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 responses »

  1. coolkidandy says:

    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

  2. Scott Levine says:

    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

  3. @breakerofthings says:

    Possible debate point here – it’s not losing weight, it’s gaining weight!. The problem is the variation in gain across the various naitional standards vs Le Grand K…

    Liked by 1 person

    • J.S. Pailly says:

      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

      • @breakerofthings says:

        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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