In most of the world, the metric system reigns supreme. Officially known as *Le Système International d'Unités*, or the International System of Units, it's more simply known the world over by its abbreviated name, the SI. For the general populace in the United States and a couple of other countries, however, the SI is little practiced and largely ignored.

But many think that's a mistake, among them are those who have ordered and served on the various panels and commissions that have tried to promote metric use in the U.S. Those efforts went largely unheeded by the American public, and the metric system failed to become the dominant measurement system -- outside of a few professions. Because of this, there are many things about the metric system that remain unknown by the average American -- and a few that may even come as a surprise to those who mastered the metric system early in life and use it every day.

It might seem like the SI has been around forever, but that's hardly the case. Many different measurement systems have been used over centuries, and it wasn't until the 1790s and the Revolution in France that the French devised the primitive basis for the modern metric base-10 system. Easier than the unwieldy and nonstandardized systems that had preceded it, this new system had several things going for it:

- Fractions and long strings of zeros were unnecessary.
- It was simpler to manipulate than systems that relied on conversions between different units of measure applied to the same characteristic.
- Hundreds of disparate units were made obsolete through the set system of prefixes.

As these new, simpler units of measure slowly gained momentum, they evolved as well. In fact, the SI is frequently examined and tweaked to make it more accurate or more applicable. It wasn't until 1960, at the General Conference on Weights and Measures, that the system's current incarnation with the seven standard base units was hammered out and *Le Système International d'Unités* was once again affirmed as the world's gold standard.

The units for different types of measures -- such as length and volume -- correlate with one another. For example, 1 milliliter (a unit of volume) has the same proportions as 1 cubic centimeter (centimeters being a measure of length). A liter of water, by extension, is the equivalent of 1 cubic decimeter. It also has a mass approximately the same as 1 kilogram. This even works by expansion; 1 cubic meter can hold approximately 1,000 liters of water and has about the same mass as 1 metric ton -- or 1,000 kilograms.

And apart from the seven base units in the metric system -- the meter, the kilogram, the second, the ampere, the kelvin, the mole and the candela -- other meaningful units of measurement can be derived from those. Consider velocity. It's typically denoted by a unit of length (kilometers) divided by a unit of time (hours).

From language to language, the names of the different units of the SI measure are often slightly different, but the symbols stay the same. So while in English, we know of the metric unit of mass as kilograms, in Spanish, they're called *kilogramos*. But in both languages, you know a person is referring to a kilogram if the number is followed by the abbreviation kg. Those base units we talked about on the last page all have letters assigned to them, so it's clear the amount being discussed no matter what country you're in.

Even in countries where Roman characters aren't used, such as Japan and China, you will still see the familiar sight of m, km, g, mg, kcal and all the rest. Capitalization is very important, too, otherwise a milliwatt (mW) could easily be confused with a megawatt (MW).

The word "ton" is ambiguous, because it can be used in reference to multiple weights. There is the short ton, for example, which is equal to 2,000 pounds or about 907 kilograms. Then there is the long ton, which is 2,240 pounds, or about 1,016 kilograms. Finally, the word "ton" can also be used in reference to what is commonly known in the United States as the metric ton, which is assigned the official symbol "t" in the International System of Units. That's 1,000 kilograms, or about 2,204 pounds [source: U.S. Metric Association]. You may also see the word "ton" spelled as "tonne" although technically, the correct metric term for 1,000 kilograms is a megagram -- denoted by the symbol Mg.

Speaking of kilograms, let's look closer at them on the next page.

You might think of metric values as set in stone, but in some situations, that's not the case. Take kilograms. They've long been defined as the mass of a cylinder of certain dimensions crafted from platinum and iridium, rather than in terms of a fundamental constant of nature. That cylinder is an actual object; it's known as the international prototype of the kilogram and it's housed in Sèrves, France, at the headquarters of the International Bureau of Weights and Measures (the *Bureau International des Poids et Mesures*, or BIPM).

Unfortunately, over time, minor variations have been measured between the weight of the international prototype and its official replicas. Metrologists have been working on the issue, but have yet to find a way to define the kilogram in a manner that correlates exactly to known constants, such as Planck's constant and Avogadro's constant [source: Brumfiel]. Until then, a kilogram's precise weight depends on a physical artifact -- although to be fair, the BIPM has the weight nailed down to the parts per billion level. However, metrologists are an exacting bunch, and they haven't been able to fully reconcile the two -- yet. Find out more about the interesting world of numbers on the next page.

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### Sources

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- "Practical Guide to the International System of Units. Colorado State University. (September 9, 2011) http://lamar.colostate.edu/~hillger/pdf/Practical_Guide_to_the_SI.pdf
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- "Weights and Measures." The CIA World Factbook. (September 9, 2011) https://www.cia.gov/library/publications/the-world-factbook/appendix/appendix-g.html

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