5 Things You Didn't Know About the Metric System

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:

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 andLe Système International d'Unitéswas once again affirmed as the world'sgoldstandard.

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 ofwater, 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 oftime(hours).

Fromlanguage语言,不同的名称ent 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 calledkilogramos. 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 amilliwatt(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 theUnited Statesas 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 (theBureau 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 interestingworld of numberson the next page.