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  • This is the kilogram. Not a kilogram, the kilogram.

  • The British National Standard Kilogram,

  • at the National Physical Laboratory in Teddington.

  • It's a lump of platinum-iridium alloy,

  • it's kept in a very very secure safe just under here,

  • and it weighs... well, one kilogram.

  • Mass measurement around the world needs to be equivalent.

  • So if I buy something in America that's a milligram, or a kilogram, or 100 tonnes,

  • I need to know that it's equivalent to a milligram,

  • or a kilogram, or 100 tonnes in the UK.

  • The real key areas of interest, where the uncertainties are very important,

  • are, for example, the pharmaceutical industry

  • where you're dealing with very, very small quantities of active ingredients

  • and you have to measure them very, very accurately.

  • So when, say, your bathroom scales were manufactured,

  • they were tested against equipment that was tested against equipment that,

  • several chains down the line later,

  • was tested against something like this.

  • And this was tested against the International Prototype Kilogram in Paris.

  • And this is a really weird thing to wrap your head around,

  • but that one in Paris? It always weighs a kilogram.

  • It doesn't matter if it's gained a tiny amount of mass from adsorbing air contaminants,

  • or lost a tiny amount when it was cleaned.

  • It is one kilogram.

  • We know that the International Prototype is probably not stable,

  • because all its copies are unstable relative to itself.

  • But there's nothing above the International Prototype that you can compare it with

  • to tell whether it's changing or not.

  • Defining international constants by a physical thing is... not ideal,

  • so in the next few years, scientists round the world

  • are deciding what the 21st century standard should be.

  • The formal definition will be based on physics, likely Planck's Constant,

  • but how do you make that into a physical thing?

  • There are two options: first, a fixed number of atoms.

  • You can manufacture 1kg sphere of almost-pure,

  • single-crystal silicon relatively easily.

  • What you have to measure here is the lattice spacing,

  • ie how far the atoms are apart in the sphere,

  • and the volume of the sphere. And both of those

  • are measurements you can make at a level of uncertainty

  • where you can generate a sphere which has an accuracy

  • of a part in 10-to-the-8, roughly.

  • The downside of that is it costs you €2.5million for a sphere.

  • Finding the kilogram that way seems to make sense.

  • We're measuring mass, after all.

  • But there is an alternative which could be cheaper and easier,

  • although it isn't quite as obvious.

  • The watt balance is an electrical way of measuring mass.

  • This is a way of demonstrating simply how a watt balance works.

  • We have a loudspeaker here

  • and if I pass a current through the coil of the loudspeaker,

  • you can see that the cone will move up and move down.

  • If I was to put a mass on here,

  • if I restore the position of the mass,

  • the current I need to do that is a measure of the weight of that mass.

  • The problem I've got is that I don't know the strength of the field of the magnet,

  • or the number of turns on the coil.

  • And I can do a second experiment by moving the cone of the speaker, like that.

  • If I was to measure the velocity with which I was moving that,

  • and also measure the voltage which was produced,

  • that would tell me that quantity and allow me to eliminate it from the two experiments.

  • Which, if done accurately, is enough to work out the weight of that mass.

  • Whichever approach gets chosen,

  • there will still be physical reference kilograms in the world.

  • They're not going away. But for the first time,

  • we're going to have something absolute to compare them against.

  • Thank you to everyone at the National Physical Laboratory!

  • They have a YouTube channel that you can check out,

  • and they occasionally hold open days. Details are on their web site.

  • [Translating these subtitles? Add your name here!]

This is the kilogram. Not a kilogram, the kilogram.

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B1 中級

誰もが今、キログラムがどのくらいなのかを正確に把握していない (Nobody's Exactly Sure How Much A Kilogram Is Right Now)

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    林宜悉 に公開 2021 年 01 月 14 日
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