Scientists have re-defined the kilogram and France’s ‘Le Grand K’ retires

Sorry to say, you still weigh the same number of kilos now as you did before

Today's expression: Seal someone’s fate
Explore more: Lesson #106
November 26, 2018:

For over a hundred years, the official definition of a kilogram was a metal cylinder held under three locks in France. But scientists have voted to change the kilo's official definition to a mathematical equation based on unchanging laws of physics. For this reason, the prototype kilogram, called "Le Grand K" will no longer be the official definition of the kilogram. Plus, learn the English phrase "seal someone's fate."

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The world has a new definition for what, exactly, precisely, is a kilogram

Scientists at the International Bureau of Weights and Measures have approved a new definition for the kilogram—but, sorry to say, you still weigh the same number of kilos now as you did before.

Hey everyone, it’s Jeff, JR is the producer in Dallas, and you are listening to Plain English, the best podcast for learning English through current events. Today is Episode number 106, so that means you can find an interactive transcript of the episode online at PlainEnglish.com/106. And that’s especially helpful if you speak Portuguese, Spanish, French, Italian, Chinese, or Japanese because we have instant translations from English to those languages. About a hundred words an episode are defined for you, so you can see the definitions of the hardest words without ever having to press pause. It’s pretty cool; PlainEnglish.com/106.

And of course we have a new partner that I told you about in a special episode last week, MosaLingua. They have tons of courses in English, including some on the TOEFL, business English, English pronunciation, all kinds of things. You can explore all their resources at PlainEnglish.com/learn – that will take you right to MosaLingua’s web site and you can explore all they have to offer. PlainEnglish.com/learn.


New definitions of a kilogram and three other measurements

The General Conference on Weights and Measures, held in Versaille, France, is generally not a headline-grabbing meeting. But last Friday, the International Bureau of Weights and Measures voted to re-define a kilogram, and three other official measurements. That vote sealed the fate of “Le Grand K,” which had been the official measure of the kilogram from 1889 up until now. Le Grand K is a platinum-alloy cylinder held under three bell jars, locked with three keys, only two of which are in France, in an environmentally-controlled basement in the outskirts of Paris. If that sounds like a lot of security, that’s because this one cylinder—it looks like a cookie jar—this one cylinder was the world’s standard, official definition of a kilo. There are several replicas around the world, and once every 40 years, the prototype kilo was taken out to compare it to the other copies.

But Le Grand K can officially go into retirement because the world has a new, much more precise definition of a kilogram. This was necessary because any physical object will change over time; it’s just a fact of the universe. The variation in mass of the prototype kilogram was just about the weight of an eyelash—about 50 parts per billion. But as the world gets more and more advanced, it needs a more precise and constant definition than a physical mass can provide. That is why scientists are now defining all the fundamental measures—not just the kilogram—in terms of the unchanging physical world.

You know, I always thought we had a consistent, scientifically-based definition of a kilogram. To an extent, we did. It was supposed to be one one-thousandth of the mass of a cubic meter of pure water. That’s how the mass of a kilogram was defined. But I guess the only way to have a single unchanging definition was to have that in solid form, which was Le Grand K.

Now here is where I am reaching the limits of my comprehension of an issue. I can talk all day about Brexit—I practically did talk all day on the last episode—but I don’t really understand all that much about how a kilo is now defined. But I’ll give it a shot.

There is a special kind of a scale called a Kibble Balance. On one side of this scale is a weight, like a kilogram or something, and on the other side is an electromagnet. The electromagnet pushes down on its side as you increase the electrical current running through the magnet. So the new definition of a kilogram is a specific amount of electrical current running through the magnet on a Kibble Balance.

This is interesting now because anywhere in the world, if you want the exact, precise measurement of a kilogram, you only need very accurate equipment. You do not need access to any individual thing. The definition of a kilo is not a physical thing, but really a mathematical equation. And any two people with accurate enough equipment can precisely measure the world’s official definition of a kilogram—no help from Paris necessary.

The International Bureau of Weights and Measures also redefined the ampere, which measures of electrical current, the kelvin, the official scientific definition of temperature, and the mole, which is how they measure the amount of a chemical substance. I am not going to even attempt to explain the new definitions of a mole or a kelvin; I barely know what they are conceptually, much less how they are now scientifically defined.

But here is what I do (kind of) understand. All the official measures in the metric system, including the meter and the second, are now defined by unchanging constants of the universe. When traders first started using measurements in ancient times, they were based on the human body or common relationships in nature. The foot, the pound, the yard, the inch, the carat, the league—these were all things based on common everyday experiences. They varied from place to place, but over time became somewhat standardized. Then the metric system emerged in the late 1800s as the world’s standard system of measurement. Though it was much more scientific, it was still based on earthly measurements. The meter was one forty-thousandth of the Earth’s circumference; the second was based on a fraction of the Earth’s rotation about its axis.

But now these are all based on things like the speed of light in a vacuum, how an electron moves, things that are the same anywhere in the universe. This was a long time coming, I think, for the kilogram. The meter followed the same evolution. It was once defined by a prototypical rod, held in the same place, made of the same compound as the kilogram. But the meter was actually re-defined way back in 1983.


If any of you brave souls want to take a shot at explaining Plank’s constant or the Boltzmann constant, send me a note [email protected]. But I’ll warn you—the only ‘C’ I ever got in high school was in physics.

I did hear from a number of you about another topic I struggle to understand, a topic just slightly less complicated than quantum physics—that would be European football. Vinicius from Curitiba, Brazil, thinks the best football teams’ own fans won’t let them leave their national leagues, after all. Gabriel from Sao Paulo has an interesting idea—he thinks the champions of the national leagues should be able to play the lower-ranked teams from the Super League and be able to earn their way into the Super League that way. That might just accomplish the unthinkable, which is to make it even more complicated than it is today!

I also want to say hi and thank you to Bruno from Minneapolis—originally from Brazil but now living up north in Minnesota—and Júlia from Portugal. They both left nice comments for us on Facebook. You can find us on Facebook and Twitter with the user name PlainEnglishPod. You can also stay in touch with us via our e-mail program. JR and I send out an email summary of each episode with links to English articles about the main topic and an extra English vocabulary word for you. To get that, just visit PlainEnglish.com/mail.

I mentioned earlier about MosaLingua, our new partner. They’ve got tons of content for English learners and I really encourage you to go explore all they have to offer. If you’re looking for an affordable and effective way to learn new words, practice your pronunciation—so many people ask me about that—or, hey, just have fun while absorbing some English, go to PlainEnglish.com/learn and explore what MosaLingua has on their web site. They’ve got a great course on English pronunciation, for example, called MosaSpeak. There’s a lot there, and I’ve been using their Spanish resources, too. Actually, that’s a good point—so many of you are on your third or even fourth language, MosaLingua has you covered, too, if you want to learn or practice yet another language. So check out PlainEnglish.com/learn and see how much you can learn today.

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Expression: Seal someone’s fate