WEBVTT
Kind: captions
Language: en

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

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The British National Standard Kilogram,

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at the National Physical Laboratory in Teddington.

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It's a lump of platinum-iridium alloy,

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

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and it weighs... well, one kilogram.

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Mass measurement around the world needs to
be equivalent.

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

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

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or a kilogram, or 100 tonnes in the UK.

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

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are, for example, the pharmaceutical industry

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

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and you have to measure them very, very accurately.

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So when, say, your bathroom scales were manufactured,

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

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several chains down the line later,

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was tested against something like this.

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

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

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

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

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or lost a tiny amount when it was cleaned.

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It is one kilogram.

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We know that the International Prototype is
probably not stable,

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because all its copies are unstable relative
to itself.

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

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to tell whether it's changing or not.

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

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so in the next few years, scientists round
the world

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are deciding what the 21st century standard
should be.

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

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but how do you make that into a physical thing?

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

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You can manufacture 1kg sphere of almost-pure,

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single-crystal silicon relatively easily.

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What you have to measure here is the lattice
spacing,

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ie how far the atoms are apart in the sphere,

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and the volume of the sphere. And both of
those

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are measurements you can make at a level of
uncertainty

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where you can generate a sphere which has
an accuracy

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

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

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Finding the kilogram that way seems to make
sense.

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We're measuring mass, after all.

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

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although it isn't quite as obvious.

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The watt balance is an electrical way of measuring
mass.

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

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We have a loudspeaker here

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

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

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If I was to put a mass on here,

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if I restore the position of the mass,

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

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

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or the number of turns on the coil.

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

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

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and also measure the voltage which was produced,

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

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

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Whichever approach gets chosen,

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there will still be physical reference kilograms
in the world.

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

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

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Thank you to everyone at the National Physical
Laboratory!

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They have a YouTube channel that you can check
out,

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

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