View source for S.I. Units

[[File:SIbaseunits.png|300px|right]]
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The modern metric system is the [https://en.wikipedia.org/wiki/International_System_of_Units '''International System of Units'''] ''(French: Système International d'Unités, SI)''.  The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now seven internationally agreed units plus units derived from them to cover all aspects of technology together with a set of [https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes '''prefixes'''] which serve as multipliers or fractions of them.


The preferred prefixes multiply or divide the unit names by 1000 ''(or 10<sup>-3</sup>)'' thus, 10mm is preferred to a centimetre, especially in construction and engineering.

----
<span style="color: green">'''Note:'''
[http://en.wikipedia.org/wiki/List_of_scientific_units_named_after_people '''''Units named after people'''''] use capital letters for the letter symbol - but not for the unit name itself.
</span>
----

=====Measurement of Length=====
{|
|-
! colspan="5"|[https://en.wikipedia.org/wiki/Metre '''Metre''']
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| kilometre
| km
| 10<sup>3</sup>
| 1000
| A thousand metres
|-
| metre
| m
| 10<sup>0</sup>
| 1
| A metre
|-
| centimetre
| cm
| 10<sup>-2</sup>
| 0.01
| A hundredth of a metre
|-
| millimetre
| mm
| 10<sup>-3</sup>
| 0.001
| A thousandth of a metre
|-
| micrometre
| μm
| 10<sup>-6</sup>
| 0.000001
| A millionth of a metre
|}


=====Units used in Mechanics=====

{|
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| gram
| g
| 10<sup>-3</sup>
| 1/1000
| Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a [https://en.wikipedia.org/wiki/Kilogram '''kilogram''']
|-
| kilogram
| kg
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Mass''' - equal to the mass of the ''International Prototype of the Kilogram''
|-
| tonne
| t
| 10<sup>3</sup>
| 1000
| Known as '''Metric Ton''' in USA ''(i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs)''
|-
| megatonne
| Mt
| 10<sup>9</sup>
| 1,000,000,000
| Used for the energy of nuclear explosions and other similar events.
|-
| Newton
| N
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Force''' named after [https://en.wikipedia.org/wiki/Isaac_Newton '''Sir Isaac Newton'''] - the amount needed to accelerate a 1kg mass at 1 metre per second each second ''(1N = 1kg X ms<sup>2</sup>)''
|-
| Newton Metre
| Nm
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Torque or Moment''' - resulting from applying a force of one newton perpendicularly to a moment arm which is one metre long ''(1Nm)''. It is calculated as for a joule but is preferred for Torque to avoid confusion with Work and Energy.
|-
| Joule
| J
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Work or Energy''' named after [https://en.wikipedia.org/wiki/James_Prescott_Joule '''James Prescott Joule'''] - the amount needed to apply a newton over the distance of a metre ''(1J = 1Nm)''
|-
| Pascal
| Pa
| 10<sup>0</sup>
| 1
| '''S.I. Base Unit of Pressure or Stress''' named after [https://en.wikipedia.org/wiki/Blaise_Pascal '''Blaise Pascal'''] - the pressure exerted by applying a newton over a square metre ''(1Pa = 1Nm<sup>2</sup>)''
|}


=====Units used in Electronics=====

{|
|-
! colspan="5"|[https://en.wikipedia.org/wiki/Ohm '''Ohms (resistance)'''] - named after [https://en.wikipedia.org/wiki/Georg_Ohm '''Georg Simon Ohm''']
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| mega-ohm
| MΩ
| 10<sup>6</sup>
| 1000000
| A million ohms
|-
| kilo-ohm
| kΩ ''(or K)''
| 10<sup>3</sup>
| 1000
| A thousand ohms
|-
| ohm
| Ω ''(or R)''
| 10<sup>0</sup>
| 1
| An ohm
|-
| colspan="5"|<span style="color: green">'''Note:'''
The unit or its prefix may be used to replace a decimal point - e.g. 2.2mega-ohm = 2M2; 2.2kilo-ohm = 2K2; and 2.2ohm = 2R2.
</span>
|-
! colspan="5"|[https://en.wikipedia.org/wiki/Ampere '''Ampere (electric current)'''] - named after [https://en.wikipedia.org/wiki/Andr%C3%A9-Marie_Amp%C3%A8re ''André-Marie Ampère''']
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| ampere
| A
| 10<sup>0</sup>
| 1
| Amp
|-
| milli-amp
| mA
| 10<sup>-3</sup>
| 0.001
| A thousandth
|-
! colspan="5"|[https://en.wikipedia.org/wiki/Farad '''Farads (electric capacitance)'''] - named after [https://en.wikipedia.org/wiki/Michael_Faraday '''Michael Faraday''']
|-
! '''Prefix Name'''
! '''Symbol'''
! '''10<sup>n</sup>'''
! '''Decimal'''
! '''Description'''
|-
| farad
| F
| 10<sup>0</sup>
| 1
| A Farad
|-
| micro-Farad
| μF
| 10<sup>-6</sup>
| 0.000001
| A millionth
|-
| nano-Farad
| nF
| 10<sup>-9</sup>
| 0.000000001
| A thousand millionth
|-
| pico-Farad
| pF
| 10<sup>-12</sup>
| 0.000000000001
| A trillionth
|}


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