Kind: captions
Language: en
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 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 US signed that the US signed it as
if they were going to become metric yes
it's a little known secret and I'll 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 doesn't that seem crazy yes is
that insane that like you base all of
these measurements on the metric system
and then you add a conversion factor yes
and then later some people have to
convert back yes it's
stupid agree are you allowed to say that
it's
true so can we see it sure sure what do
we have to do this uh requires that we
go through some high security GE and
also that we be U somewhat clean in 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 that did I do
it yeah there you go straight back all
the way there you go boom this is the
first um booty Butler that I've ever I'm
I'm a big fan
so this is the first
layer and
see the
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 a more modern
version this is
K20 oh my goodness so 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 mask that
has been accurately weighed in the US
over the past 130 years
can trace its measurement back to this 1
kg hunk of 90% Platinum 10% iridium
idium 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 see K20 doesn't
have a very nice finish on it compared
to the the more modern prototypes here's
K92 so it's got a 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 K20 K4 with uh about 130
years of History they're they're they're
Priceless you you could never replace
them the purpose of this room then is to
share the precise mass of K20 with
anyone who wants to make a measurement
without sharing K20 itself what we do in
this Suite here is we transfer the
definition of the kilogram from the
Platinum idium prototypes to stainless
steel secondary standards and you can
tell that they're a lot bigger than the
Platinum meridium prototypes and that's
because of the relative difference in
density the density of stainless steel
is about 8 G per cubic cm whereas the
den density of platinum meridium is
about
21.5 G per cubic cm so there's almost a
three times difference in density which
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 Mig 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 and the humidity the
same pressure is the 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
kilogram were returned to Paris for a
weighin including K20 how does it get
there it gets there uh by a person hand
carrying it each prototype gets taken
out of its Bell jar and put in its
Container makes an ugly
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
storage oh no no no no no no it stays
with us at all times like it's a you
know the nuclear football the codes for
setting off the nuclear weapons and
you've never had any scares while you're
carrying the uh the only scare comes if
somebody wants to see it like a customs
official I've never had to open it
although I I had a kind of a SC moment
at one time when they asked me what it
was made of and I told them it was
platinum aridium and somebody heard the
word aridium and connected that with
radioactivity and kind of you know flew
off the handle a little bit and I had to
calm them down and assure them that it
wasn't radioactive there was no threat
going on here the real threat was the
unreliable weight of 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 1
kilogram now over the course of a 100
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 you're
telling me for several decades it was
gaining mass and all of a sudden it
stopped which you know I find that hard
to explain but it seemed like all of the
40 masses seem to be somewhat diverging
at that weighin 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 all
be changing a lot but because all you
can tell is the difference between them
if you know M all that matters is the
difference between them and the
international prototype because the
international prototype is the only
thing you need only thing that you
really know because it's defined as 1
kilogram right but I mean outside of
that definition there's a chance it
could have gained you know five grams as
long as they all gained five gr you know
obviously it's ridiculous obviously it's
ridiculous relative but the point is
they could have all been gaining or they
could have 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 if you want to find out how the
kilogram will be redefined be sure to
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