Sciency Words: Kilogram (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, K is for:


We’ve already met the International Astronomy Union and the International Commission on Stratigraphy. There are lots of international science organizations like these, and a big part of their job is to set official definitions for scientific terms, so that the use of these terms doesn’t cause confusion in scientific discourse.

Today we’ll get to know the International Bureau of Weights and Measures, which is in charge of defining all the units of measurement for the metric system. Originally, all the metric system units were based on physical prototypes. So for example, there was a prototype meter stick. A meter was equal to however long that meter stick was, and all other meter sticks had to be cut to match the prototype.

And if something happened to the prototype meter stick, if it got shorter or longer somehow, then by definition the meter would get shorter or longer too. As you can imagine, this caused problems.

Over the years, the International Bureau of Weights and Measures has been redefining all the metric system units using universal constants like the speed of light or other fixed values like the triple point of water. They’ve been able to do this for every unit except one: the kilogram.

The kilogram is still based on a protoype: a cylinder of platinum/iridium alloy made in the late 1880’s.

Actually, most people call it Le Grand K because it’s located in France. On very, very rare occasions, Le Grand K is taken out of its high security vault and compared to other weights, which are then used to calibrate measuring instruments all around the world.

Unfortunately, it seems Le Grand K has lost a little weight. A very, very little amount of weight. Its total mass appears to have decreased by 0.05 milligrams. You’d need to be doing some extremely precise measurements before the change in Le Grand K’s mass would matter, but of course there are scientists and engineers out there who are doing those kinds of extremely precise measurements. Or at least they’re trying to.

But a fix for the kilogram may be on its way, using Planck’s constant and Einstein’s famous E = mc2 equation. Assuming the math checks out, the International Bureau of Weights and Measures might be able to retire Le Grand K by the end of 2018.

Next time on Sciency Words: A to Z, let’s get ready to librate!

18 thoughts on “Sciency Words: Kilogram (An A to Z Challenge Post)

  1. Aha, we were taught that a kilogram was based on something but didn’t remember that until now. And if Le Grand K lost a meager 0.05 kilogram, then I weigh less now! LOL. I hope my weight remains the same in 2018 when they figure out how light/heavy a kilogram is 😛

    Liked by 1 person

      1. Especially for those body conscious and health freaks! Hahaha I’ll let them know that, “Hey, the kilogram lost weight!”

        Liked by 1 person

  2. I saw an interesting video about the kilogram (seems like such an odd thing to say) a few months ago. I’ll post a link if I can find it again. There’s a lot going on behind the scenes with measures like these. Good stuff. Good post.

    Liked by 1 person

    1. Oh, might it be the video by Veritasium? I love that channel, and their piece on the kilogram was awesome! (Yeah, that does feel like a weird thing to say, but it was pretty awesome.)

      That was the first time I heard about this problem, and I immediately thought if I’m doing A to Z this year, that has to be my pick for K.

      Liked by 1 person

      1. Yes! Well… Maybe! I think it was either Veritasium (I agree; he’s great at what he does) or Tom Scott. I *think*. I know you’ll be waiting on the very edge of your chair until I post, but I’ll track it down. This is what’s great about the English/US system. It’s all arbitrary, so we don’t need to worry about such trifling things as accuracy! Huzzah!

        Liked by 1 person

      2. I guess we Americans can be a little smug that we don’t have these metric system problems… except apparently our units of measure are defined in relation to the metric system. So… someone please fix the kilogram so our pounds don’t get lighter too.

        Liked by 1 person

      3. Wow… huh. I had no idea. Thanks. I might never have learned that if not for you and your post. Thanks. I like that, though, it makes it sort of semi-arbitrary. It’s arbitrary, but scientifically fixed.

        Liked by 1 person

    1. Fascinating. So everything I’d read leading up to this post said Le Grand K had lost weight, but I’d sort of been wondering how we could even know. What do you compare Le Grand K to to determine how much it’s changed? So it makes sense to me that there’d be disagreement about whether Le Grand K’s mass had increased or decreased.

      Liked by 1 person

      1. I think there have been a number of theories about what might cause weight loss, but I think that variation in weight gain is a lot more plausible. It is interesting that kg remains as the last SI unit that is defined by a physical object.

        Liked by 1 person

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