A kilogram sample at Sandia’s Primary Standards Laboratory in Albuquerque, NM. Standard kilograms are polished cylinders made of 90 percent platinum, ten percent iridium, and stand 1.5 inches tall and 1.5 inches wide.
Scientists may be nearing the end of a 15-year global effort to reinvent the kilogram, the only unit in the International System of Units that is still based on a physical weight. The original kilogram, a hunk of platinum-iridium alloy about the size of a golf ball, sits in a vault in Paris. Forty replicas are used around the world for precise measurement. However, the platinum-iridium kilograms have begun to vary in weight as they collect dust from the atmosphere, and regular steam cleanings aren't enough to keep them all consistent. It's estimated that the kilograms are seeing an average "relative drift" from the international prototype of .5 micrograms a year.
That's why the Swiss Federal Office of Metrology (METAS) issued a call for a new method, one that doesn't rely on a physical object. Teams of researchers are developing agnostic ways to measure the unit, including counting atoms and measuring against gravity. Today, one such team announced a breakthrough that they believe will allow them to develop a new standard for the kilogram by the 2015 deadline.
2015 is the deadline for the new kilo
Researchers at Mettler Toledo, a global maker of precision measuring instruments, have developed a tiny, ultraprecise weighing cell that allows for measurements accurate within .1 micrograms, significantly more precise than the 1 microgram requested by METAS. Mettler Toledo is collaborating with Federal Institute of Technology in Lausanne (EPFL) and the European Organization for Nuclear Research (CERN) to develop the so-called "watt balance," an experimental scale that will allow the kilogram to be measured electronically according to the voltage required to lift the prototype in an electromagnetic field. The new weighing cell is a critical component of the watt balance.
There is no guarantee that Mettler Toledo's method will be adopted by the International Bureau of Weights and Measures, as this team isn't even the only one working on the watt balance method. However, the new weighing cell may be the edge they need to be the next arbiters of the kilogram.