Mass Confusion

To do physical science, one needs uniform references for fundamental quantities: length, duration, mass, and so forth. Over time, as the need for accuracy has increased, attempts have been made to place the fundamental units on ever more precise footing. For example, the reference meter, which was declared in 1791 by the French Academy of Sciences to be one ten-millionth of the distance between the North Pole and the Equator along a meridian running through Paris, is now taken to be “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.”

The kilogram, however, has to this day been referred to the mass of the International Prototype Kilogram, a platinum-iridium cylinder that resides in a chateau in Sevres, France. It is the only SI unit that is still defined by reference to an artifact.

In a puzzling turn of events, however, it appears that this master kilogram — the kilogram — is, for some inexplicable reason, losing mass.

Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.

“The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart,” he said.

“We don’t really have a good hypothesis for it,” Davis said in a phone interview Wednesday.

While 50 micrograms isn’t much — it certainly won’t get you far along your way to that Life Membership at Weight Watchers — in the rarefied world of international physical standards, it’s a pretty big deal, especially in light of the fact that nobody knows why it’s happening.

Learn more here.

Related content from Sphere
  1. By the way, the measurement of this distance by a seven-year surveying expedition undertaken by Jean Baptiste Joseph Delambre and Pierre François André Méchain is a fascinating story, and is told in the recent book The Measure of All Things, by Ken Alder.


  1. Charles says

    Fascinating stuff. But I have a stupid question.

    If the meter is defined as “one ten-millionth of the distance between the North Pole and the Equator along a meridian running through Paris,” how did they measure this original distance? You obviously can’t use anything metric because you can’t use a system of measurement to define a distance that you are using to define that system. Did they measure it using the Imperial system and then convert it?

    Oh, and hi, by the way. Don’t know if I’ve commented here before, but I found your site through Kevin and have been visiting for a while now.

    Posted September 13, 2007 at 10:09 pm | Permalink
  2. Malcolm says

    Hi Charles, and welcome. I’ve been visiting your site for some time as well, having found it also through Kevin. Your visit reminds me to add a link. And you ask an excellent question.

    For the survey to be of any use, an accurate baseline was required. For that purpose, the Chevalier de Borda, the maker of the exquisitely accurate “repeating circle” instruments used by Mechain and Delambre for the survey of the meridian, provided a special ruler, which Delambre used for a painstaking measurement of a baseline near Paris. The unit in use at the time was the toise, which was about double the length of what was to become the meter.

    Delambre joined Laplace at Melun to fix the ends of the baseline near Paris. Borda had devised special rulers for the purpose, which were not ready until the spring of 1798. Their employment was extremely tedious. Delambre could make only about ninety lengths (about 360 m) a day; it took thirty-three days to cover the entire distance of 6075.90 toise.

    [from The Quantifying Spirit in the 18th Century, section 233]

    This really is a fascinating tale. The two men were of very different temperaments, and laying out an interlocking set of triangles from Dunquerque to Barcelona was an arduous and adventurous undertaking, which they managed with astonishing precision. I enjoyed Alder’s book very much, and recommend it highly, although if your curiosity requires immediate satisfaction I’m sure you could find much of the story online.

    Posted September 14, 2007 at 12:36 am | Permalink
  3. Well, I hesitate to say this — and nobody will believe me — but about those 50 missing micrograms…

    I took them.

    I broke into that chateau in Sevres, France and shaved off a bit of that platinum-iridium cylindrical kilogram with a very sharp and tough razor. Who knew it would be precisely 50 micrograms, eh?

    I’m sorry to have caused such alarm, but the scientific community can relax, for there’s now a rational explanation.

    Also, I hid all of Saddam Hussein’s WMDs, which explains why no one ever found any.

    I’m also responsible for some other mysteries, but I’ll leave those for some other time…

    Jeffery Hodges

    * * *

    Posted September 14, 2007 at 3:53 am | Permalink
  4. Charles says


    Thanks for the explanation. Laying down interlocking triangles? That must have been some journey. I love books about stuff like this, so I will have to add this one to my wishlist.

    And, as I always do when someone tells me these days that they have been visiting my site, I feel I should apologize for the lack of new content these days. I’ve been going through a bit of a dry spell.

    Posted September 14, 2007 at 7:35 am | Permalink
  5. Malcolm says


    Thanks for clearing that up. That car pulling up outside will be the Sûreté, by the way.

    I took 50 micrograms myself once; spent the next six weeks as a potted plant in the lobby of the Waldorf.

    Posted September 14, 2007 at 11:44 am | Permalink
  6. Malcolm says


    I know about those dry spells. I fill them in with drivel. It’s to your credit that when you feel you have nothing to say, you don’t say it.

    Posted September 14, 2007 at 11:47 am | Permalink
  7. Potted plant, eh? Nice pun. Did you catch any offenders?

    Jeffery Hodges

    * * *

    Posted September 14, 2007 at 7:12 pm | Permalink