Tag Archives: international prototype kilogram

In This Blog: Wikipedia Gets Sassy about the Kilogram

I’ve talked about the kilogram quite a bit on here, but I want to revisit it a bit. Mainly because of Wikipedia’s sassy little remark about the International Prototype Kilogram (IPK).

The other six base SI units (second, meter, ampere, Kelvin, mole, and candela), which used to be based on physical artifacts, are now defined in terms of physical constants for precision’s sake. For example, the meter was originally defined as 1/10,000,000 of the meridian through Paris between the North Pole and the Equator. It is currently defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second.

Only the kilogram remains to be redefined in this new, more precise way.

The IPK itself is a small little cylinder of platinum/iridium and is stored in Sèvres, France. It is the internationally recognized artifact that is defined as having a mass of exactly one kilogram. So it is, in essence, the kilogram.


But of course, there are copies of it. Sèvres holds three; other nations have national prototypes. And that’s where things get interesting.

Wiki: “By definition, the error in the measured value of the IPK’s mass is exactly zero; the IPK is the kilogram. However, any changes in the IPK’s mass over time can be deduced by comparing its mass to that of its official copies stored throughout the world, a process called ‘periodic verification.'”

“Beyond the simple wear that check standards can experience, the mass of even the carefully stored national prototypes can drift relative to the IPK for a variety of reasons, some known and some unknown.”

And what does this drift look like? the IPK itself is the line at zero denoted with the K. The other lines represent the mass changes in various national prototype kilograms.


Well, hell. And the IPK itself, Wiki notes, may be changing as well (and likely is), though it is still the “base” against which every other national prototype is compared.

Yup, humans have been to the moon but can’t figure out how to keep the kilogram constant. I find that hilarious.

But here’s where I lost it when reading this article:

“The magnitude of many of the units comprising the SI system of measurement, including most of those used in the measurement of electricity and light, are highly dependent upon the stability of a 134-year-old, golf ball-size cylinder of metal stored in a vault in France.”

Ooooh, Wiki…a bit of attitude, eh?

My mom can attest to the fact that I laughed about this for like 10 minutes straight.

(all pics from Wiki’s Kilogram page)