Is America Actually Metric?

I'm here at the National Institute for Standards and Technology, and I'm about to see some of the original kilogram standards.

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?

Patrick: 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?

Patrick: The U.S. signed it.

As if they were going to become metric?

Patrick: Yes. A little-known secret, and I'll tell you something else: 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?

Patrick: 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?

Patrick: Yes, it's stupid.

[Laughter] Are you allowed to say that?

Patrick: It's true.

So can we see it?

Patrick: Sure, sure.

What do we have to do?

Patrick: This requires that we go through some high-security gear and also that we 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.

Patrick: All right, so, did I do it?

Patrick: 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.

Patrick: This is the first layer.

Secret code?

Patrick: 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 K20.

Patrick: 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 and 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 K20 doesn't have a very nice finish on it compared to the more modern prototypes. Here's K92, so it's got a much higher polish on it—different manufacturing techniques, absolutely no swirls.

It is a really beautiful-looking specimen.

Patrick: 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?

Patrick: Well, monetarily, the new ones are about $100,000 a piece if you were to buy one.

Oh my goodness!

Patrick: So, but if you think about K20, K4, 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 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-iridium prototypes to stainless steel secondary standards. You can tell that they're a lot bigger than 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.

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 and 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?

Patrick: 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?

Patrick: 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.

Patrick: So you actually, like, while you're on the plane, do you put it in like the overhead?

Patrick: No, it stays just at all times like it's a, you know, the nuclear football codes for setting off a nuclear weapon. And you never have any scares while you're carrying it?

Patrick: 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.

Patrick: Somebody heard the word "iridium" and connected that with radioactivity.

Patrick: No, no! And kind of, you know, flew off the handle a little bit, and I had to calm them down, assure them that it wasn't radioactive, there were 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, alright? But the interesting thing is, if one looks at the data that was recently taken at the end of 2013, the International prototype kilogram did not show an increasing mass.

Patrick: 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?

Patrick: Yes, they went in different directions.

So that some changed a lot, some changed little, some hardly changed at all, but it's hard to tell. I mean, they could have all been changing a lot, but because all you can tell is the difference between them.

They've already marred; 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.

Patrick: 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 gain five grams, you know?

Patrick: I'm having these ridiculous—obviously, it's ridiculous—but the point is, they could have all been gaining or they could all have been losing, right?

It's a relative measurement, and that's the weakness of the system.

Patrick: And that's why, as of 2018, the kilogram will no longer be defined as the mass of a metal cylinder.

Why?

Patrick: If you want to find out how the kilogram will be redefined, be sure to subscribe to Veritasium.