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History of the International System of Units (SI)

The story of the SI begins after the French Revolution in the late 1700s. Before this, every region had its own measurement units (like feet or pounds), which caused confusion for trade and science.

1. The Birth of the Metric System (1790s)

· In France, scientists developed a new, rational system based on nature rather than human body parts.
· They defined the metre as one ten-millionth of the distance from the North Pole to the Equator.
· They defined the gram as the mass of one cubic centimeter of water.
· This new "metric system" was decimal (based on 10), making calculations much easier.

2. The Treaty of the Meter (1875)

· The metric system's success led many countries to adopt it.
· To ensure global consistency, 17 nations signed the Treaty of the Meter.
· This treaty established:
· The International Bureau of Weights and Measures (BIPM) to oversee the system.
· The General Conference on Weights and Measures (CGPM) to make decisions.
· New, precise physical prototypes for the metre and kilogram made from a durable platinum-iridium alloy.

3. The Modern SI is Born (1960)

· As science advanced, definitions based on physical objects were no longer precise enough.

· In 1960, the system was officially renamed the Système International d'Unités (International System of Units), or SI for short.
· It was rebuilt around seven base units (metre, kilogram, second, ampere, kelvin, mole, candela).

4. Basing Definitions on Universal Constants (2019)

· The most recent major step happened in 2019.
· All SI units are now defined by fundamental constants of nature, not physical objects.

· For example:
· The metre is defined by the speed of light.
· The second is defined by the vibration of a cesium atom.
· Even the kilogram is now defined by a fundamental constant of quantum physics (the Planck constant).

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