The kilogram: A weighty topic

The metric system was designed to be the measurement system for all people, in all places, for all time. The constancy of the natural world was at stake—the weight of water, the length of a day. But the way we originally measured the most basic of standards, length and weight, now seems mind-boggling. The system relied on sophisticated physical artifacts that were locked away for their own (and our own) protection. To measure distance, for example, the French Academy of Sciences once defined the meter as one ten-millionth of the distance from the North Pole to the equator. This was enshrined in a physical rod, [the ur-meter]( from which all other meters could be measured. The kilogram, meanwhile, took the form of a small metal cylinder made of platinum and iridium. This was the International Prototype Kilogram (IPK), locked in a vault on the outskirts of Paris, with dozens of copies around the world. After the meter was [redefined based on natural phenomena in 1983,]( it was the last constant of its kind. Unfortunately, the absolute weight measure had a weight problem. While it technically always weighed a kilogram—it had one job!—for reasons no one understood, its mass fluctuated slightly but measurably, relative to the mass of the copies. A shifting standard isn’t just an oxymoron—it’s a physicists’ nightmare. That’s why on Nov. 16, delegates at the annual General Conference on Weights and Measures unanimously voted to redefine the kilogram using [terms of electrical energy]( stripping the IPK of its title. In other words, weight will now be defined using a fundamental constant of nature, rather than a mysteriously inconsistent chunk of metal. The kilogram is dead. Long live the kilogram. 🐦 [Tweet this!]( 🌐 [View this email on the web]( Join the next chapter of Quartz by downloading [our new app]( or becoming a [member](. It’s where the future gets its news. Sponsored by [Quartz Obsession] The Kilogram November 28, 2018 Goodbye to the IPK --------------------------------------------------------------- The metric system was designed to be the measurement system for all people, in all places, for all time. The constancy of the natural world was at stake—the weight of water, the length of a day. But the way we originally measured the most basic of standards, length and weight, now seems mind-boggling. The system relied on sophisticated physical artifacts that were locked away for their own (and our own) protection. To measure distance, for example, the French Academy of Sciences once defined the meter as one ten-millionth of the distance from the North Pole to the equator. This was enshrined in a physical rod, [the ur-meter]( from which all other meters could be measured. The kilogram, meanwhile, took the form of a small metal cylinder made of platinum and iridium. This was the International Prototype Kilogram (IPK), locked in a vault on the outskirts of Paris, with dozens of copies around the world. After the meter was [redefined based on natural phenomena in 1983,]( it was the last constant of its kind. Unfortunately, the absolute weight measure had a weight problem. While it technically always weighed a kilogram—it had one job!—for reasons no one understood, its mass fluctuated slightly but measurably, relative to the mass of the copies. A shifting standard isn’t just an oxymoron—it’s a physicists’ nightmare. That’s why on Nov. 16, delegates at the annual General Conference on Weights and Measures unanimously voted to redefine the kilogram using [terms of electrical energy]( stripping the IPK of its title. In other words, weight will now be defined using a fundamental constant of nature, rather than a mysteriously inconsistent chunk of metal. The kilogram is dead. Long live the kilogram. 🐦 [Tweet this!]( 🌐 [View this email on the web]( Join the next chapter of Quartz by downloading [our new app]( or becoming a [member](. It’s where the future gets its news. Reuters/Christian Hartmann By the digits [6.62607015×10−34 Js:]( The numerical value of Planck’s constant—a fundamental constant related to the amount of energy a photon carries and the frequency of its electromagnetic wave [0:]( Error in the measured value of the IPK’s mass. In other words, it always weighed a kilogram. [2.2046226218:]( Weight in pounds of one kilogram [6:]( Number of sister copies of the IPK; the original was located in Saint-Cloud, France [10:]( Number of working copies, also located in Saint-Cloud. Eight were for “routine use” and two for special occasions. [39:]( Height in millimeters of the IPK [40:]( Total number of official replicas of the IPK around the world [1,093:]( Approximate number of M&Ms in a kilogram [84,446,8893 × 83⅓:]( Number of carbon-12 atoms that make up a kilogram Origin story Remembering the “grave” --------------------------------------------------------------- The story of the kilogram began with the French king Louis XVI. Amid rampant cheating towards the end of the 1700s, he ordered a new system of measurement based on the natural world. At its heart would be the grave: the mass of a liter of water at its “ice point,” weighed in a vacuum. After the French Revolution, it was eventually renamed the kilogram, with the gram—one thousandth of its weight—as the key unit. But there were a few problems. To begin with, it’s essentially impossible to weigh an open vessel of liquid in a vacuum—and air pressure has a substantial influence on mass. When scientists attempted to reweigh a decimeter of water in 1799, for instance, the final result was [just 99.92072% of the mass]( of the provisional kilogram made four years earlier. Finally, in mid-1799, a physical, platinum-alloy based kilogram was chosen in its stead. And that was that—until the kilo started to waste away. Sponsored by Synchrony Alexis Ohanian, co-founder of Initialized Capital and Reddit, is getting all up in other people’s businesses. --------------------------------------------------------------- The Silicon Valley guru is on a mission to find out who is helping make the economy tick. Hear from an innovative generation of entrepreneurs on what it takes to run a successful company in Business Schooled, a podcast by Synchrony.[Tune in to get schooled]( Giphy Quotable “If aliens ever visit Earth, what else would we talk about other than physics? … If we say our unit of mass is based on a lump of metal we keep in Paris, we’ll be the laughing stock of the universe.” —[Stephan Schlamminger]( a physicist at the US National Institute of Standards and Technology Explain it like I’m 5! Why didn't Le Grand K stay? --------------------------------------------------------------- The kilogram was made out of a platinum-iridium alloy [for stability and accuracy](. Platinum can be machined to a smooth surface to prevent contamination. It’s also dense, which means Le Grand K is small, reducing surface area to be contaminated. Finally, its density means that its buoyancy in air—remember, extreme accuracy matters—is reduced. But it just wasn’t stable enough. Contamination is one of the possible culprits; Le Grand K was only taken out every 40 years, but its copies were more widely used. The act of exposing one to the air could have been enough to cause changes in mass—[around one-millionth of a gram over a century]( which is a big difference at small scales. Perhaps [cleaning the Ks]( to remove impurities might have done it. Or gases present at the creation of Le Grand K were [ever so slowly escaping](. Giphy Pop quiz How many of the world’s countries do not use the metric system? 1271043 Correct. The three countries are Burma, Liberia, and the United States. Incorrect. If your inbox doesn’t support this quiz, find the solution at bottom of email. Explain it like I’m 5! What’s the big deal about a little weight? --------------------------------------------------------------- The difference in weight between Le Grand K and the US copy, for example, was about the same as an eyelash. When that was discovered, manufacturers that made weights to the US standard [had to make new ones.]( And the kilogram is the basis for other measurements, so the problem can ripple. Back in 2012, [one physicist warned Mental Floss]( that the demands of technology would run up against the kilo problem in “a decade or two.” That’s why scientists raced to fix it. They’ve settled on a scale—the steampunk-looking [Kibble balance]( which [translates mass to electrical energy]( using [the Planck constant](. There are currently only two working Kibble balances in the world. And Le Grand K leaves a legacy: both the Kibble and the sphere were calibrated with it. As part of the General Conference, the ampere, kelvin, and mole were also redefined based on universal constants, fulfilling the ideal of the metric system. “Now for the first time in the history of this endeavor, anyone can theoretically create their own primary reference of mass,” [the NIST’s Jon Pratt told Spectrum](. “It takes a lot of know-how, like knowing quantum electrical standards, but in principle anyone can make it.” Reuters/Christian Hartmann Brief history [1668:]( The English philosopher John Wilkins publishes a book advocating for “universal measure.” His suggestion for a standard weight? A volume of about 1 meter cubed, filled with rainwater. [1795:]( The French National Convention issues a decree officially adopting the term “gram.” [1799:]( A platinum-alloy artifact is fashioned to define the kilogram, to be known as the kilogram of the archives. [1875:]( 17 different countries sign the Treaty of the Meter to establish new international prototypes for mass and length. [1889:]( The most recent IPK is manufactured, then promptly placed in a vault. [1960:]( The General Conference on Weights and Measures replaces the ur-meter bar with “a definition based upon a wavelength of krypton-86 radiation.” [1983:]( The GCWM updates its definition of the meter to “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.” [2018:]( A unanimous vote of delegate metrologists declares the IPK obsolete. [2019:]( On May 20, the new standard for the kilogram will come into effect, and the old IPK will be retired Fun fact! The word “kilogram” is what’s called a learned coinage, where one part comes from one language and another from a completely separate one. In this case, it’s “khilioi,” the Greek word for “a thousand,” tacked onto the Latin “gramma,” meaning “a small weight.” Watch this! The world’s roundest object --------------------------------------------------------------- Although scientists may have opted for electrical current to define the kilogram, there are other constants that might have been a good fit. One of the most dazzling, produced by National Institute of Standards and Technology, is an almost perfectly spherical lump of a certain kind of silicon—Silicon-28. The raw materials alone cost $1 million—though now that it’s been sculpted into a near-perfect sphere, it’s priceless. This video explains why, and its role in the redefinition of the kilogram. take me down this 🐰 hole! A kilogram world tour --------------------------------------------------------------- Checking the weight of the world’s kilogram measure isn’t a question of simply plonking it on a scale. Instead, as [American physicist Patrick Abbott told Wired]( “It’s a real cloak-and-dagger affair.” To take an American measure to Paris, Abbott used tongs to place it into a custom-built container, wrapped the item in bubble wrap, and then carried it aboard the plane as his carry-on. It came with a letter of introduction from the director of NSIT, “to keep customs and TSA officials’ grubby hands from opening the container.” [This deep dive]( explores the long journey to Paris—complete with a visit to the airplane bathroom. Giphy Poll Do you use the kilogram regularly? [Click here to vote]( I use it to weigh myself—you get a smaller number than with pounds.Sorry, I don’t speak European.Of course. Doesn’t everyone? 💬let's talk! 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