International System of Units
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The International System of Units, symbolized SI, is the simplified modern version of the metric system. Congress has adopted SI as the preferred measurement system for the United States because of its many advantages:
- No conversions (only one unit for each quantity)
- No numbers to memorize (derived units are defined without numerical factors)
- No fractions (decimals only)
- No long rows of zeros (prefixes eliminate them)
- Only 30 individual units (compared to hundreds of traditional units)
- Easy to pronounce and write (short names; simple letter symbols)
- Based on natural standards (size of Earth, water, laws of physics)
- Coherent system (symbols can be manipulated algebraically)
- World standard (even traditional U.S. units are defined by it)
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SI base units
These seven base quanteties are the foundation of the SI and are assumed to be mutually independent. The remaining SI units can be algebraically derived from the base units.
| Base quantity | Name | Symbol |
|---|---|---|
| length | meter | m |
| mass | kilogram | kg |
| time | second | s |
| electric current | ampere | A |
| thermodynamic temperature | kelvin | K |
| amount of substance | mole | mol |
| luminous intensity | candela | cd |
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