Topics Numbers and logic

How was the metric system invented?

The metric system was invented in France in the 1790s, during the Revolution. Scholars were asked to replace a huge number of local units, which varied from town to town, with one decimal system meant for everyone. They chose the meter as the base unit, planned to tie it to the size of the Earth, and built everything else on powers of ten.

The story is stranger than the tidy version. A provisional meter was set in 1793 from earlier measurements of the Earth. Then two astronomers, Jean-Baptiste Delambre and Pierre Méchain, spent years surveying the distance from Dunkirk to Barcelona, and their data was used to fix the final meter in 1799. Their work contained small errors, and the meter ended up very slightly off its original idea. France also went back to older units for a while, from 1812 to 1837, before committing for good.

The definitions kept changing after that. The meter was redefined by the speed of light in 1983, and the kilogram by the Planck constant in 2019, which retired the last metal artifact. An episode would follow the story from tangled markets to those modern definitions, and you can press Talk to ask a question. bre's hosts are AI and can be wrong, so check anything that matters.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. A world of local unitsBefore the 1790s, measures varied by region, trade and even the goods being sold. That made commerce confusing and invited disputes and cheating.
  2. Scholars choose natureRevolutionary France asked scientists for a system based on something shared. They settled on a fraction of the Earth's size and on counting in tens.
  3. Delambre, Méchain and the surveyAfter a provisional meter in 1793, two astronomers measured an arc of the meridian through a country at war. Their data fixed the final meter of 1799.
  4. Naming the unitsPrefixes such as kilo, centi and milli made one set of names work at every scale. Mass was tied to water, giving the gram and kilogram.
  5. Resistance, retreat and returnPeople kept using old units, and France allowed them again from 1812 to 1837 before adopting the metric system firmly. Other countries joined over the following century.
  6. From a bar to constantsThe meter was redefined by the speed of light in 1983, and the kilogram by the Planck constant in 2019. The goal stayed the same: units anyone can reproduce.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Picture a market in France in the 1780s. You buy cloth by the ell, grain by the measure, and both change size when you cross a few miles. A huge number of local units, many sharing names. How does anyone trade fairly?

  2. ArloAI host

    Badly. Everyone gets cheated.

  3. breAI host

    Often, yes. So after the Revolution, scholars proposed a unit taken from nature rather than from a ruler's body. A ten-millionth of the distance from the pole to the equator.

  4. ArloAI host

    Nobody measured the pole. Who counted that?

  5. breAI host

    Good catch. Nobody did. They surveyed an arc from Dunkirk to Barcelona and worked out the rest. And a provisional meter came first, from older measurements.

  6. ArloAI host

    So they guessed first, checked later.

  7. breAI host

    Roughly, yes. The survey took years, and it still had small errors.

  8. ArloAI host

    So: nobody's foot. Just a slightly wrong Earth.

Questions people also ask

Who invented the metric system?
No single person did. French scholars working for the revolutionary government designed it in the 1790s, including a commission of scientists from the Academy of Sciences. Astronomers Delambre and Méchain carried out the survey that fixed the final meter in 1799.
Was the meter really based on the Earth?
Yes, in design. It was meant to be one ten-millionth of the distance from the North Pole to the equator, along a line through Paris. The survey contained small errors, so the meter differs slightly from that ideal. The designers aimed for a standard that belonged to no king or country.
Why do some countries still not use it?
Habit, cost and history. The United States, for example, still uses customary units in daily life, though its science and many industries are metric. Changing road signs, tools, recipes and schooling is expensive, and people resist changing what they know.
How is the meter defined today?
Since 1983 the meter has been defined through the speed of light: the distance light travels in a tiny fraction of a second. The kilogram followed in 2019, defined by the Planck constant instead of a metal cylinder kept in France.

Related topics

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bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.