The birth of the SI

Dr Peter Saunders explains how the first three SI units – the metre, litre and kilogram – were created.

Discussion point: In 1791, the first of the modern metric measurements were introduced, along with the quote “For all time, for all people”. Do you think this quote has stood the test of time? Why?

Transcript

PETER SAUNDERS

The French revolution was a time of great change – this is sort of the early 1800s or a bit before that in France. So France was uniting itself into a republic and so it became really important that the different territories in France could talk to each other, and so it was decided in 1791 to commission the French Academy of Sciences to come up with a unified rational system of measurements based on a decimal system. And the philosophy of the committee can be summed up a quote by the permanent secretary, who was the Marquis de Condorcet, and it’s been commemorated on a French postage stamp, and his quote is, “À tous les temps, à tous les peuples,” which means “For all time, for all people”. So, ideally, the system should be available to everybody, and it should persist over time and not change.

And so it was decided at that time that they would define the metre to be one 10 millionth of the distance between the equator and the North Pole. So the distance from the equator to the North Pole should be 10,000 km, and so they set about to measure that. And so the line had to pass through Paris, through the quadrant that passes through Paris, and it took maybe 8 or 9 years to do this measurement, and they didn’t really measure the whole thing – they measured sort of from northern Spain up to north of Paris but they managed to extrapolate the rest. And they must have done a pretty good job because we know today that the circumference of the Earth is pretty close to 40,000 km, so they got the metre right.

So once they defined that metre, then they defined a litre, which is one-tenth by one-tenth by one-tenth of a metre – that’s a litre – and if you fill that volume up with water at 4°C, that defined a kilogram. And so based on these measurements in 1799, they manufactured a platinum artefact mass and a platinum artefact metre bar and they deposited those into the French archives.

Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology development.