Link This |
Email this |
Blog This |
Comments (0)
What's a kilogram, anyway, and what's electronics got to do with it?
October 18, 2005
Metrology got some coverage in the MSM (mainstream media) in the form of an article in the New York Times. A slide show accompanying the article presents views of measurement equipment ranging from a granite 1-cubit ruler (the cubit is thought to be the length from an Egyptian pharaoh's elbow to the end of his middle finger) to the latest electronic kilogram machine at the National Institute of Standards and Technology. You can read the article and view the slide show here; it's free as of this posting although registration is required.
Why an electronic kilogram? As NIST puts it, "The kilogram is the only remaining base unit in the International System of Units (SI) whose definition is based on a physical artifact rather than on fundamental properties of nature." That artifact is a platinum-iridium cylinder in Paris, whose mass was in 1889 declared to be the official kilogram.
The kilogram was originally conceived as the mass of one liter of water, but it proved to be impractical to obtain water of sufficient purity. Now, however, the platinum-iridium cylinder itself is becoming an impractical means of maintaining the kilogram at accuracy levels modern metrologists are pursuing. NIST estimates that factors such as environmental contamination or material loss due to surface cleaning can cause the mass of the kilogram to vary by about 3 parts in 108 per century.
Hence NIST's electronic implementation, which will define kilogram in terms of electrical forces needed to balance gravitational forces, with measurements depending only on length, time, voltage, and resistance.
The Times article reports that the electronic kilogram now achieves 99.999995 percent accuracy and that the five will need to be brought up to an eight to satisfy the international body that sets measurement standards.
Posted by Rick Nelson on October 18, 2005 | Comments (0)
