The next time you pick up a bag of spuds from the supermarket or fill up the car with petrol, you can thank a treaty signed in 1875 for the metric system that underpins daily life.
The next time you pick up a bag of spuds from the supermarket or fill up the car with petrol, you can thank a treaty signed 150 years ago for the metric system that underpins daily life.
On May 20, 1875, delegates from 17 countries assembled on a Parisian spring day and signed the Metre Convention, also known as the Treaty of the Metre.
At the time, it wasn't uncommon for countries, states and even cities to have entirely different ways of measuring distance and mass, hampering trade and holding back progress in science.
Loading...To standardise and unify these definitions, the Treaty of the Metre established the International Bureau of Weights and Measures, which initially defined the metre and kilogram.
Over the years, more countries signed the Treaty of the Metre, including Australia in November 1947.
A handful of other units of measurement were also included to form the International System of Units, the basis of the metric system.
But the metre's inception predates the treaty that bears its name by nearly 100 years.
And its story begins during the French Revolution.
The metre through history
During the late 1700s, revolutionaries shaping the new republic of France shed old traditions bound to royalty and religion.
The Unity Festival in Revolution Square, as depicted by French artist Pierre-Antoine Démachy, was held on August 10, 1793 to promote the values of the French Republic. (Getty Images: Philippe Lissac)
This reinvention included creating a new system of measurement.
This system would be available to everyone, and be tied to fundamental properties of nature, "not from the length of the king's arm, or something that changed over time", Bruce Warrington, CEO and chief metrologist at the National Measurement Institute, says.
So mathematicians and scientists of the time decreed that the length of a metre — from the Greek word "metron", meaning "a measure" — was equal to one 10-millionth of the distance from the North Pole to the equator through the Paris Observatory.
This line through Paris, called the Paris meridian, featured in Dan Brown's novel The Da Vinci Code. (Wikimedia Commons: US Government, CC0)
It fell to a pair of astronomers to calculate this distance, and after seven years, in 1799, they presented their final measurement to the French Academy of Sciences which made a "Metre of the Archives" in the form of a platinum bar.
(It was later found the astronomers were a bit off in their calculations, and the metre as we know it is 0.2 millimetres shorter than it should've been.)
The Metre of the Archives and its copies were eventually replaced by around 30 metre bars made of a stable platinum-iridium alloy. They were distributed around the world in the late 1890s, and remained the "standard" metre for decades.
National metre prototype 27 was sent to the US and received by President Benjamin Harrison on January 2, 1890. (Supplied: NIST Research Library & Museum)
But as science progressed, the definition of the metre changed too.
This change started early last century, when scientists discovered they could measure distances using light.
Light travels in waves. If you know the distance between each wave — called the wavelength, literally the length of the wave — it's possible to use light "as a very fine ruler", Dr Warrington says.
And in 1960, the platinum alloy bars were out and a new definition of the metre was introduced.
Pass an electrical current through a lamp filled with krypton gas and the krypton atoms throw off light, including a reddish orange wavelength.
Krypton-86 gas is odourless, colourless and exists in tiny quantities in the atmosphere. (Supplied: National Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899)
One metre equalled 1,650,763.73 times the wavelength of this specific reddish orange light.
Meanwhile, electronics fabrication kicked off and the scale of manufacturing shrunk to the incredibly tiny. Think transistors in a smartphone integrated circuit, which are only a few billionths of a metre wide.
"So you need rulers that can check for and control the quality of that manufacturing at that level," Dr Warrington says — something the krypton lamp metre definition could not do.
Since 1960, there'd been a lot of progress made on measuring time accurately with atomic clocks. Their "ticking" is produced by oscillations of radiation emitted when atoms are bathed in laser light.
And they can tick billions of times every second. This new ability to divvy up the second into increasingly tinier slices, coupled with a universal physical constant, the speed of light, redefined the metre.
From 1983, a metre was considered the distance that light travels in a vacuum in 1/299,792,458 of a second (because light travels 299,792,458 metres per second).
This new definition incorporating time and the speed of light opened up new ways of measuring length, Dr Warrington says.
For instance, scientists use it to accurately measure Earth's distance to the Moon.
"The Apollo astronauts left a kind of fancy mirror on the surface of the Moon, and to this day, you can still fire a laser at that reflector, and time the round trip for the light to go all the way to the Moon and back," Dr Warrington says.
"And you can turn that into a very careful measurement of the distance between the Earth and the Moon."
These measurements show the Moon is slowly pulling away from Earth at around 3.8 centimetres each year.
Of the five reflecting panels on the Moon, two were delivered by Apollo 11 and 14 crews in 1969 and 1971, respectively. (Supplied: NASA)
Slow adoption of the metric system
Even as the metre and other units of measurement were being redefined, it was up to each Metre Treaty signatory to adopt the metric system in their own time.
It took Australia more than 20 years after signing the Metre Treaty to officially adopt the metric system when the Metric Conversion Act was passed in 1970.
Other countries have been far slower to go metric. One of the original signatories of the Metre Treaty was … the US.
Today, while day-to-day life in the US tends to use imperial units, the metric system is legally recognised and is "at the core of its civil measurement", Dr Warrington says.
"So its national standards [for mass and distance] are the kilogram and the metre, just like everybody else's."
Even in Australia today you don't have to look far to see imperial units in, for example, men's trouser waistbands and television screen size.
One area that still suffers inconsistencies in measurement is in the kitchen, Dr Warrington says.
"I find it slightly frustrating as a professional measurement nerd that an Australian tablespoon is four teaspoons, whereas almost everywhere else in the rest of the world, it's three teaspoons.
"That means if you're cooking something, then you might need to know whether the recipe came from Australia or from somewhere else to get the tablespoon right."
For more on the history of the metre, check out the full episode of Lab Notes.
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Comments
Another fun fact dating from French revolution is the 10 hour-day, each hour had 100 minutes and each minutes 100 seconds : https://historyfacts.com/world-history/fact/france-had-a-cal...
By paulorlando 2025-05-2316:371 reply Fun fact... or not so fun?
For 12 years of the revolutionary era, France did use decimal time. And the calendar and clocks were organized around a 10 day week and a 10 hour day. But those changes, coupled with the loss of Sunday worship, had other effects on the population.
Here’s an assessment of what was really meant and then lost by the elimination of Sunday:
“‘The elderly ladies took advantage of the long journey (to church) to exchange old stories with other old gossips … they met friends and relatives on the way, or when they reached the county town, whom they enjoyed seeing … there then followed a meal or perhaps a reciprocal invitation, which led to one relative or another….’ But if that was the way it was for the old ladies, what did Sunday mean to ‘young girls, whose blood throbbed with the sweetest desire of nature!’ We can well understand their impatience, ‘they waited for each other at the start of the road they shared,’ they danced.
“Now, however, when the Tenth Day came around, ‘the men were left to the devices they always had:’ the old men went to the tavern, and they bargained. The young men drank and, deprived of their ‘lovely village girls’, they quarrelled. As for the women, they had nothing left to do in village. The mothers were miserable in their little hamlets, the daughters too, and out of this came their need to gather together in crowds. If the need for recreation is necessary because of moral forces… there is absolutely no doubt that village girls find it very hard to bear privations which are likely to prolong their unmarried state: ‘in all regions the pleasure of love is the greatest pleasure.'”
– from The Revolution Against the Church, From Reason to the Supreme Being, by Michel Vovelle, pp 158-159.
By kitten_mittens_ 2025-05-2317:494 reply If you’re interested in a what an analog clock in decimal time might look like: https://decimal-time.netlify.app/
By cafard 2025-05-2320:18 As I habitually mention when the revolutionary calendar comes up, emacs calendar mode will give you the date with p-f. For what it's worth, today is Quartidi 4 Prairial an 233 de la Révolution, jour de l'Angélique. (Prairial I had heard of, jour de l'Angélique is news to me.)
[edit: corrected spelling of Quartidi]
By linguistbreaker 2025-05-2317:242 reply I hadn't heard of this and it's fun to think about.
It's 100,000 s/day as opposed to our current 86,400 s/day which is not far off.
Hours, however, were twice as long.
They had time pieces that displayed both together.
Another "fun fact" somewhat more relevant to the article is the gradient (aka. grad, or gon), it is a unit of angle equal to 1/400 of a turn, slightly smaller than a degree.
It goes well with the metre because 1 km is 1/100 grad of latitude on earth. It mirrors the nautical mile in that 1 nautical mile is 1/60 degree (1 arcminute) of latitude on earth.
The grad is almost never used on a day to day basis, even in France. It is still used in specialized fields, like surveying.
By lxgr 2025-05-2421:59 Ah, this brings back fond memories of Swatch's attempt [1] at a decimal division of the day at the height of the dotcom boom.
I still must have one of these digital wristwatches in some box in a closet, with a big button that starts a glorious monochrome LCD animation of "going online" (while of course the watch stayed as offline as any other Quartz watch).
The thought of a watch that could actually go online seemed ridiculously utopian back then, even when everybody was otherwise dreaming of cyberspace. But only a few weeks ago, in a moment of closure spanning a quarter of a century, I finally downloaded a "Swatch Internet Time" complication – from the Internet, directly onto my wristwatch.
By nancyminusone 2025-05-2316:521 reply Sadly, the 100 day year never worked quite right.
By ucarion 2025-05-2319:12 And every other month was named after a coup d'etat!
By mjevans 2025-05-247:14 Given the difference of ( 10 00 00 ) / 86400 ; they made their second ~1.1574 times faster? ( 125 : 108 , or 5 * 5 * 5 : 2 2 3 3 3 )
By jacquesclouseau 2025-05-2316:45 inb4 we still have the 8 hour workday
> (It was later found the astronomers were a bit off in their calculations, and the metre as we know it is 0.2 millimetres shorter than it should've been.)
That's actually impressively good accuracy for the time! Hats off to the astronomers.
I was just about to post same quote but you beat me to it.
I'd go further, I think their work was a remarkable feat for the late 1790s. That they achieved that accuracy given the primitive equipment of the day says much for their abilities and understanding.
Also at the time France was in turmoil, numbers of its scientists were victims of the French Revolution—Antoine Lavoisier, probably the greatest chemist of his time—was beheaded by guillotine in 1794, so the political environment was anything but stable.
Look back 225+ years ago: there was no electricity, no material science to speak of to make precision instrumentation—journal bearings on lathes, etc. couldn't be made with the accuracy of today, backlash would have been a constant worry. All instrumentation would have been crafted by hand.
And the old French pre-metric system of units was an imperial system similar to the British (France even had an inch that was similar in length to British Imperial unit). All instrumentation up to that point would have relied on the less precise standards of that old system.
Traveling was by horse and sailing ship, and so on. Surveying would have been difficult. There wasn't even the electric telegraph, only the crude optical Chappe telegraph, and even then it was only invented in the 1790s and wasn't fully implemented during the survey.
They did a truly excellent job without any of today's high tech infrastructure but they made up for all these limitations by being brilliant.
In today's modern world we often underestimate how inventive our forefathers were.
I always think about what a cool adventure it must have been, for Pierre Méchain and Jean-Baptiste Delambre to roam for 7 years, go wherever they need thanks to an official letter, make calculations and come back successful to Paris. To think that they were only off by .2mm !
“The Measure of All Things” by Ken Adler[0] is a good, extremely readable book about their adventure, which was indeed wild.
[0] https://en.wikipedia.org/wiki/Special:BookSources/978-0-349-...
