
Name  Symbol  Quantity  Definition other units 
Definition SI base units 

radian  rad  plane angle  −  
steradian  sr  solid angle  −  
hertz  Hz  frequency  s^{−1}  
newton  N  force, weight  m∙kg∙s^{−2}  
pascal  Pa  pressure, stress  N/m^{2}  m^{−1}∙kg∙s^{−2} 
joule  J  energy, work, heat  N∙m  m^{2}∙kg∙s^{−2} 
watt  W  power, radiant flux  J/s  m^{2}∙kg∙s^{−3} 
coulomb  C  electric charge  s∙A  
volt  V  voltage, electrical potential difference, electromotive force  W/A J/C 
m^{2}∙kg∙s^{−3}∙A^{−1} 
farad  F  electrical capacitance  C/V  m^{−2}∙kg^{−1}∙s^{4}∙A^{2} 
ohm  Ω  electrical resistance, impedance, reactance  V/A  m^{2}∙kg∙s^{−3}∙A^{−2} 
siemens  S  electrical conductance  1/Ω  m^{−2}∙kg^{−1}∙s^{3}∙A^{2} 
weber  Wb  magnetic flux  J/A  m^{2}∙kg∙s^{−2}∙A^{−1} 
tesla  T  magnetic field strength  Wb/m^{2} V∙s/m^{2} N/A∙m 
kg∙s^{−2}∙A^{−1} 
henry  H  inductance  Wb/A V∙s/A 
m^{2}∙kg∙s^{−2}∙A^{−2} 
degree Celsius  °C  temperature relative to 273.15 K  T_{K} − 273.15  K 
lumen  lm  luminous flux  cd∙sr  cd 
lux  lx  illuminance  lm/m^{2}  m^{−2}∙cd 
becquerel  Bq  radioactivity (decays per unit time)  s^{−1}  
gray  Gy  absorbed dose (of ionizing radiation)  J/kg  m^{2}∙s^{−2} 
sievert  Sv  equivalent dose (of ionizing radiation)  J/kg  m^{2}∙s^{−2} 
katal  kat  catalytic activity  s^{−1}∙mol 
Prefixes
Very large or very small measurements can be written using prefixes. Prefixes are added to the beginning of the unit to make a new unit. For example, the prefix kilo means "1000" times the original unit and the prefix milli means "0.001" times the original unit. So one kilometre is 1000 metres and one milligram is a 1000th of a gram.
Multiples  Name  deca  hecto  kilo  mega  giga  tera  peta  exa  zetta  yotta  

Prefix  da  h  k  M  G  T  P  E  Z  Y  
Factor  10^{0}  10^{1}  10^{2}  10^{3}  10^{6}  10^{9}  10^{12}  10^{15}  10^{18}  10^{21}  10^{24}  
Fractions  Name  deci  centi  milli  micro  nano  pico  femto  atto  zepto  yocto  
Prefix  d  c  m  μ  n  p  f  a  z  y  
Factor  10^{0}  10^{−1}  10^{−2}  10^{−3}  10^{−6}  10^{−9}  10^{−12}  10^{−15}  10^{−18}  10^{−21}  10^{−24} 
References
 ↑ "Appendix G: Weights and Measures". The World Facebook. Central Intelligence Agency. 2013. https://www.cia.gov/library/publications/theworldfactbook/appendix/appendixg.html. Retrieved 5 April 2013.
 ↑ International Bureau of Weights and Measures (1954), Système pratique d'unités de mesure (Practical system of units), http://www.bipm.org/en/CGPM/db/10/6/. 9^{th} session, Resolution 6.
 ↑ International Bureau of Weights and Measures (1971), Unité SI de quantité de matière (SI unit of amount of substance), http://www.bipm.org/en/CGPM/db/14/3/. 14^{th} session, Resolution 3.
 ↑ McKenzie, A.E.E (1961). Magnetism and Electricity. Cambridge University Press. p. 322. http://books.google.co.uk/books?id=m7c6AAAAIAAJ&pg=PA322&lpg=PA322&dq=cgs+unit+of+current+1881&source=bl&ots=Lw5kV5jd4&sig=mytqNION410y2qYnGEGnHcbu6o&hl=en&sa=X&ei=qaWkUKfiFeeZ0QXpvYC4CA&ved=0CCAQ6AEwADgK#v=onepage&q=cgs%20unit%20of%20current%201881&f=false.