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I'm here at the National Institute for Standards and Technology, and
I'm about to see some of the original kilogram standards. Is that right?
Patrick: You are, you are.
When were these made?
Well the originals were made in the 1880s.
There were 40 of them that were brethren of the
International prototype kilogram
and these 40 were distributed to the signatories
of the Treaty of the meter and the
United States was given two, K4 and K20.
What was this meter agreement?
The treaty of the meter?
Yeah, what is that?
It's the modern-day foundation of the metric system.
So the U.S. signed that?
The U.S. signed it.
As if they were going to become metric?
Yes.
A little known secret and I'll tell you something else
that all the units that we commonly use
like feet and gallons and so on are
actually defined in terms of metric units.
So it's just a little translation
that we do here but our country is
actually on the metric system.
[Laughter]
Doesn't that seem crazy?
Yes!
Isn't that insane that like you base all of these measurements on the metric
system and then you add a conversion
factor and then later some people have
to convert back.
Yes, it's stupid.
[Laughter]
Are you allowed to say that?
It's true.
So can we see it?
sure sure
What do we have to do?
This requires that we go through
some high-security gear and also that
would be somewhat clean in what we do
so I'll have to ask you to put on some
booties here.
Let me see if I can do this
all right so, did I do it?
Yep, there you go.
straight back, all the way there you go.
This is the first
BootieButler that I've ever...
I'm a big fan.
This is the first layer.
Secret code
I'm going to show you two I'm going to
show you one of the originals and I'm
going to show you a more modern version
This is K 20.
Oh my goodness that is like the
original kilogram mass standard of the US.
That's right.
I never thought I would
get this close to it
virtually every mass that has been
accurately weighed in the US over the
past hundred and thirty years can trace
its measurement back to this one
kilogram hunk of 90 percent platinum 10 percent iridium.
Iridium makes the alloy much
harder than pure platinum and both
elements were selected for their high
density and resistance to oxidation this
kilogram was created in the same way at
around the same time as an object which
to this day remains the definition of
the kilogram an identical cylinder
stored in a basement vault on the
outskirts of Paris.
If you look straight
down from the top you can maybe see K 20
doesn't have a very nice finish on it
compared to the more modern prototypes
here's K 92 so it's got a much higher
polish on it different manufacturing
techniques absolutely no swirls it is a
really like beautiful looking specimen
yes it is I mean is there a reason why
you want it to look so beautiful?
well you want it not to be very rough
because roughness increases surface area
surface area increases the probability
that you will get contaminants on that
will change the mass of the entire thing
how much is it worth
well monetarily the new ones are about
$100,000 a piece if you were to buy one
oh my goodness so but if you think about
K 20 K for with about 130 years of
history they're priceless you could
never replace them the purpose of this
room then is to share the precise mass
of K 20 with anyone who wants to make a
measurement without sharing K 20 itself
what we do in this suite here is we
transfer the definition of the kilogram
from the platinum-iridium prototypes to
stainless steel secondary standards and
you can tell that they're a lot bigger
and the platinum-iridium prototypes and
that's because of the relative
difference in density the density of
stainless steel is about 8 grams per
cubic centimeter whereas the density of
platinum iridium is about 21.5 grams per
cubic centimeter so there's all of the
three times difference in density which
tells you why this is so much bigger in
volume and that creates a problem
ordinarily we don't worry much about the
buoyant force that is the upward force
on every object in the atmosphere equal
to the weight of air it displaces but
since the volumes of these masses are so
different a stainless steel object that
has around the same mass as k20 can have
its weight reduced due to buoyancy by
around 110 milligrams
the precise amount depends on the
temperature pressure and humidity of the
air that's why the mass comparator
itself is inside of a chamber here
that's isolated from the outside world
so that the temperature remains
relatively stable I'm the humidity the
same pressure is by far the biggest
contributor to air density so we don't
want it changing all over the place with
the weather the problem scientists are
having with the kilogram now is much
bigger than weather fluctuations it's
something they discovered when all the
original kilograms were returned to
Paris for a weigh-in including k20 how
did it get there it gets there by a
person hand carrying it each prototype
gets taken out of the bell jar and put
in its container how do we sound after
we get it in there all nicely secured
then we wrap the whole thing in bubble
wrap and put it in a camera bag and
sling it over our shoulders don't let it
out of our sight it sounds a little
casual so you actually like while you're
on the plane do you put it in like the
overhead stretch
know it stay just at all times like it's
a you know the nuclear football codes
were setting off a nuclear weapon and
you never have any scares while you're
carrying the only scare comes that
somebody wants to see it like a customs
official I've never had to open it
although I had a kind of a scary moment
at one time when they asked me what it
was made of and I told them it was
platinum iridium and somebody heard the
word iridium and connected that with
radioactivity no no and kind of you know
flew off the handle a little bit and I
had to calm them down on the shoreland
that it wasn't radioactive there was no
threats going on here
the real threat was the unreliable
weight and the kilogram mass standard
what they do is a series of comparisons
they compare every one of those with the
International prototype kilogram using
that data and plotting it it looked as
if there had been a change in the
International prototype that made it
about 50 micrograms heavier than one
kilogram now over the course of a
hundred years all right
but the interesting thing is if one
looks at the data that was recently
taken at the end of 2013 international
prototype kilogram did not show an
increasing mass telling me for several
decades it was gaining mass and also on
the stop which you know I find that hard
to explain but it seemed like all of the
40 masses in to be somewhat diverging at
that weigh-in they went different ways
they went in different directions yes so
that some changed a lot some changed
little some hardly changed at all but
it's hard to tell I mean they could
we'll all be changing a lot but because
all you can tell is the difference
between them they've already married
all that matters is the difference
between them and the International
prototype because the International
prototype is the only thing you need the
only thing that you really know because
it's defined as one kilogram
right but I mean the outside of that
definition there's a chance it could
have gained you know five grams as long
as they all gain five grams you know I'm
having these ridiculous obviously it's
ridiculous but the point is they could
have all been gaining or they could all
been losing right it's a relative
measurement and that's the weakness of
the system
and that's why as of 2018 the kilogram
will no longer be defined as the mass of
a metal cylinder why if you want to find
out how the kilogram will be redefined
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