« This Day In History: June 18 | Main | This Day In History: June 19 »

The Perfect Sphere

prototype.jpgTomorrow, June 19, a representative of the International Bureau of Weights and Measures (BIPM) will deliver a big cylinder of silicon to a group of Australian scientists. This might not seem like the most thrilling event of the week, but over the next three months those scientists will whittle (so to speak) that cylinder into a perfect sphere. And that perfect sphere will be used to "improve" the definition of the kilogram.

The kilogram is the only standard unit of measure that is still defined by a physical object—the International Prototype (right), a lump of platinum-iridium metal that has been stored in a BIPM vault since 1889. The meter used to be similarly defined by a platinum-iridium bar, but is now defined as the length of the path travelled by light in vacuum during 1/299,792,458 of a second.

Just like the existing International Prototype, the new silicon sphere will be used to calibrate real-world equipment. But it will also serve another purpose: silicon_sphere.jpg

While a physical object will still be necessary for calibrating scales and balances, the silicon atoms in the sphere will always remain the same. It is for this reason that the scientists working on what’s known as the Avogadro Project are collaborating to determine what is effectively the number of atoms in a sphere. Once the number of atoms is known, the definition of the kilogram can be based on it from then on.

More news below, plus some historical background on standard measurements and their real-world reference points.

Australia Weighs In To Make The Perfect Kilogram (ScienceDaily.com)
Definitions and Historical Context of SI Base Units (National Institute of Standards & Technology)

Photos: BIPM (top), CSIRO Industrial Physics (bottom)


TrackBack URL for this entry:

Post a comment

(If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)


This page contains a single entry from the blog posted on June 18, 2007 4:00 PM.

The previous post in this blog was This Day In History: June 18.

The next post in this blog is This Day In History: June 19.

Many more can be found on the main index page or by looking through the archives.

Creative Commons License
This weblog is licensed under a Creative Commons License.