Units and Standards - History, The Metric System, Le Système International D'unités (the Si System), Derived Units
measurement object quantity meter
A unit of measurement is some specific quantity that has been chosen as the standard against which other measurements of the same kind are made. For example, the meter is the unit of measurement for length in the metric system. When an object is said to be 4 m long, that means that the object is four times as long as the unit standard (1 m).
The term "standard" refers to the physical object on which the unit of measurement is based. For example, for many years the standard used in measuring length in the metric system was the distance between two scratches on a platinum-iridium bar kept at the Bureau of Standards in Sèvres, France. A standard serves as a model against which other measuring devices of the same kind are made. The meter stick in your classroom or home is thought to be exactly 1 m long because it was made from a permanent model kept at the manufacturing plant that was originally copied from the standard meter in France.
All measurements consist of two parts: a scalar (numerical) quantity and the unit designation. In the measurement 8.5 m, the scalar quantity is 8.5 and the unit designation is meters.
Additional Topics
The need for units and standards developed at a point in human history when people needed to know how much of something they were buying, selling, or exchanging. A farmer might want to sell a bushel of wheat, for example, for 10 dollars, but he or she could do so only if the unit "bushel" was known to potential buyers. Furthermore, the unit "bushel" had to have the same…
In an effort to bring some rationality to systems of measurement, the French National Assembly established a committee in 1790 to propose a new system of measurement, with new units and new standards. That system has come to be known as the metric system and is now the only system of measurement used by all scientists and in every country of the world except the United States and the Myanmar Repub…
A set of prefixes is available that makes it possible to use the fundamental SI units to express larger or smaller amounts of the same quantity. Among the most commonly used prefixes are milli- (m) for one-thousandth, centi- (c) for one-hundredth, micro- (æ) for one- millionth, kilo- (k) for one thousand times, and mega- (M) for one million times. Thus, any volume can be expressed by using …
One characteristic of all of the above units is that they have been selected arbitrarily. The committee that established the metric system could, for example, have defined the meter as one one-hundredth the distance between Paris and Sèvres. It was completely free to choose any standard it wished. …
For many years, an effort has been made to have the metric system, including SI units, adopted worldwide. As early as 1866, the United States Congress legalized the use of the metric system. More than a hundred years later, in 1976, the Congress adopted the Metric Conversion Act, declaring it the policy of the nation to increase the use of the metric system in the United States. In fact, little pr…
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