WEBVTT - How Do We Define the Kilogram?

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<v Speaker 1>Welcome to brain stuff from how Stuff Works. Hey, brain Stuff,

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<v Speaker 1>I'm Lauren Vogelbaum. And for nearly a hundred and fifty years,

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<v Speaker 1>the official weight of a kilogram was determined by a

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<v Speaker 1>shiny cylinder of platinum locked away in a French vault.

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<v Speaker 1>The kilogram, like the meter and the second, is one

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<v Speaker 1>of the seven fundamental units of measurement, also known as

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<v Speaker 1>the International System of Units or the metric system the

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<v Speaker 1>SI for short. These were first formalized in the eight

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<v Speaker 1>seventy five Treaty of the Meter. Back then, the best

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<v Speaker 1>way to agree on the weight of a kilogram was

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<v Speaker 1>to forge a single hunk of metal and dubit le

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<v Speaker 1>grand K, And for more than a century, all scientific

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<v Speaker 1>scales were calibrated back to that one physical reference point,

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<v Speaker 1>with copies stored in a dozen countries. But even solid

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<v Speaker 1>objects can change over time. When le grand K was

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<v Speaker 1>weighed in the nineteen eighties, it was a couple of

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<v Speaker 1>micrograms lighter, meaning that all highly accurate scientific scales, not

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<v Speaker 1>like the one in your bathroom, to be recalibrated, and

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<v Speaker 1>that's what nerds call a real pain in the mass. Luckily,

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<v Speaker 1>a team of metrologists was already on the case. Metrology

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<v Speaker 1>being the science of weights and measures. They were searching

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<v Speaker 1>for a universal constant that would generate a fixed value

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<v Speaker 1>for the kilogram that's true now and will still be

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<v Speaker 1>true a million years from now. They had already found

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<v Speaker 1>such a physics fix for the second, which was redefined

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<v Speaker 1>in nineteen sixty seven from its previous value of a

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<v Speaker 1>fraction of a day one six thousand, four hundredths of

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<v Speaker 1>a day to be precise, to something much more confusing,

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<v Speaker 1>but much more constant. It takes nine billion, one d

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<v Speaker 1>and ninety two million, six hundred and thirty one thousand,

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<v Speaker 1>seven hundred and seventy oscillations of a special microwave beam

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<v Speaker 1>to excite atoms of the isotope caes M one thirty

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<v Speaker 1>three to a higher energy level. Since that number will

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<v Speaker 1>never change unlike the exact length of a day, that's

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<v Speaker 1>your new second. Same for the meter. Instead of being

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<v Speaker 1>defined as the length of a single meter long metal

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<v Speaker 1>poll for orged back in eighteen eighty nine, it was

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<v Speaker 1>redefined in nineteen eighty three as the distance light travels

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<v Speaker 1>in a vacuum in a particular fraction of a second

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<v Speaker 1>one two million, seven hundred and nine two thousand, four

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<v Speaker 1>hundred and fifty eight of a second. It wasn't until

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<v Speaker 1>twenty seventeen that scientists working at the u S National

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<v Speaker 1>Institute of Standards and Technology and similar bodies worldwide finally

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<v Speaker 1>agreed on a universal constant for the kilogram. The achievement

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<v Speaker 1>required solving one of the Thorny's physics problems of the

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<v Speaker 1>last century, coming up with a numerical value for planks

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<v Speaker 1>constant without getting too technical. A physicist, Max Planck proved

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<v Speaker 1>in nineteen hundred that matter releases energy in discrete chunks

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<v Speaker 1>called quanta. His equation for measuring these packets of energy

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<v Speaker 1>included a constant called h hitherto known as planks constant.

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<v Speaker 1>Thanks to Einstein, we know that energy and mass are

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<v Speaker 1>mathematically related, that whole equals mc squared thing, so physicists

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<v Speaker 1>figured out the planks constant, being a fixed unit of energy,

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<v Speaker 1>could yield the world most accurate measurement of mass. Calculating

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<v Speaker 1>the exact value of plunks constant took decades and some

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<v Speaker 1>serious technological innovation, specifically a nifty device called a Kibble balance,

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<v Speaker 1>but they did that work, and we now know that

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<v Speaker 1>plunks constant is six point six to six zero seven

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<v Speaker 1>one zero times ten to the power of negative thirty

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<v Speaker 1>four jewels per second I mean duh. In mid November,

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<v Speaker 1>at the annual meeting of the International Bureau of Weights

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<v Speaker 1>and Measures in Versailles, France, representatives from more than sixty

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<v Speaker 1>countries voted to approve a new and everlasting definition of

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<v Speaker 1>the kilogram, as calculated by the plank constant. No more

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<v Speaker 1>hunk of metal. The kilograms mass is now tied to

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<v Speaker 1>plunks constant. New definitions were also announced for SI units,

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<v Speaker 1>the ampire electrical current, the kelvin for temperature, and the

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<v Speaker 1>mole the number of molecules or atoms in an element.

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<v Speaker 1>These new definitions will take effect on nineteen The original

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<v Speaker 1>platinum kilogram prototype will remain in that underground French vault,

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<v Speaker 1>while countless generations of scientists will make world change in

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<v Speaker 1>discoveries using the kilogram to point oh. Today's episode was

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<v Speaker 1>written by Dave Ruse and produced by Tyler Clang. For

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<v Speaker 1>more on this and lots of other weighty topics, visit

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<v Speaker 1>our home planet, how Stuff Works dot com.