| The metric system of measurement is an | | | | defined as the mass of a certain bar of |
| internationally agreed-upon set of units for expressing | | | | platinum-iridium alloy that has been kept since 1889 at |
| the amounts of various quantities such as length, | | | | the International Bureau of Weights and Measures in |
| mass, time, and temperature. The metric system has | | | | Sèvres, France. The kilogram is equivalent to 2.2046 |
| several advantages over the English system which is | | | | pounds. |
| still in place in the U.S. However the scientific | | | | The metric unit of time is the same second that has |
| community has adopted the metric system almost | | | | always been used, except that it is now defined in a |
| from its inception. In fact, the metric system missed | | | | very accurate way. It no longer depends on the |
| being nationalized in this country by one vote in the | | | | wobbly rotation of our planet (1/86,400th of a day), |
| Continental Congress in the late 1700's or early 1800's. | | | | because Earth is slowing down. Days keep getting a |
| As of 1994, every nation in the world has adopted | | | | little longer as Earth grows older. So the second is |
| the metric system, with only four exceptions: the | | | | now defined in terms of the vibrations of a certain |
| United States, Brunei, Burma, and Yemen (which use | | | | kind of atom known as cesium-133. One second is |
| the English units of measurement). Because of its | | | | defined as the amount of time it takes for a |
| convenience and consistency, scientists have used | | | | cesium-133 atom to vibrate in a particular way |
| the metric system of units for more than 200 years. | | | | 9,192,631,770 times. Because the vibrations of atoms |
| Originally, the metric system was based on only three | | | | depend only on the nature of the atoms themselves, |
| fundamental units: the meter for length, the kilogram | | | | cesium atoms will presumably continue to behave |
| for mass, and the second for time. Today, there are | | | | exactly like cesium atoms forever. The exact number |
| more than 50 officially recognized units for various | | | | of cesium vibrations was chosen to come out as |
| scientific quantities. | | | | close as possible to what was previously the most |
| | | | | accurate value of the second. The metric unit of |
| Nearly all early units of size were based on the | | | | temperature is the degree Celsius, which replaces the |
| always-handy human body. In the Middle Ages, the | | | | English system's degree Fahrenheit. It is impossible to |
| inch is reputed to have been the length of a medieval | | | | convert between Celsius and Fahrenheit simply by |
| king's first thumb joint. The yard was once defined | | | | multiplying or dividing by 1.8, however, because the |
| as the distance between English king Henry I's nose | | | | scales start at different places. That is, their |
| and the tip of his outstretched middle finger. The | | | | zero-degree marks have been set at different |
| origin of the foot as a unit of measurement is | | | | temperatures. |
| obvious. Eventually, ancient "rules of thumb" gave | | | | In the metric system, there is only one basic unit for |
| way to more carefully defined units. The metric | | | | each type of quantity. Smaller and larger units of |
| system was adopted in France in 1799. | | | | those quantities are all based on powers of ten |
| The metric system of units defines seven basic units: | | | | (unlike the English system that invents different-sized |
| one each for length, mass, time, electric current, | | | | units with completely different names based on |
| temperature, amount of substance, and luminous | | | | different conversion factors). To create those |
| intensity. (While, amount of substance refers to the | | | | various units, the metric system simply attaches a |
| number of elementary particles in a sample of | | | | prefix to the name of the unit. Latin prefixes are |
| matter, luminous intensity has to do with the | | | | added for smaller units, and Greek prefixes are |
| brightness of a light source.) But only four of these | | | | added for larger units. The basic prefixes are: kilo- |
| seven basic quantities are in everyday use by | | | | (1000), hecto- (100), deka- (10), deci- (0.1), centi- |
| nonscientists: length, mass, time, and temperature. | | | | (0.01), and milli- (0.001). Therefore, a kilometer is 1,000 |
| Their defined units are the meter for length, the | | | | meters. Similarly, a millimeter is one-thousandth of a |
| kilogram for mass, the second for time, and the | | | | meter. |
| degree Celsius for temperature. (The other three | | | | |
| basic units are the ampere for electric current, the | | | | Minutes are permitted to remain in the metric system |
| mole for amount of substance, and the candela for | | | | even though they don't conform strictly to the rules. |
| luminous intensity.) | | | | The minute, hour, and day, for example, are so |
| The meter was originally defined in terms of Earth's | | | | customary that they're still defined in the metric |
| size; it was supposed to be one ten-millionth of the | | | | system as 60 seconds, 60 minutes, and 24 |
| distance from the equator to the North Pole. Since | | | | hours—not as multiples of ten. For volume, the |
| Earth is subject to geological movements, this | | | | most common metric unit is not the cubic meter, |
| distance does not remain the same. The modern | | | | which is generally too big to be useful in commerce, |
| meter, therefore, is defined in terms of how far light | | | | but the liter, which is one-thousandth of a cubic |
| will travel in a given amount of time when traveling at | | | | meter. For even smaller volumes, the milliliter, |
| the speed of light. The speed of light in a | | | | one-thousandth of a liter, is commonly used. And for |
| vacuum—186,282 miles (299,727 kilometers) per | | | | large masses, the metric ton is often used instead of |
| hour—is considered to be a fundamental constant | | | | the kilogram. A metric ton (often spelled tonne) is |
| of nature that will never change. The standard meter | | | | 1,000 kilograms. Because a kilogram is about 2.2 |
| is equivalent to 39.3701 inches. The kilogram is the | | | | pounds, a metric ton is about 2,200 pounds: 10 |
| metric unit of mass, not weight. Mass is the | | | | percent heavier than an American ton of 2,000 |
| fundamental measure of the amount of matter in an | | | | pounds. Another often-used, nonstandard metric unit |
| object. Unfortunately, no absolutely unchangeable | | | | is the hectare for land area. A hectare is 10,000 |
| standard of mass has yet been found on which to | | | | square meters and is equivalent to 0.4047 acre. |
| standardize the kilogram. The kilogram is therefore | | | | |