Tuesday, March 01, 2005
Kilogram: Out of fashion?
The Kilogram (as with the meter*), the basic unit of mass in the SI system, equal to 1000 grams or 2.2046 lbs is moving towards a new definition based on a universal constant. The intention of the scientists working on this is to switch the kilogram from being defined by a physical model to a constant. A paper released on Monday proposes redefining the unit via fixing the values of one of two well-known universal constants. Avogadro’s Constant Matter is made of molecules, which are the smallest division that share all the chemical properties of the bulk material. Those molecules are made of atoms that are comprised of protons and neutrons. To a very good approximation, protons and neutrons have the same mass which is one AMU (Atomic Mass Unit). Since atoms and molecules are built from these protons and netrons, their masses are expediently measured in these units. Simply put Avogadro’s number is nothing more than a conversion factor between AMUs and grams. Planck Constant Planck constant on the other hand states the mathematical relationship between the frequency of an electromagnetic wave and the energy in that wave, and is often used to explain the sizes of quanta, which are tiny electromagnetic packets. Planck is represented by the letter h and has a value of 6.63 × 10^−34 J-sec. The combination h/2π, denoted by h (called “h-bar”), occurs frequently. So what’s wrong with our current model? In 1889, a cylinder of a platinum-iridium alloy was declared the international standard of measurement for the kilogram. It's kept at the Bureau International des Poids et Mesures in France, and several copies were distributed around the world. It's unclear if the principal cylinder's mass is increasing or decreasing, scientists said, because it is the object used to measure others. Still, this poses a concern. Another worry is the possibility of the main model's destruction, come a natural disaster So the idea is to have some kind of definition to be able to construct the kilogram just given this information, without an object. Interesting. You can read more about it http://www.iop.org/EJ/journal/Met --------------------------------- * Back in the days when the meter was a pair of lines on a bar kept in a box in Paris, the speed of light was a number that was measured. When it became clear that our distance standard (the physical bar) was limiting the precision with which we could measure the speed of light, we changed the standards to something more precise and repeatable than the use of this physical artifact. The first change was to specify a certain number of wavelengths of a particular transition frequency, but this defintion was found wanting. So eventually the meter was defined in terms of the speed of light, because that allowed for the most precise and repeatable definition of the meter.