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, yeah, yeah. This flashlight will help. I feel like I need to get video of this. [Dr. Shaw] I don't know how. (Dr. Shaw laughing)
It kinda looks like a hair, like a tiny,-
Yeah. like a smaller than an eyelash. If you wanna 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, a
platinum-iridium cylinder stored in a vault on
the outskirts of Paris. (gentle music) 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 0.000001%. 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 gram test weight. So it's a reference mass. Can I pick it up? Maybe with tweezers? Yeah, if you don't mind? Sure. [Dr. Shaw] We try 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? [Dr. Shaw] This is 10 grams here. Yeah, that's pretty light. That's pretty light. And paperclip here is about one gram. And so, how you might use
one of these test masses. So, if you're working in a
laboratory or something like that you could take one of these little weights and put it on here and you
could look at the scale and say, oh, okay, my scale is
reasonably well calibrated here. That last digit might 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, these are obviously much
smaller than a kilogram. How do we think about getting
from that large object down to something that's 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 two 500 gram masses and you make sure that
they're equal on your balance. And then you put both
of those on your balance and compare that against a kilogram. Would you go like two 500's and then two 250's and then
two 125's? like you just... Yeah, yeah, yeah. So you could do like
one common arrangement is something like we have here. This is a 500 milligram mass, these are two 200 milligram masses, and that's a 100 milligram mass. So these three sum up to this. Okay. 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? [Dr. Shaw] That is a
milligram right there. What are they made out of? Okay, these are made
outta stainless steel. And we have to do things like you see this little lint free brush here. If you have a speck of dust on
here or something like that, it's can be a problem, if
it's a big speck of dust. So every time we do weighings with these, we'll take a lint free cloth and a brush and clean them off a little bit. Why are they this sort of curious wire kind of shape? The shapes? Yeah. It helps you remember which one is which. The five-sided one is 500 milligrams. One of the interesting things from my perspective about this is that you can really subdivide
over a large range of these masses from a kilogram. this is one millionth of a
kilogram is a milligram. So you can subdivide the
kilogram by a million times. 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? If you do it with a subdivision, it will be maybe a part in 10 to the four, one part in 10 to the four. So like 0.01 percent-ish range. But there is a way to do better, and that's thanks to the fact the kilogram is no longer defined by the platinum-iridium 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 micrograms. 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's dependence
on a physical object and instead define it based
on a constant of nature, Planck's constant. So how does that work? Well, Planck's constant is best known for relating the frequency of a photon to its energy by E=hf. But energy and mass are
related through E=mc2. So you can see how mass is
related to Planck's constant. In 2019, scientists
officially set the value of Planck's constant to be
this number in Joule-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 quantities used in this balance can be
read out very accurately, and in units of Planck's 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 electrostatic
force balance or the EFB. And this is a balance that was designed, specifically to measure mass
sort of in the milligram range. The Kibble balance uses an
electromagnet, I use a capacitor, which is basically 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 an electrostatic force
by applying a voltage here at this you can see the cylinder here. 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 degrees Celsius to avoid any thermal 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 absolute gravity with an absolute perimeter
9.801-ish meters per second. Does this lab measure small forces the most accurately in the world? At the milligram level, so 10
micro newtons-ish of force. Yes, this measures force the
most accurately in the world. I'm confident in saying that, but of course, 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 a kilogram, when you take Planck's
constant at a Kibble balance and you realize the kilogram, you're at that level
of about 10 micrograms. And that is what 10
micrograms, I mean you can't, 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 micrograms. [Dr. Shaw] I don't know how. It kinda looks like a hair, like a tiny, like a smaller than an eyelash, right? [Dr. Shaw] It is really much, yeah, that's about the...
Much thinner. [Dr. Shaw] Yeah, that's about the scale you're looking at there, yeah. I'm almost certain I can
capture this on video. We brought a special lens with us. Ooh yes. We brought a special, Special lens. 2024 mill macro, 'cause I was like, we're
gonna 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. When you have that thing, 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 wanna measure
a force smaller than that, so I have these little tiny chips here. These are atomic force
microscope cantilevers. There're little tiny force
sensors on the end of these. There're little tiny cantilever
beams with a sharp tip. And you can use that sharp
tip to press against things and apply nanonewton to piconewton forces. But I mean the tip is so small it's very, very difficult to see. It really requires a microscope. Is it like there's a little
diving board on there? Yeah, exactly. There's a little diving
board, that's right. It looks like a little diving board. 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? Do you have a... Yeah, I can show you, I can show you. This is one of the sensors we
used to do the smallest forces that I can confidently say we've measured that are traceable in some way to the International System of units. And that is a femtonewtons of force at about a piconewton would be like if you're stretching
out a DNA molecule. So, if you take it a DNA molecule and stretch it out end to
end, that's a piconewton. So back to a factor of a
thousand less than that was what we were measuring. So, this is an example
of one of the sensors we use to get to the sort
of femtonewton level. This is a fused silica
parallelogram flexor. You can't see it really well. So let me, I have a
big version right here. So what we can do is we
can set this vibrating and it'll vibrate with really pure tone. And we can see very, very
small changes in force based on how far this
vibrates up and down. I would have a little
laser interferometer, which measures the motion of this. So we measure the
displacement of this end here and then right next to it we would have a 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 force sinusoidally, 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 femtonewtons in our force. So you're saying you
could measure the force from a laser pointer? Yeah, oh yeah, yeah, definitely. Yeah, we've done that. My pointer was shining on that. That's right, that about approximately seven piconewtons of force. And once again, 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?
Yeah, that's a good question. So a couple of things. 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 exhausts, particularly in diesel systems. Particulate contamination is
a really kind of a 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. Pharmaceuticals, you have milligram doses microgram doses sometimes. The other 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 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. (electronic music) If you, like me, are fascinated
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