Metric system

The metric system of measurement is an internationally agreed-upon set of units for expressing the amounts of various quantities such as length, mass, time, and temperature. As of 1994, every nation in the world has adopted the metric system, with only four exceptions: the United States, Brunei, Burma, and Yemen (which use the English units of measurement).

Because of its convenience and consistency, scientists have used the metric system of units for more than 200 years. Originally, the metric system was based on only three fundamental units: the meter for length, the kilogram for mass, and the second for time. Today, there are more than 50 officially recognized units for various scientific quantities.

Measuring units in folklore and history

Nearly all early units of size were based on the always-handy human body. In the Middle Ages, the inch is reputed to have been the length of a medieval king's first thumb joint. The yard was once defined as the distance between English king Henry I's nose and the tip of his outstretched middle finger. The origin of the foot as a unit of measurement is obvious.

Eventually, ancient "rules of thumb" gave way to more carefully defined units. The metric system was adopted in France in 1799.

The metric units

The metric system defines seven basic units: one each for length, mass, time, electric current, temperature, amount of substance, and luminous intensity. (Amount of substance refers to the number of elementary particles in a sample of matter; luminous intensity has to do with the brightness of a light source.) But only four of these seven basic quantities are in everyday use by nonscientists: length, mass, time, and temperature. Their defined units are the meter for length, the kilogram for mass, the second for time, and the degree Celsius for temperature. (The other three basic units are the ampere for electric current, the mole for amount of substance, and the candela for luminous intensity.)

The meter was originally defined in terms of Earth's size; it was supposed to be one ten-millionth of the distance from the equator to the North Pole. Since Earth is subject to geological movements, this distance does not remain the same. The modern meter, therefore, is defined in terms of how far light will travel in a given amount of time when traveling at the speed of light. The speed of light in a vacuum—186,282 miles (299,727 kilometers) per hour—is considered to be a fundamental constant of nature that will never change. The standard meter is equivalent to 39.3701 inches.

The kilogram is the metric unit of mass, not weight. Mass is the fundamental measure of the amount of matter in an object. Unfortunately, no absolutely unchangeable standard of mass has yet been found on which to standardize the kilogram. The kilogram is therefore defined as the mass of a certain bar of platinum-iridium alloy that has been kept since 1889 at the International Bureau of Weights and Measures in Sèvres, France. The kilogram is equivalent to 2.2046 pounds.

Metric System

MASS AND WEIGHT

LENGTH

AREA

VOLUME

CAPACITY

The metric unit of time is the same second that has always been used, except that it is now defined in a very accurate way. It no longer depends on the wobbly rotation of our planet (1/86,400th of a day), because Earth is slowing down. Days keep getting a little longer as Earth grows older. So the second is now defined in terms of the vibrations of a certain kind of atom known as cesium-133. One second is defined as the amount of time it takes for a cesium-133 atom to vibrate in a particular way 9,192,631,770 times. Because the vibrations of atoms depend only on the nature of the atoms themselves, cesium atoms will presumably continue to behave exactly like cesium atoms forever. The exact number of cesium vibrations was chosen to come out as close as possible to what was previously the most accurate value of the second.

The metric unit of temperature is the degree Celsius, which replaces the English system's degree Fahrenheit. It is impossible to convert between Celsius and Fahrenheit simply by multiplying or dividing by 1.8, however, because the scales start at different places. That is, their zero-degree marks have been set at different temperatures.

Bigger and smaller metric units

In the metric system, there is only one basic unit for each type of quantity. Smaller and larger units of those quantities are all based on powers of ten (unlike the English system that invents different-sized units with completely different names based on different conversion factors: 3, 12, 1760, etc.). To create those various units, the metric system simply attaches a prefix to the name of the unit. Latin prefixes are added for smaller units, and Greek prefixes are added for larger units. The basic prefixes are: kilo- (1000), hecto- (100), deka- (10), deci- (0.1), centi- (0.01), and milli- (0.001). Therefore, a kilometer is 1,000 meters. Similarly, a millimeter is one-thousandth of a meter.

Minutes are permitted to remain in the metric system even though they don't conform strictly to the rules. The minute, hour, and day, for example, are so customary that they're still defined in the metric system as 60 seconds, 60 minutes, and 24 hours—not as multiples of ten. For volume, the most common metric unit is not the cubic meter, which is generally too big to be useful in commerce, but the liter, which is one-thousandth of a cubic meter. For even smaller volumes, the milliliter, one-thousandth of a liter, is commonly used. And for large masses, the metric ton is often used instead of the kilogram. A metric ton (often spelled tonne) is 1,000 kilograms. Because a kilogram is about 2.2 pounds, a metric ton is about 2,200 pounds: 10 percent heavier than an American ton of 2,000 pounds. Another often-used, nonstandard metric unit is the hectare for land area. A hectare is 10,000 square meters and is equivalent to 0.4047 acre.

[See also Units and standards ]

metric system system of weights and measures planned in France and adopted there in 1799; it has since been adopted by most of the technologically developed countries of the world. It is based on a unit of length, called the meter (m), and a unit of mass, called the kilogram (kg).

The system has changed somewhat since it was first developed; e.g., the definition of the meter has changed, and the unit for mass is different. The meter was originally intended to be 1/10,000,000 of the distance on the earth's surface between the equator and either pole; however, because of errors in the original survey for determining the meter and because of the impracticality of referring to such a standard, the meter was later redefined in terms of the standard prepared and kept at Sèvres, France, near Paris. Long defined as the distance between two scratches on a bar of platinum-iridium alloy, the meter in 1960 was first redefined in terms of an atomic standard. In 1983 the meter was officially redefined as the distance traveled by light in vacuum during 1/299,792,458 of a second.

The original unit of mass, the gram , was first defined as the mass of pure water at maximum density that would fill a cube whose edges are each 0.01 m. The unit of mass is now the kilogram, defined as the mass of a platinum-iridium cylinder kept at Sèvres. (A gram is now defined as a mass 1/1,000 kg.) Other metric units can be defined in terms of the meter and the kilogram. For example the are, the unit of area, is equal to the area of a square whose edges are each 10 m long. The liter, the metric unit of volume, is equal to the volume of a cube whose edges are each 1/10 m long.

Fractions and multiples of the metric units are related to each other by powers of 10, allowing conversion from one unit to a multiple of it simply by shifting a decimal point, and avoiding the lengthy arithmetical operations required by the English units of measurement. Standard prefixes (found in the table entitled Prefixes for Basic Metric Units ) have been accepted for designating multiples and fractions of the meter, gram, are, and other units. Thus, 1,000 grams are a kilogram, 100 ares are a hectare , and 1/100 of a meter is a centimeter.

Several other systems of units based on the metric system have been in wide use. The cgs system is based on the centimeter of length, the gram of mass, and the second of time. The mks system is based on the meter of length, the kilogram of mass, and the second of time. Units in the mks system are larger than the corresponding cgs units. Electric and magnetic units have been defined for both of these systems; in fact, two different sets of electric units are defined in the cgs system. The mks system serves as the basis for the International System of Units , a comprehensive system of units for all physical quantities adopted in 1960 by the 11th General Conference on Weights and Measures.

See also decimal system .

Bibliography: See L. V. Judson, Weights and Measures Standards of the United States: A Brief History (1976; U.S. National Bureau of Standards Special Publication 447); K. Alder, The Measure of All Things (2002).

metric system Decimal system of weights and measures based on the metre (m) and the kilogram (kg). Larger and smaller metric units relate by powers of 10. Devised in 1791, the metric system is used internationally by scientists (particularly as SI units) and has been adopted for general use by most Western countries, although the imperial system is still commonly used in the USA and for certain measurements in Britain.

met·ric sys·tem • n. the decimal measuring system based on the meter, liter, and gram as units of length, capacity, and weight or mass. The system was first proposed by the French astronomer and mathematician Gabriel Mouton (1618–94) in 1670 and was standardized in France under the Republican government in the 1790s.