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Different Systems of Units
Historically, many different systems of units have been used, where a system of units is defined as a collection of units of measurement with rules that relate them to each other. A unit of measurement is a defined magnitude of a quantity used as a standard for measurement for the same kind of quantity — such as measurements of length, weight, and volume.
In the past, many systems of measurement were defined on a local level, and could be based on factors as arbitrary as the length of a king's thumb. While this may work locally, when considering trade and science, having systems based on units that others cannot easily relate to makes interaction difficult. As such, more universal systems developed over time.
Today, the primary systems in use include the metric system, the imperial system, and United States customary units (USC). The International System of Units (SI) is the standard metric system currently used globally, consisting of seven SI base units covering length, mass, time, temperature, electric current, luminous intensity, and amount of substance.
Brief History of the Metric System
In 1668, John Wilkins proposed a decimal system in which length, area, volume, and mass were linked to each other based on a pendulum with a one-second beat as a base unit. By 1790, standardization of weights and measures was seen as necessary by nations engaged in trade and scientific exchange. France formally defined the metric system in law in 1795, and officially adopted it in 1799.
The International System of Units (SI), currently the world's most widely used measurement framework, was published in 1960 and has been adopted by all developed countries except the United States — though SI is used heavily in US science and military applications.