Cold Open

It’s May 8th, 1794, in Paris, France, five years into the French Revolution.

On the back of a battered cart, 50-year-old scientist Antoine Lavoisier hunches down, trying to shield his face.

As he passes, the angry crowd lining the street shouts insults at him and hurls rotten vegetables, rocks, and clumps of mud in his direction.

Despite being one of France’s greatest scientists, Antoine has been locked away in prison for months. And now, his past ties to the deposed king Louis XVI have sealed his fate. Antoine has just been convicted as a traitor to the revolution. And now, execution awaits.

The cart bumps and rattles over the cobblestones, drawing closer and closer to the scaffold.

Finally, it jerks to a halt in a wide public square seething with spectators.

Antoine stumbles as he’s pulled from the cart. The guards grip his arms, guiding Antoine’s trembling legs to the scaffold. There, the executioner and his guillotine await. Antoine is pushed to the ground and forced to kneel. Then his head is placed beneath the blade.

An eerie silence falls over the crowd. And for a brief moment, everything goes still. But then comes a creak of wood, a rustle of rope, and the blade falls, cutting short Antoine’s life, work, and legacy, in one swift, unforgiving stroke.

The execution of the influential Antoine Lavoisier will send shockwaves through Paris’s scientific community. Antoine had discovered and named the element oxygen. And he had tried to push science toward more precise methods through the use of a new system of measurements. His death will cast a dark shadow over the future of both chemistry and the metric system that he championed. But Antoine’s work will not be in vain. It will take nearly a century, but his dream of a shared international system of measurement will finally be realized on May 20th, 1875.

Introduction

From Noiser and Airship, I’m Lindsay Graham and this is History Daily.

History is made every day. On this podcast—every day—we tell the true stories of the people and events that shaped our world.

Today is May 20th, 1875: The Meter Convention.

Act One

It’s June 1792 on the streets of Paris, two years before the execution of the scientist Antoine Lavoisier.

42-year-old Jean-Baptiste Delambre climbs into a carriage packed with instruments, its wheels groaning under the weight of the equipment. He tugs the door shut, the horses lurch forward, and Jean-Baptiste feels a jolt of excitement. If all goes to plan, this is the start of a historic scientific expedition.

Jean-Baptiste is one of two astronomers who have been commissioned to measure the globe. More precisely, they’re measuring a slice of it: a meridian arc, a curved line that runs from Dunkirk in Northern France, through the capital Paris, and all the way to Barcelona. By calculating its length, they can estimate the size of the Earth. And from that, they’ll define a new unit of measure: the meter, one ten-millionth of the distance from the North Pole to the equator.

Jean-Baptiste and his peers at Paris’s Academy of Sciences hope that the meter will become a universal measure, the base for a new “metric system” that all of France, and even the world, can share. It’s an ambitious goal—and it's also one that’ll plunge Jean-Baptiste into danger.

Because France is in turmoil. King Louis XVI has been imprisoned in his palace, many of his aristocrats have been exiled or executed, and a republican movement is growing in strength. Amid the chaos, scientists like Jean-Baptiste have become vulnerable. For years, they’ve relied on the patronage of the upper classes to support their work, but now their ties to the establishment risk turning them into potential targets.

And as Jean-Baptiste crisscrosses the French countryside taking measurements, he’s met with suspicion everywhere he goes. His strange equipment sparks accusations of espionage, and Jean-Baptiste is repeatedly forced to defend his mission.

In village after village, Jean-Baptiste explains how there are currently hundreds of measurements in France. Different trades and different regions use different standards. It’s a recipe for confusion and fraud. But a single shared measuring system could transform people’s lives, facilitating commerce and connection between all French people.

This argument seems to sway his doubters, and for several months, Jean-Baptiste continues his work, interrupted only by poor weather. But while he takes the careful measurements needed to create the meter, on the streets of Paris, the revolution is gaining momentum.

In January 1793, the French king is executed, and a republican government takes his place. Soon after, radical politicians call for the dissolution of all royal institutions, including the Academy of Sciences.

It seems that the Academy’s meridian survey seems doomed. But then it gets an unexpected reprieve. The new French Government not only embraces the survey, it passes a new law mandating the use of the metric system. The people of France are given one year to prepare, with a provisional estimated meter to be used until Jean-Baptiste and his team produce the definitive measurement.

But Jean-Baptiste has not dispelled all suspicions about his work, and, in the first week of 1794, he is removed from the meridian survey. In the eyes of the Revolutionaries now in power, Jean-Baptiste has failed to prove his loyalty to the new regime, and he must be purged. Jean-Baptiste is ordered to hand over all of his field notes, calculations, and instruments.

The news devastates him, but Jean-Baptiste does as he’s told, too scared to defy France’s new leadership.

Because back in Paris, the new government rules with fear and bloodshed. And upon his return, Jean-Baptiste learns that the acclaimed scientist and fellow proponent of the metric system, Antoine Lavoisier, has already been arrested. His imprisonment sends a clear message to Jean-Baptiste and his colleagues: in Revolutionary France, no amount of scientific achievement can protect you.

In this climate of fear, the meridian survey is no longer a priority. The leaders of the Revolutionary government decide they can make do with the provisional but less accurate meter. What matters most is getting the entire country working from one system of measurement.

But by the time the mandatory adoption date arrives, few French citizens know or care about the new metric system. Even public officials continue filing reports in the old units.

The government tries to turn things around, condemning the previous system of measures as obsolete and a relic of the old regime. It urges citizens to embrace the new metric system. But their appeals are largely ignored.

So for a time, the metric system seems destined for obscurity. But then, in 1795, Jean-Baptiste receives an unexpected opportunity to revive it.

France’s leaders have a change of heart. They decide that if the meter is to have real authority, it must be based on fresh, precise data, not the outdated estimates behind the provisional meter. So they order Jean-Baptiste to resume his work, this time with their full support.

Jean-Baptiste will spend three more years on the road taking precise measurements before his mission is complete. And upon his return to Paris, France will prepare to reintroduce the meter — and not just to its citizens, but to the entire world

Act Two

It’s June 22nd, 1799, in Paris, seven months after the meridian survey was completed.

Inside a lecture hall in France’s National Archives, a grand ceremony is underway. Physicist Pierre-Simon Laplace stands before a room of French lawmakers and scientists from around the world. In front of Pierre-Simon, resting on a velvet-draped table, is a newly forged platinum bar, exactly one meter long.

Pierre-Simon gestures proudly to the bar as he speaks. Hailing it as the crowning achievement of a decade of labor, he describes the long expeditions that were needed to create this new unit of measurement. Then, with a sweep of his hand, he pulls away a sheet to reveal a second set of bars. These are made of iron, but each is also precisely one meter long.

Pierre-Simon smiles and invites the foreign visitors to take a bar home, carrying the metric system beyond France’s borders and out into the world.

The metric system promises to transform the economy and simplify daily life, if it's adopted. So in the months after the ceremony, the French government pushes hard to ensure that the new meter is embraced. The legislature orders its citizens to start learning the metric system, and schools are forced to teach the new measurements.

But most French citizens still resist. Not because they want to cling to royalist traditions, but because old habits are hard to break. Even France’s rising young leader, Napoleon Bonaparte, refuses to adopt the metric system, claiming it’s too difficult to think in metric units.

Meanwhile, a new problem emerges. In 1804, Pierre Mechain dies. Pierre was part of the team tasked with surveying the meridian back in the 1790s. And while Jean-Baptiste Delambre was responsible for measuring between Dunkirk and Rodez in the south of France, Pierre traced the rest of the arc to Barcelona.

And after Pierre’s death, Jean-Baptiste goes through his former colleague’s data and records from the expedition. That's one he discovers some discrepancies. These inconsistencies mean that their calculations were inaccurate. The meter that Pierre and Jean-Baptiste spent nearly a decade defining is wrong. It isn’t one ten-millionth of the distance from the North Pole to the equator. It’s off by about 0.02 percent.

Disappointed, Jean-Baptiste shares his findings with his colleagues. But in the end, they agree that it’s better to leave the error buried. Another revision to the meter could destroy what little faith the public has in the metric system.

So instead, they focus on boosting adoption. The government ramps up production of metric rulers to 300,000 a year. Police are ordered to punish those who refuse to use metric measures. And new, more detailed conversion charts are printed and circulated to help those more accustomed to the old ways adapt.

But all of these efforts fail. Most people continue trading in the old units, and soon rumors spread that the government will abandon the metric system all altogether.

In 1812, that gossip becomes reality. With his attention focused on foreign conquest, Napoleon Bonaparte needs to minimize economic turmoil at home. So, on February 12th of that year, he introduces a compromise: the “ordinary measures.”

This new, new system officially keeps the meter as its foundation, but for everyday use, it allows people to draw from more familiar, pre-revolutionary units.

Pierre-Simon is one of the scientists who lobby against this decision. He pleads with Napoleon to preserve the metric system. He even tries to win him over by offering to rename the system as the “Napoleonic measures.”

But Napoleon refuses.

And even after Napoleon’s reign collapses in 1815. And the French monarchy returns to power. The metric system stays buried. For over a decade, the meter remains an unloved and almost forgotten relic of the Revolutionary era.

But in 1830, a new uprising rocks France. Angry over political repression and economic hardship, the people rise against their king once again. And after three days of fierce fighting in the streets of Paris, the king abdicates.

This 1830 Revolution will bring a wave of reform to France—and with it, a fresh look at the metric system. And for the new government, this system will be more than a tool of measurement. It will be a symbol of equality, science, and progress—values that this new generation of revolutionaries longs to restore.

So, in 1837, France will officially reintroduce the metric system. And this time, it will stick.

Act Three

It’s May 20th, 1875, in Paris, France, 38 years after France reintroduced the metric system.

Inside a conference room, delegates from seventeen countries across Europe and the Americas take their seats around a long table, eager to discuss a radical initiative.

Since France made the metric system mandatory again in 1837, other nations have slowly begun to adopt it. But without a single authority to oversee standards, discrepancies remain. Different countries have slightly different standards for the meter and kilogram. And these minor variations have caused major confusion, especially as international trade and scientific research have expanded.

So today, the gathered delegates hope to solve that problem.

Together, they propose the creation of an international body: a permanent organization tasked with maintaining universal standards for measurement. They will call it the International Bureau of Weights and Measures.

By the end of the day, the diplomats sign the Treaty of the Meter, agreeing to establish a set of exact, shared measures. Under the treaty, the new Bureau will build official standards of the meter and kilogram, store them under strict conditions, and distribute identical copies to participating countries.

This convention will mark a profound shift toward a truly global system of measurement—one born from revolution, refined through science, and now made possible by diplomacy.

Over the next few decades, the metric system will continue to expand its reach, driven by the needs of science, industry, and global trade. Countries not represented in the convention will begin the transition to the meter. But there’ll be one major exception.

Out of all the nations that did sign the Treaty of the Meter, only the United States will fail to fully adopt the metric system. While the rest of the world continues to move toward at least partial metrication, the US, Liberia, and Myanmar will cling to their traditional measures. Of the three, America alone will carry its resistance into the 21st century, remaining a stubborn exception to a global movement that was launched in Paris on May 20th, 1875.

Outro

Next on History Daily. May 21st, 1924. In an attempt to commit the “perfect crime,” Nathan Leopold and Richard Loeb murder a 14-year-old boy.

From Noiser and Airship, this is History Daily, hosted, edited, and executive produced by me, Lindsay Graham.

Audio editing by Muhammad Shahzaib.

Sound design by Mollie Baack.

Supervising Sound Designer Matthew Filler.

Music by Thrumm.

This episode is written and researched by Alexandra Currie-Buckner.

Edited by Scott Reeves.

Managing producer, Emily Burke.

Executive Producers are William Simpson for Airship and Pascal Hughes for Noiser.