Everyone knows how to measure, for example, 1cm or 1 kg of something, but how did people decide how much length is going to be 1 cm or how much weight is going to be 1kg? And how did they make most of the world accept those measurements? Questions applies to other measurements aswell, these two were just examples.
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One of the problems with older forms of measurement like pounds, cubits, and feet was the lack of standardization. What was defined as a foot varied depending on where you lived. The British later implemented the Imperial measuring system to standardize this.
When the metric system was defined in France they decided that measurements would be defined as repeatable standards. Meaning that you define a particular measurement based on a repeatable test so that it can be regularly checked and be consistent.
The meter was originally defined as 1 millionth the distance between the equator and the pole.
Since the metric system is all about fractions of 10 the meter was easily transformed into the KM, CM, and MM, etc
A KG was originally defined as a 10 cm x 10 cm x 10 cm cube of water.
These were later re-defined with standard weights and lengths. Devices made of metal that would be used to calibrate measuring tools. Essentially a stick made of metal that is the standard meter that everyone else copies from, and a block of brass that is the standard KG.
This was again later re-defined to a more precise measurement, today the meter is defined by the speed of light in a vacuum. While weights like the KG are defined by the mass of a certain number of atoms.
When the SI units were initially adopted, there was an attempt to have them be based on easily reproducible things. The distance measurement was the “base” of the system – the intention was to have the distance between the Equator and the North Pole be exactly 10,000 km. When that distance was measured, a metal bar representing 1 metre was created. There was some slight error in the measurement that created the definition of the metre, which is why the distance between the Equator and the North Pole is actually more like 10,002 km.
Once the metre was defined, other measurements were defined off of it. The kilogram, for example, is the mass of water in a container that is 1 metre by 1 metre by 1 metre.
Nowadays all of the SI base units have been redefined off of fundamental constants of the universe. So the metre is no longer based off of a survey on the Earth, instead it’s based off of how far light travels in a vacuum in a very specific amount of time.
Napoleon!!
The metric system was invented during Revolutionary France, as a way to modernize and standardize units of measurement. While most old/local measurement systems were made to be practical for regular people (one mile is roughly 1,000 paces, one fathom is roughly the length of your outstretched arms, one foot is, well,), the metric system was always supposed to come from universal constants. So, a meter was defined as a particular fraction of the circumference of the earth, one kilogram is the mass of one liter of water, one calorie is the energy to heat one gram (also one cm^(3)) of water by one degree. It’s all very nice and tidy and makes for easy calculations.
Napoleon then went and conquered most of Europe, and modernized lots of things wherever he went. Legal codes, infrastructure, and the metric system. These things long outlived his reign, and in 1875 the International Bureau of Weights and Measures was founded in Paris to officially coordinate all these units. Until recently, the Bureau’s building held *the* official kilogram weight and meter length, that defined these units.
This also *happened* to be the height of French (and European) global power, and imperial powers spread these units to the countries they conquered/colonized. By the time decolonization happened, no one was going back. Same with the Gregorian calendar, the way we divide days into hours/minutes/seconds, and to a lesser extent the use of French and English as international languages of business.
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