Summary
Highlights
Initially, measurements were based on human body parts or arbitrary metrics like a man's forearm (cubit), waist length (yard), or a Roman soldier's pace (mile). These systems were inconsistent, leading to thousands of different units even within a single country like France.
During the French Revolution, the need for a standardized, rational measurement system became clear. In 1790, French scientists devised a decimal-based system, defining the meter as one ten-millionth of the distance from the North Pole to the Equator. This measurement was calculated by Pierre Méchain and Jean-Baptiste Delambre, though a slight error in their calculation meant the meter was slightly shorter than intended. The kilogram was defined based on the mass of water with a specific volume, and these new standards were made into platinum prototypes.
France officially adopted the metric system in 1795, and other European countries, including Portugal, the Netherlands, and Spain, followed suit. Through colonialism, the metric system spread globally. However, early meter bars used by different countries were prone to wear and tear, leading to inconsistencies, which undermined the goal of a universal standard.
In 1875, the International Bureau of Weights and Measures was created to establish a new, more durable international standard meter, marked with lines to prevent wear. This improved standard lasted until 1960.
In 1960, the International Bureau developed the International System of Units (SI), consisting of six base units: meter (length), kilogram (mass), second (time), Ampere (electric current), Kelvin (temperature), and Candela (light intensity). A seventh unit, the mole, was later added. The SI system aimed to peg these units to universal, unchanging constants, such as the speed of light, to ensure absolute precision and stability over time. The meter was redefined based on the speed of light in 1975, and eventually, all seven base units were pegged to constants by 2019, completing the modern SI system.
The SI system is now almost universally used in scientific and professional fields. However, a few countries still haven't fully adopted the metric system, and some metric countries still retain elements of their old imperial measurements.