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A system of measure initially based on: The circumference of the Earth, the freezing and boiling points of water under 1 atm, the density of water at 4oC, the specific heat capacity of water, rotational period of the Earth, and a number of other things. It uses powers of ten for the relationships between as many units as possible. It was developed during the French Revolution. The two main variants of Metric are mks/SI and cgs. Only the most backward of countries cling to antiquated systems like Imperial and it's derivative US Standard.

Devised by French scientists in the late 18th century to replace the chaotic collection of units then in use. The goal of this effort was to produce a system that did not rely on a miscellany of separate standards, and to use the decimal system rather than fractions.

To obtain a standard of length a quadrant of the earth (one-fourth of a circumference) was surveyed (actually only in part) along the meridian that passes through Paris. This distance was divided into ten million parts to become the meter (spelled metre in some countries - including France (mètre)). The definition of the meter has become more and more precise through the years since. Currently the meter is the distance light travels in a vacuum in 1/299 792 458 second. The nautical mile used in modern navigation, in relation to which boat speeds and wind velocities are measured (one knot is one nautical-mile-per-hour), is defined as one minute of latitude. A degree of latitude therefore is 60 nautical miles. The quadrant of the earth measured by the French, being 90 degrees, measures 90x60 or 5400 nautical miles. Therefore: 5400 nautical miles exactly equal 10-million meters, or 10 000 kilometers.

The units most commonly used from the metric system are listed below:
• m - meter/metre, for length.
• kg - kilogram, for mass/weight.
• t - ton, 1000 kg.
• s - second, for time.
• °C - degrees Celsius, for temperature. Water freezes at 0°C and boils at 100°C.
• m2 square metre, for area.
• ha - hectare, 10,000 m2.
• l - liter/litre, for volume. 1 l of water at 4°C weighs 1 kg.
• m3 - cubic metre, 1,000 litres.
• m/s - metres per second - for wind speeds.
• km/h - kilometres per hour - 3.6 metres per second - for velocities of cars/planes/bicycles/joggers
• N - newton, for force (the force of a 1 kg weight lifted form the ground (on Earth) is about 9.82 N .. BUGGER!!)
• kPa - kilopascal, for pressure.
• J - joule, for energy. J=N*m.
• W - watt, for power. 1 W = 1 J/s.
• 1 km=0.62 mile 1 mile=1.61 km
• 1 m=1.09 yards 1 yard=0.914 m
• 1 m=3.28 feet 1 foot=0.305 m
• 1 m=39.37 inches
• 1 cm=0.394 inch 1 inch=2.54 1 foot=30.5 cm
• 1 mm=0.039 inch

Area:

• 1hectare(ha)=10000m2
• 1 ha=2.47 acres 1 acre=0.0405 ha
• 1 m2=1.196 square yards(SY) 1 SY=0.8361 m2
• 1 m2=10.764 square feet (SF) 1 SF=0.0929 m2
• 1 cm2=0.155 square inch (SI) 1 SI=6.4516 cm2

Mass:

• 1 metric ton (t)= 1000 kilograms (kg)
• 1 metric ton (t)= 1.103 tons 1 ton=0.907 t
• 1 kg=1000 grams (g)
• 1 kg=2.205 pounds 1 pound=0.4536 kg
• 1 g =1000 milligrams (mg)
• 1 g =0.0353 ounce 1 ounce=28.35 g
• 1 g =15.432 grains
• 1 mg=approx. 0.015 grain
• 1 cubic meter (m3)=1000000 cubic cm (cm3)
• 1 m3=1.308 cubic yards 1 cubic yard=0.7646 m3
• 1 m3=35.315 cubic feet 1 cubic foot=0.0283 m3
• 1 cubic centimeter (cm3)=0.000001 m3
• 1 cm3=0.061 cubic inch 1 cubic inch=16.387 cm3
• 1 kiloliter (kl or kL)=1000 liter
• 1 kL=264.17 gallons
• 1 liter (L)=1000 milliliters (mL)
• 1 L=0.264 gallons 1 gallon=3.785 L
• 1 L=1.057 quarts 1 quart=0.946 L
• 1 mL=0.034 fluid ounce 1 fluid ounce=229.57 mL
• 1 mL=approx. 1/4 teaspoon 1 teaspoon=approx. 4 mL
• 1 quart=946 mL
• 1 pint=473 mL
• 1 second (s)=1/60 minute
• 1 millisecond (ms)=0.001 second
• F(Fahrenheit) = 9/5 C(Celsius) + 32
• C = 5/9 (F-32)

*note:
the absolute 0 is at -273 Celsius, which is the same as 0 Kelvin. Kelvin and Celsius have the same size degrees.

• The benefit of using metric over other systems is in its utility at conversion, since most people don't like to be bothered to have to do math without a calcucrutch.

That's fine and dandy. I can admit that, even as an American, the metric system makes sense, except for one glaring exception that I adamantly refuse to budge on:

Temperature is better measured using the Fahrenheit system.

"But OUR system is based on the boiling and freezing points of water!" they say. SO WHAT? With Fahrenheit the degrees of latitude are more expressive in terms of our own physical tolerances. The difference of a mere 10 degrees means changing from a 20°C jacket to 30°C shorts. If the measurement is based on something so arbitary as the physical properties of water, why not make it more useful by comparing it to degrees of human livability? Fahrenheit accomplishes this nicely. 0°F and 100°F are approximately the lower and upper limits that the human body can comfortably handle, taking into account the normal range of clothing offered to us. It's a realistic, and more to the point, useful measurement.

The benefits of metric conversion are lost on temperature. The only advantage is that you don't have to think when someone asks you at what temperature water freezes or boils (and honestly, when was the last time you cared what the temperature of boiling water was?).
More precise definition of a few metric (SI) base units:

• s - second: One second is 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
• m - meter/metre: One meter is the distance light travels in 1/299 792 458 of a second.
• A - ampere: One ampere is the constant current which would produce a force of 2*10^-7 newton per meter of length when the currant is maintained in two straight parallel conductors of infinite length and neglectible circular cross section placed one meter apart in vacuum.
• K - kelvin: The kelvin, unit of thermodynamic temperature, is the fraction of 1/237.16 of the thermodynamic temperature of the triple point of water.
• kg - kilogram: A kg is equal to the mass of the international prototype of the kilogram. (A particular cylinder of platinum-iridium alloy that is preserved in a vault at Sevres, France.) (Duh!)
• cd - candela: The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 * 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
• mol - mole: The amount of a substance that contains as many elementary entities as there are carbon atoms in 0.012 kg of carbon 12

And so on... most of the others are derived from these.
Sources:
• Tabeller og formelsamling / Gyldendal norsk forlag AS 2001
• University Physics / Young & Freedman / Addison-Wesley Publishing Company, Inc. 1996
(Thanks to Professor Pi for correcting some horrible errors)
The metric system is a necessity when dealing and trading with other people. Consider that once upon a time the yard was measured as half the distance from finger tip to finger tip of the King's outstreched arms, and that a pound was the weight of "7,000 grains of barley chosen from the middle ear". Understandably, problems arose when trading with anyone but a friend or neighbour.

Merchants developed their own system (the British system) which was more accurate, but not very logical or easy to understand. Consider:
• 2 pints to the quart
• 4 quarts to the gallon
• 8 furlongs to the mile
• 12 inches to the foot
• 16 ounces to the pound
• 20 hundredweight to the ton
• 22 yards to the chain

Take into account that during the Middle Ages there were no less than five very different weights for the pound. This follows us through to today, as an ounce of precious metals is a different weight to an ounce of food, and an American gallon is different to a British gallon.

In 1799, following the French Revolution, standards for a metric system were set. These were:
• A metre was one ten-millionth of the distance from the North Pole to the equator.
• A litre was exactly 1,000 cubic centimetres.
• A kilogram was the weight of one litre of water with a temperature of four degrees celsius.

While this system was definitely a step in the right direction, scientific work requires that the same measurements are able to be reproduced anywhere in the world - a fact that the system did not satisfy. An international conferance in 1875 led to the standards of length and weight in 1889, in the form of a metre ruler and a kilogram weight. These measurements are considered to be exactly correct. The weight is kept in Paris, France, and copies of these international standards were distributed throughout the world. The weight is made out of an alloy of platinum and iridium. Unfortunately, perhaps due to the cleaning processes, either the foreign weights are gaining mass or the French one is losing mass.

In 1960, an international conferance approved the International System of Units or S.I., which is explained in detail above. Also in 1960, the metre ruler was retired, and the metre was redifined in terms of light emitted from the gas krypton. This makes the system almost infinitely accurate, and you can be sure that all over the world the metric system shouldbe the same.

The people in charge of regulating all of this are called the International Bureau of Weights and Measures.
• The metric system is a system of measurement in which each unit is named by combining a prefix denoting a power of 10 with the name of a base unit. For example, the centi- prefix represents 10-2; thus, a centimetre is 1/100 the length of a metre.

The base units are as follows:

Concept measured Name Symbol Definition
Time Second s The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at rest at 0K1
Distance Metre (spelt meter in America m The length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.2
Area Are a 100 square metres
Volume Litre (spelt liter in America l (lowercase L) Equal to 1dm3
Mass Kilogram kg The mass of a lump of metal in Paris after being cleaned in a certain way3
Temperature Kelvin K The fraction 1/273.16 of the thermodynamic temperature of the triple point of water4 (Easier definition: 1/100 of the difference in temperature between the freezing and boiling points of water at standard pressure)
Electric current Ampère A That constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length5 (Easier definition: 96,500 electrons per second)
Substance Mole mol The amount of substance of a system which contains as many elementary entities as there are atoms in 12g of carbon 126
Luminous intensity Candela cd The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 W per steradian6

The system has plenty of other units as well, but they are all defined in terms of the above; for example, the joule is defined as kg m2 s-2.

Note that the base unit of mass, unlike the others, has a prefix; namely kilo-. When the metric system was originally devised, the base unit of mass was called the grave, and was equal in magnitude to what is today called the kilogram. Gram was a slang term for what was then called the milligrave. However, grave also meant count; after the French Revolution, aristocracy was rather unpopular, and so the term was dropped in favour of kilogram.

Under the SI system, the are and litre are deprecated, with area and volume given in square metres and cubic metres respectfully. However, litres and hectares are commonly used in everyday life due to greater convenience..

Prefixes are denoted by, in all but one case, a single letter placed immediately before the symbol for the base unit. Each represents come power of 10. It is important to always use the prefix symbol in its correct case - there is a big difference between mm and Mm.

Prefix Symbol Magnitude
yocto- y 10-24
zepto- z 10-21
atto- a 10-18
femto- f 10-15
pico- p 10-12
nano- n 10-9
micro- µ 10-6
milli- m 10-3
centi- c 10-2
deci- d 10-1
deca- da 101
hecto- h 102
kilo- k 103
mega- M 106
giga- G 109
tera- T 1012
peta- P 1015
exa- E 1018
zetta- Z 1021
yotta- Y 1024

These prefixes can be added to any base unit you care to name (though note that in the case of mass, the prefix is applied to the gram and not kilogram). Thus, for example, 100 metres is a hectometre (1 hm), and one millionth of a joule is a microjoule (µJ). They can even be used for non-metric, non-SI units of measurement; for example, the American telecommunications industry traditionally measured wires in thousands of feet, and so it was only a matter of time before they started referring to kilofeet.

### References

1. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html
2. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/metre.html
3. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kilogram.html
4. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html
5. http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html