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Hey, rain stuff on Vogel bomb and for nearly one hundred fifty years, the official weight of a kilogram was determined by a shiny. Cylinder of platinum locked away and a French vault. The kilogram like the meter and the second is one of these seven fundamental units of measurement also known as the international system of units or the metric system. The sl for short. These were first formalized in the eighteen seventy five treaty of the meter back. Then the best way to agree on the weight of a kilogram was to forge a single hunk of metal and debit, Legrand K and for more than a century all scientific scales were calibrated back to that one physical reference point with copies stored in a dozen countries. But even solid objects can change over time when Legrand K was weighed in the nineteen eighties. It was a couple of micrograms lighter meaning that all highly accurate scientific scales. Not like the one in your bathroom had to be recalibrated. And that's what nerds call a real pain in the mass. Luckily, a team of metrology was already on the case a metrology being the science of weights and measures. They were searching for a universal constant that will generate a fixed value for the kilogram. That's true now and will still be true a million years from now they already found such a physics fixed for the second which was redefined in nineteen sixty seven from its previous value of a fraction of a day. One eighty six thousand four hundred of a day to be precise to something much more confusing, but much more constant it takes nine billion one hundred ninety two million six hundred thirty one thousand seven hundred and seventy also nations of a special microwave beam to excite atoms of the isotope cesium one thirty three to a higher energy level since that number will never change. Unlike the exact length of a day that's your new second same for the meter instead of being defined as the length of a single meter long. Metal pole forged back in eighteen eighty nine. It was redefined in nineteen Eighty-three as the. Since light travels in a vacuum in a particular fraction of a second. One two hundred ninety nine million seven hundred ninety two thousand four hundred and fifty eighth of a second. It wasn't until twenty seventeen that scientists working at the US National Institute of standards, and technology and similar bodies worldwide. Finally, agreed on universal constant for the kilogram the achievement required solving one of the thorniest physics problems of the last century coming up with a numerical value for punks constant without getting too technical a physicist Max Planck proved in nineteen hundred that matter releases energy in discrete chunks called Quanta his equation for measuring. These packets of energy included, a constant called H hitherto known as planks constant thanks to I'm Stein. We know that energy and mass are mathematically related that whole E equals MC squared thing. So physicists figured out the plugs constant being fixed unit of energy could yield the world's most accurate measurement of mass calculating the exam. Value of plugs, constant took decades and some serious technological innovation specifically a nifty device called a Kibble balance. But they did that work, and we now know that punks constant is six point six two six zero seven one five zero times ten to the power of negative thirty four jewels per second. I mean, da in mid November twenty eighteen at the annual meeting of the international bureau of weights and measures in verse I France representatives for more than sixty countries voted to approve a new and ever lasting definition of the kilogram. As calculated by the Planck, constant no more hunk of metal the kilograms masses. Now tied to planks constant new definitions were also announced for sl units. The Ampere electrical current the Kelvin for temperature and the mole the number of molecules atoms in an element. These new definitions will take effect on may twentieth. Two thousand nineteen. The original platinum kilogram. Prototype will remain in that underground French vault while countless generations of scientists will make world. Changing discoveries using the kilogram two point. Oh. Today's episode was written by Dave rou and produced by Tyler clang for more on this and lots of other weighty, topics. Visit our home planet. How stuff works dot com. Hey, brain stuff listeners today. I wanted to tell you about another podcast how to money, which is not your typical personal finance podcast, the hosts Matin Joel are best friends. Aiming not to lecture you but to make conversations about money. Interesting informative even fun every Wednesday. They cover real life money topics like ways to cut your grocery Bill. Why your house is an awful investment, and how to achieve financial independence, if you kind of suck at money, or if you just want to learn more about how you can support yourself and your future, you can listen and subscribe on apple podcasts the iheartradio app or wherever you get your podcasts. Just search for how to money."