The Roundest Object in the World.

The Avogadro Project

In determining the Avogadro constant, the preferred method has been to use one of the high-precision spheres fabricated here at the ACPO. These come in the form of a highly polished 1 kg single crystal silicon sphere, fabricated with a roundness in range of 60 nm.

The nominal diameter of a 1 kg Si sphere is 93.6 mm. In order to obtain an accuracy of 0.01 ppm in volume, the diameter must be known to a range of 0.6 nm. In other words, within one atom spacing.

Such high accuracy requires specialised equipment and one such procedure is by optical interferometry using a precision etalon through a stabilised laser light. The measurements are sensitive to many parameters, particularly to those of temperature and pressure. An instability within the range of 2 mK would be sufficient to cause the silicon to expand by more than the allowable uncertainty. The refractive index of air (and hence the wavelength of the light) is sensitive to the surrounding air pressure. It is therefore necessary to carry out the measurements in a controlled environment.

Corrections must be applied for surface impurities such as oxides and absorbed water. Typically, silicon has an oxide layer 3 to 4 nm thick, which is a mixture of SiO and SiO2. It is also possible for the surface to absorb some monolayers of water. Since much of the absorbed water is removed in a vacuum, a number of the key measurements are made in a vacuum environment. A further correction must then be applied for the difference in bulk modulus between the air and vacuum.


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8 Responses:

  1. J. Peterson says:

    The contact is Dr. Bob Oreb (the orb Dr.)

  2. MattF says:

    Sunday's NYT has an article on the standard kilogram:

    • From that article:

      It is here that the kilogram - the universal standard against which all other kilograms are measured - resides in controlled conditions set out in 1889, in an underground vault that can be opened only with three different keys possessed by three different people.

      I take it Capcom designed the vault? Once you get the cylinder, is there a glass cabinet somewhere else which will only open when an object weighing precisely one kilogram is placed on the adjacent pedestal?

      (as for the original article: the only thing missing from making this the most SCIENCE story ever is a couple of Tesla coils. Avogadro's number is a kick-ass universal physical constant.)

  3. jope says:

    Phantasm V: The Kill-o-gram

  4. andrew hutchison says:

    The take-home point of all this is the unsuitability of the artifact kilogram. Despite the (admittedly) compelling photograph, pretty much everyone I know in the field disfavors atom-counting techniques like the one described as opposed to the electronic kilogram in resolving that problem. wikipedia treatment. nerd treatment.

    • says:

      That seems much neater. I think. If I understand it correctly, the trick is that macroscopic currents (1 A is 6.2e18 electrons per second, and that's too many to count) have microscopic effects on microscopic objects-we can fix the ampere by measuring the effect on a test charge, which might as well be microscopic, and won't have to count anything to get to the kilogram.

      And the alternative is to try really hard to count to 1e23 without losing your place.

  5. SpaceHobo says:


  6. vvill says:

    I wonder how much Tigger would get fined if he spate on one of those...

    I could use a pair like that...