From DT Online
The modern metric system is the 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 prefixes which serve as multipliers or fractions of them.
The preferred prefixes multiply or divide the unit names by 1000 (or 10-3) thus, 10mm is preferred to a centimetre, especially in construction and engineering.
Note: Units named after people use capital letters for the letter symbol - but not for the unit name itself.
Measurement of Length
|kilometre||km||103||1000||A thousand metres|
|centimetre||cm||10-2||0.01||A hundredth of a metre|
|millimetre||mm||10-3||0.001||A thousandth of a metre|
|micrometre||μm||10-6||0.000001||A millionth of a metre|
Units used in Mechanics
|gram||g||10-3||1/1000||Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a kilogram|
|kilogram||kg||100||1||S.I. Base Unit of Mass - equal to the mass of the International Prototype of the Kilogram|
|tonne||t||103||1000||Known as Metric Ton in USA (i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs)|
|megatonne||Mt||109||1,000,000,000||Used for the energy of nuclear explosions and other similar events.|
|Newton||N||100||1||S.I. Base Unit of Force named after Sir Isaac Newton - the amount needed to accelerate a 1kg mass at 1 metre per second each second (1N = 1kg X ms2)|
|Newton Metre||Nm||100||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||100||1||S.I. Base Unit of Work or Energy named after James Prescott Joule - the amount needed to apply a newton over the distance of a metre (1J = 1Nm)|
|Pascal||Pa||100||1||S.I. Base Unit of Pressure or Stress named after Blaise Pascal - the pressure exerted by applying a newton over a square metre (1Pa = 1Nm2)|
Units used in Electronics
|Ohms (resistance) - named after Georg Simon Ohm|
|mega-ohm||MΩ||106||1000000||A million ohms|
|kilo-ohm||kΩ (or K)||103||1000||A thousand ohms|
|ohm||Ω (or R)||100||1||An ohm|
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.
|Ampere (electric current)' - named after André-Marie Ampère|
|Farads (electric capacitance) - named after Michael Faraday|
|nano-Farad||nF||10-9||0.000000001||A thousand millionth|