By aeiffert, 9/8/2018

If you've looked at some of the experiments or facts on Experimonkey,
you may have noticed that we don't use some of the measurements you're used to,
like **pounds, inches, or gallons**. Instead, we use measurements from something
called the **International System of Units**, or **SI** for short.
(Why not "IS"? Well, "SI" comes from the french *Système international*.)

SI is the modern, updated version of the **metric system**,
the most common measurement system in the world. It is unlike the **United States
Customary System**, which uses different conversion factors for each unit:

1 mile | 880 fathoms | 1,760 yards | 5,280 feet | 63,360 inches |

1 US ton | 17.857 long hundredweights | 20 hundredweights | 2,000 pounds | 32,000 ounces |

1 gallon | 4 quarts | 8 pints | 16 cups | 128 ounces |

Sure we're used to understanding things like, "I ran a mile," or "add a cup of flour," but what happens when we need to convert say 2/3 cups to ounces, or calculate someone's running speed in inches/second? And what if we want to discuss our findings with a scientist from a different country?:

1 kilometer | .621 miles | 1,093.61 yards | 3,280.84 feet | 39,370.1 inches |

1 metric ton | 1.102 US tons | 2204.62 pounds | 19.684 hundredweights | 35,274 ounces |

1 liter | .264 gallons | 1.057 quarts | 4.227 cups | 33.814 ounces |

Believe it or not, US Engineers still have to deal with such complicated conversions. Confused? It would be surprising if you weren't! That's where SI comes into play. SI starts off with 7 base units:

- The Ampere (A) - Measures electric current
- The Kelvin (K) - Measures temperature
- The Second (s) - Measures time
- The meter (m) - Measures length
- The Kilogram (kg) - Measures mass (not weight!)
- The Candela (A) - Measures the strength of light
- The Mole (A) - Amount of a substance

And also gives 22 more derived units, or units that can be expressed from the base units. Click here to see them.

- The Hertz (Hz) - Measures frequency
- The Radian (rad) - Measures angles
- The Steradian (sr) - Measures a field of view
- The Newton (N) - Measures force (and weight)
- The Pascal (Pa) - Measures pressure
- The Joule (J) - Measures energy
- The Watt (W) - Measures power
- The Coulomb (C) - Measures lectric charge
- The Volt (Hz) - Measures voltage
- The Farad (F) - Measures electrical capacitance
- The Ohm (Ω) - Measures electrical resistance
- The Siemens (S) - Measures electrical conductance
- The Weber (Wb) - Measures magnetic flux
- The Tesla (T) - Measures the strength of a magnetic field
- The Henry (H) - Measures electrical inductance
- The Degree Celsius (C) - Measures temperature
- The Lumen (lm) - Measures luminous flux
- The Lux (lx) - Measures illuminance
- The Becquerel (Bq) - Measures radioactivity
- The Gray (Gy) - Measures absorbed ionizing radiation
- The Sievert (Sv) - Measures the health effect of ionizing radiation on the human body
- The Katal (kat) - Measures catalytic activity

Then comes the best part about SI units: to convert between them,
we use multiples, or **bases** of 10. Each base of 10 has a corresponding
prefix and abbreviation. For example, "Kilo-" or "kg" means 1,000. So, a
"Kilogram" means 1,000 grams. A "Kilo-monkey" means 1,000 monkeys! (That's
just a joke, don't worry.) This system is much easier to understand and
remember than the US units (which have different conversions
for every unit)! Some of the most commonly used prefixes are:

Prefix | Abbreviation | Base 10 | Full number |
---|---|---|---|

Nano- | n | 10^{-9} |
.000000001 |

Milli- | m | 10^{-3} |
.001 |

Centi- | c | 10^{-2} |
.01 |

Kilo- | k | 10^{3} |
1,000 |

Because of the prefix system, SI is much easier to use than other systems like the US Customary System. And since it's silly to go back and forth between different systems (remember that second crazy table?), scientists typically choose to work with SI. And so, that's why it's very important to learn and get comfortable with the SI system!

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