About six and a half billion people use the metric system every single day. That's more than the citizens of any single nation, the followers of any single religion or the speakers of any single language. Sociologist Hector Vera has called the metric system “more popular than Jesus.”
In our “≤ kg” episode, we told a bit of its epic history. How it dates back to revolutionary-era France, where over 250,000 measures were in regular use. How the hungry and angry French peasants demanded a unified set of weights and measures. And how the revolutionary government wanted their system to be natural and timeless, and thus based it on the most fundamental physical object they could think of: the earth. As Napoleon later said, “It was not enough for them to make the forty million people [of France] happy, they wanted to sign up the whole universe.”
No historian has told this story better than Ken Alder.
Alder’s book The Measure of All Things zooms in on the two astronomers who were enlisted to survey the world. We couldn't squeeze their story into our episode, but it’s a doozy — so we figured we'd give you a small taste of it here.
As Alder explains, the duo set out in opposite directions from Paris - in tricked-out carriages, carrying the most advanced scientific equipment of their day - hoping to measure the fraction of the meridian between Dunkerque, France and Barcelona, Spain.
From their measurements, the French Academy of Sciences could extrapolate an exceedingly precise length: one ten-millionth of the distance between the equator and the north pole. They’d consecrate this distance as the meter.
For all the sophisticated surveying tactics and all the noble utopian idealism, the expedition - conducted in a time of revolution and war - turned out to be a bit of a farce. As the men climbed church steeples and peered through their instruments, angry mobs fingered them as spies. They were hauled before local officials, only to present papers signed by the now-deposed King. They were repeatedly detained. “At every turn,” Alder writes, “they encountered suspicion and obstruction.”
Of the two, the South-going astronomer — a short and obsessive man named Pierre-François-André Méchain — fared worse.
Once he arrived in Barcelona, Méchain took 10,000 readings to get the precise latitude, and mailed it off to Paris. Before he had a chance to return to France, however, he suffered a freak accident, taking a blow to the chest so hard it left him in a coma for three days. In another twist, war broke out between France and Spain, and, as an enemy national, the convalescing Méchain was under house arrest. In his hotel room jail cell, with nothing better to do, Méchain re-did the 10,000 latitude readings.
He found, to his horror, a discrepancy. The reading didn’t match the prior reading he had already dispatched to Paris. Admittedly, the error was slight - approximately the size of two football fields over the entire length of France - yet it was enough to make a difference in the standard meter bar that was already being forged back in Paris. Méchain decided to keep the second set of calculations a secret. Alder quotes from his letters at length, allowing us to see how guilty Méchain felt after being showered with his colleagues' accolades "which they presumed I deserved."
A few years later, Méchain returned to Spain to try once more to straighten out his erroneous calculations. Again, forces beyond his control seemed to conspire against him. War began anew between Spain and France. Yellow Fever epidemics broke out up and down the coast. A fog settled, preventing him from making any observations. He contracted malaria in Valencia, writing to his friend:
“Though I do not seek death, I am far from fearing it … Never, no never … have I found myself in a situation so hopeless, so terrifying, so wrenching. This dreadful commission, whose success appears so far off and so improbable, will more than likely be the end of me.”
Soon after, as a result of his attempt to correct himself, Méchain died.
As a result of Méchain’s error - compounded by errors made in extrapolation back in Paris - the platinum meter bar is not one ten-millionth of the distance between the equator and the north pole. It’s off by about 0.2mm, the width of two sheets of paper. Crazy, right?!
By the time the world discovered this tiny error, it was too late. Everybody was already using the measures that were slightly "off." By the Metric Convention of 1875, the European powers that be decided that it was too difficult and too costly to recalibrate all the world’s metric measures, and that instead, they’d just reset the standard from “one ten-millionth of the distance from the equator to the north pole” to “the length of this metal bar, right here.” A new platinum meter bar was fashioned - the precise length of the old one - to be the true standard bearer. In this new bar, Méchain’s error was immortalized. It remains built in to the meter (and, as a result, the kilogram) to this very day.
Alder’s history renders a seemingly objective and rational system of measure into a human and flawed one. But, still, we didn’t quite know what to make of it. What are the implications for measurement in general? Is it impossible to draw a standard from the physical world? Is the metric system one giant conspiracy?!
Alder's take on all this surprised us.
“I’ll say this,” he told us. “measurement conventions are conventions. They’re what we all agree to … They’re agreements that we strike as an individual culture, as a global society. And the metric system is an incredibly successful global agreement of what measurement can and should be. It has, in a sense, made it possible to communicate over the whole world.”
In the end, Méchain’s error didn’t matter one lick. The only thing that did matter was that we all agreed to use the meter, even the USA who was one of the signatories way back in 1875. A standard is a standard is a standard. As long as we all agree to it, it’ll continue to help us - however quietly - communicate with one another, across borders, across cultures, across ages. And that seems like something worth agreeing to.