Metric System | Research & Encyclopedia Articles

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, temperature, and so on.

Whenever we measure something, from the weight of a sack of potatoes to the distance to the moon, we must express the result as a number of specific units: for example, pounds and miles in the English system of measurement (although even England no longer uses that system), or kilograms and kilometers in the metric system. As of 1994, every nation in the world has adopted the metric system, with only four exceptions: the United States, Brunei, Burma and Yemen.

The metric system that is in common use around the world is only a portion of the broader International System of Units, a comprehensive set of measuring units for almost every measurable physical quantity from the ordinary, such as time and distance, to the highly technical, such as the properties of energy, electricity and radiation. The International System of Units grew out of the 9th General [International] Conference on Weights and Measures, held in 1948. The 11th General Conference on Weights and Measures, held in 1960, refined the system and adopted the French name Système International d'Unités, abbreviated as SI.

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 SI units for various scientific quantities.

Measuring units in folklore and history

In the biblical story of Noah, the ark was supposed to be 300 cubits long and 30 cubits high. Like all early units of size, the cubit was based on the always-handy human body, and was most likely the length of a man's forearm from elbow to fingertip. You could measure a board, for example, by laying your forearm down successively along its length. 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 the nose of England's King Henry I and the tip of his outstretched middle finger. The origin of the foot as a unit of measurement is obvious.

In Renaissance Italy, Leonardo da Vinci used what he called a braccio, or arm, in laying out his works. It was equal to two palmi, or palms. But arms and palms, of course, will differ. In Florence, the engineers used a braccio that was 23 inches long, while the surveyors' braccio averaged only 21.7 inches. The foot, or piede, was about 17 inches in Milan, but only about 12 inches in Rome.

Eventually, ancient "rules of thumb" gave way to more carefully defined units. The Metric System was adopted in France in 1799 and the British Imperial System of units was established in 1824. In 1893, the English units used in the United States were redefined in terms of their metric equivalents: the yard was defined as 0.9144 meter, and so on. But English units continue to be used in the United States to this day, even though the Omnibus Trade and Competitiveness Act of 1988 stated that "it is the declared policy of the United States...to designate the metric system of measurement as the preferred system of weights and measures for United States trade and commerce."

English vs. metric units

Why do scientists and everybody else in the world except the United States and three tiny, non-industrialized nations believe that the metric system is superior to the English system? There are four main reasons.

(1) English units are based on silly standards. When that medieval king's thumb became regrettably unavailable for further consultation, the standard for the inch was changed to the length of three grains of barley, placed side by side-not much of an improvement. Metric units, on the other hand, are based on nature, not on the whims of humans.

(2) The standards behind the English units aren't reproducible. Arms, hands, and grains of barley will obviously vary in size; the size of a 3-foot yard depends on whose feet are in question. But metric units are based on standards that are precisely reproducible, time after time.

(3) There are simply too many English units. We have buckets, butts, chains, cords, drams, ells, fathoms, firkins, gills, grains, hands, knots, leagues, three different kinds of miles, four kinds of ounces, and five kinds of tons, to name just a few. There are literally hundreds more. For measuring volume or bulk alone, the English system (now more accurately called the American system) uses ounces, pints, quarts, gallons, barrels and bushels, among many others. In the metric system, on the other hand, there is only one basic unit for each type of quantity.

(4) Any measuring unit, in whatever system, will be too big for some applications and too large for others, so we must have a variety of sizes. People would not appreciate having their waist measurements in miles or their weights in tons. That's why we have inches and pounds. The problem, though, is that in the American system the conversion factors between various-sized units-12 inches per foot, 3 feet per yard, 1760 yards per mile-have no rhyme or reason to them. They're completely arbitrary. Metric units, on the other hand, have conversion factors that are all powers of ten. That is, the metric system is a decimal system, just like dollars and cents. In fact, our entire system of numbers is decimal, based on tens, not threes or twelves. Therefore, converting a unit from one size to another in the metric system is just a matter of moving the decimal point.

The metric units

The SI starts by defining 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 non-scientists: length, mass, time and temperature. Their defined SI 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.) Almost all other units can be derived from the basic seven. For example, area is a product of two lengths: meters squared, or square meters. Velocity or speed is a combination of a length and a time: kilometers per hour.

The meter was originally defined in terms of the earth's size; it was supposed to be one ten-millionth of the distance from the equator to the North Pole, going straight through Paris. But because the earth is subject to geological movements, this distance can't be depended upon to remain the same forever. The modern meter, therefore, is defined in terms of how far light will travel in a given amount of time when traveling at-naturally-the speed of light. The speed of light in a vacuum is considered to be a fundamental constant of nature that will never change, no matter how the continents drift. The standard meter turns out to be 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. The mass of a baseball won't change if you hit it from the earth to the moon, but it will weigh less-have less weight-when it lands on the moon because the moon's smaller gravitational force is pulling it down less strongly. Astronauts can be weightless in space, but they can lose mass only by dieting. As long as we don't leave the earth, though, we can speak loosely about mass and weight as if they were the same thing. So you can feel free to "weigh" yourself (not "mass" yourself) in kilograms. Unfortunately, no absolutely unchangeable standard of mass has yet been found to standardize the kilogram on. The kilogram is therefore defined as the mass of a certain bar of platinum-iridium alloy that has been kept (very carefully) since 1889 at the International Bureau of Weights and Measures in Sèvres, France. The kilogram turns out to be 2.2046 pounds.

The metric unit of time is the same old second that we've always used, except that it is now defined in a super-accurate way. It no longer depends on the wobbly rotation of our eccentric old planet (1/86,400th of a day), because Mother Earth is slowing down; her days keep getting a little longer as she 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. This may sound like a strange definition, but it is a superbly accurate way of fixing the standard size of the second, because the vibrations of atoms depend only on the nature of the atoms themselves, and 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 (^oC), which replaces the English system's degree Fahrenheit (^oF). In the scientists' SI, the fundamental unit of temperature is actually the Kelvin (K). But the kelvin and the degree Celsius are exactly the same size: 1.8 times as large as the degree Fahrenheit. You can't 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. For conversions and other characteristics of the temperature scales.

Bigger and smaller metric units

Because the meter (1.0936 yards) is much too big for measuring an atom and much too small for measuring the distance between two cities, we need a variety of smaller and larger units of length. But instead of inventing different-sized units with completely different names, as the English-American system does, we can create a metric unit of almost any desired size by attaching a prefix to the name of the unit. For example, since kilo- is a Greek form meaning a thousand, a kilometer (kil-OM-et-er) is a thousand meters. Similarly, a kilogram is a thousand grams. The complete set of prefixes that are used in the metric system to convert any unit into larger or smaller ones. For example, a gigagram is a billion grams or 10⁹ grams; a nanosecond is one billionth of a second or 10^-9 second.

Minutes are permitted to remain in the metric system for convenience or for historical reasons, 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 in other countries) is 1,000 kilograms. Because a kilogram is about 2.2 pounds, a metric ton is about 2,200 pounds: ten percent heavier than an American ton of 2,000 pounds. Another often-used, non-standard metric unit is the hectare for land area. A hectare is 10,000 square meters and is equivalent to 0.4047 acre.

Converting between English and metric units

The problem of changing over a highly industrialized nation such as the United States to a new system of measurements is a substantial one. Once the metric system is in general use in the United States, its simplicity and convenience will be enjoyed, but the transition period, when both systems are in use, can be difficult. Nevertheless, will be easier than it seems. While the complete SI is intimidating because it covers every conceivable kind of scientific measurement over an enormous range of magnitudes, there are only a small number of units and prefixes that are used in everyday life.

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