Kind: captions
Language: en
this is 10 micrograms you think that I
might be able to see I think you might
be able to oh boy it's an arrow right
there yeah the flashlight will help I
feel like I need to to get video of this
I don't know how it kind of looks like a
hair like a tiny like a smaller than an
eyelash if you want to measure a force
like the weight of an object the way it
has always been done has been to balance
it with some known standards and the
most precise standard weight people ever
created was the kilogram of platinum
idium cylinder stored in a vault on the
outskirts of
Paris replicas of this kilogram were
sent to countries around the world to
use as their Mass standards here is K20
the United States Mass standard and yes
the US is secretly metric they just
apply a conversion factor to get to
Freedom Units 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 it's
stupid the uncertainty in the mass of a
standard kilogram is on the order of
tens of micrograms so that's tens of
parts per billion or about
0.00001% it's pretty good but there's a
problem if you want to weigh something
lighter than a kilogram the uncertainty
increases this little object here is a
50 g test weight so it's a reference
Mass can I pick it up uh with tweezers
yeah if you don't mind sure we tried to
keep the fingerprints off so it's got a
little bit of heft you can feel that a
little yeah what about this one what's
this this this is 10 G
here yeah that's pretty light that's
pretty light and paper clip here is
about 1
G and so how you might use one of these
test masses so if you're you know
working in a laboratory or something
like that you could take one of these
little weights and put it on here and
you can look at the scale and say oh
okay my scale is reasonably well
calibrated here that last digit might
you know change a little bit but you
could sort of make some statement about
whether or not your scale is accurate
when we're talking about the kilogram
you know these are obviously much
smaller than a kilogram you know how do
we you know think about getting from
that large object down to something
that's you know this size here one of
the ways to do it is using conventional
Mass Metrology which is that you take
the kilogram and you use a process
called subdivision where you compare
other smaller masses against the
kilogram you take 2 500 gam masses and
you make sure that they're equal on your
balance right and then you you put both
of those on your balance and compare
that against a kilogram would you go
like two 500s and then two 250s and then
225s like you just yeah yeah yeah so you
could do like one of one common
Arrangement is something like we have
here this is a 500 mgram mess these
these are two uh 200 MGR masses and
that's a 100 mg Mass so these three sum
up to this right so you can compare
those two against each other the
smallest we have here is down there
that's a milligram that is a milligram
that is a milligram right there what are
they made out of these are made out of
stainless steel and we have to do things
like you see this little lintree brush
here you know if you have a speck of
dust on here or something like that it's
can be can be a problem if it's a big
Speck of dust you know so so every time
we do weighs with these you know we'll
take a a a lint Rec cloth and a brush
and clean them off a little bit why are
they these this sort of curious wire
kind of shape the shapes yeah uh it
helps you remember which one is which
you know the five-sided one is five 500
milligrams you know one of the
interesting things from from my
perspective about this is that you can
you can really subdivide over a lot
large range of these masses from a
kilogram you know this is 1 millionth of
a kilogram is a is a is a milligram so
you can subdivide the kilogram by a
million times you know but you sort of
pay a price for that because each time
you do this subdivision the uncertainty
increases a little bit right so what is
the uncertainty in say the milligram
there uh if you do it with a a
subdivision it will be U maybe uh a a
part in 10 to the four one part in 10
the four so like uh 0 1% is range but
there is a way to do better and that's
thanks to the fact that kilogram is no
longer defined by the Platinum aridium
cylinder in Paris over the course of a
century or so the replica kilograms were
brought back to Paris a few times to be
weighed with each other and from those
measurements it became clear that their
weights were diverging by up to 75
microgram no one could say if the
replicas were getting heavier or if the
original was getting lighter
but it was unacceptable to have mass
standards with changing masses so the
solution was to eliminate the kilogram
dependence on a physical object and
instead Define it based on a constant of
nature Plank's constant so how does that
work well Plank's constant is best known
for relating the frequency of a photon
to its energy by eal HF but energy and
mass are related through eal mc^2 so you
can see how Mass is related to Plank's
constant in 2019 scientists officially
set the value of Plank's constant to be
this number in juw seconds which along
with the definition of the meter and the
second now defines what a kilogram is
the real advantage of this definition is
how it can be applied in fancy
scales this is a kibble balance it can
balance the weight of an object with an
electromagnetic force what's great about
that is that the electrical quantity
ities used in this balance can be read
out very accurately and in units of
planks constant so you get direct
traceability by weighing something in
this
balance this is kind of the smaller
cousin of the kibble balance it's called
the uh electrostatic force balance or
the efb and this is a this is a balance
that was designed specifically to
measure mass sort of in the milligram
range the kibble balance uses
electromagnet I use a capacitor which is
basically uh two metal electrodes that
you apply a potential to and when you
apply a potential there's an attractive
force between those two electrodes I
apply a an electrostatic force by
applying a voltage here at this you can
see this cylinder here mhm there's this
cylinder and there's inside of this
there's another cylinder and they're
close together so you have this
concentric cylinder like this and when
you apply a voltage it pulls that moving
cylinder down
in there and by measuring the properties
of the capacitor and measuring the
voltage that we apply we can know
exactly how much force we get here and
then up here we drop our Mass on so we
compare our gravitational force from the
mass to the electrostatic force from our
capacitor to get the best accuracy this
lab is located deep underground and they
keep the air temperature a constant 20°
C to avoid any thermal expans expansion
or contraction of the devices and all
measurements in this balance are made in
a vacuum so there are no air currents
and no buoyant force on the object from
the atmosphere they've even carefully
measured the acceleration due to gravity
in the lab here it is it's under the
chair right there that triangle that is
where the USGS measured uh absolute
gravity with an absolute gravimeter
9.81 is m/s does this lab measure small
force is the most accurately in the
world at the milligram level so 10
Micron Newtons is of force yes this
measures Force the most accurately in
the world I'm confident in saying that
but of course you know you can go lower
than that this is the smallest weight
and you can't see it here this is 10
micrograms so when you think about the
uncertainty in in a kilogram when you
take PK constant at a kibble balance and
you realize the kilogram you're at that
level of about 10 microG and that is
what 10 microG I mean you can I I had to
put the little arrow here so you can if
you were here in the laboratory you
could look and peer down there maybe I
feel like I need to to get video of this
so people can see what 10 micr I you
know I I I don't know
how it kind of looks like a hair like a
tiny like a smaller than an eyelash
right th yeah that's about the scale
you're looking at there yeah I'm I'm
almost certain I can capture this on
video we brought a special lens with us
oh we brought a special special lens 20
24 mil macro cuz I was like we're going
to need it for
this try to find this
thing oh I can see it oh yes do you see
it there yeah yeah yeah yeah you got
it you made this yes with great trouble
and then calibrated it on a balance it
was man I'll tell you it was not easy
the when you have that thing you know
you can imagine what it's like trying to
work with something like this right and
this is about as small as you can
reasonably expect to make something as a
test weight and if you want to measure a
force smaller than that so I have these
little tiny chips here these are Atomic
Force microscope Canal levers they they
are little tiny Force sensors on the end
of these they're little tiny Canal lever
beams with a sharp tip and you can use
that sharp tip to to
to press against things and apply Nano
Newton to Pico Newton forces but I mean
the tip is so small though it's very
very difficult to to see it really
requires a microscope is it like there's
a little diving board on yeah exactly
there's a little diving board that's
right it looks like a little diving
board it it's like a spring right and if
you push on it the more it bends the
larger the force what is the smallest
force that you want to
measure yeah I can show you I can show
you uh this is one of the sensors we us
to do the smallest forces that I can
confidently say we've measured that are
traceable in some way to the the
International System of Units um and
that is a fton Newton of force at about
a picon Newton would be like if you're
stretching out a DNA molecule so if you
take a DNA molecule and stretch it out n
to n that's a picon Newton so back a
factor of a thousand less than that was
what we were measuring so you know this
is an example of one of the sensors we
use to get to the sort of fonon level
this is a fused silica parallelogram
flexure you can't see it really well so
let me I have a big big version right
here so what we can do is we can set
this
vibrating and it'll vibrate re with
really pure tone and we can see very
very small changes in force based on how
how far this vibrates up and down I
would have a little uh laser
interferometer which measures the motion
of this so we measure the displacement
of this end here and then right next to
it would have little tiny optical fiber
that would deliver a known optic laser
power to this so this would be a photon
pressure Force whereby reflecting the
light off the surface here we actually
get a very small Force if we vary that
that Force sinos we vary it in time we
can get this to move up and down and we
can get it to vibrate and we can see
differences as small as uh as Fon
Newtons in in in our Force so you're
saying you could measure the force from
a laser poter
yeah oh yeah yeah definitely Point
shining on that that's right that's
that's about uh about about
approximately seven Pico Newtons of of
force and once again you know that's
enough to stretch out a DNA
molecule can I ask you the big question
yeah yeah why does anyone need to
measure forces this small that's a good
question so a couple of things you know
there are a couple of different answers
to that one is sort of the industrial
relevance Automotive manufacturers need
to measure the mass of particulates that
come off their exhaust particularly in
diesel systems particulate contamination
is really kind of a big big deal so you
need to be able to measure 50 micrograms
of these particulates for those
environmental standards to be met the
laser power measurements for people who
are doing industrial processes with
lasers because you can actually use the
measurement of a small Force to
calibrate laser power you know
Pharmaceuticals you know you have
milligram doses microgram doses
sometimes the other um thing that's
important I think kind of goes to the
heart of why nist is so cool in my
opinion is that it really helps us push
the frontiers of science the scientific
new scientific discoveries benefit from
the new measurement capabilities which
then feed into new Precision Metrology
capabilities and so that is really one
of the things to me that makes nist
really special is that we're very good
at sort of creating that environment
where that can happen
[Applause]
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