Washington: Researchers have developed a novel system to allow a direct comparison of an object being weighed in a vacuum to one outside a vacuum.
The official kilogram, called the International Prototype Kilogram (IPK), is stored in a bureau just outside of Paris, France, and has served as the standard to which all the nations` prototypical kilograms have been compared for the last 125 years.
Every few decades, the national prototypes are carried, usually by hand, to France, where they are measured against the IPK.
But discrepancies between the national prototypes and the official specimen have been increasing at a rate of 0.050 milligrams (mg) every 100 years. And no one knows why.
Patrick Abbott of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md, said that it`s not really clear if the IPK is getting lighter or the national prototypes are getting heavier.
Loss of mass due to wear is unlikely because the IPK is hardly ever taken out of its vault.
The metrologists eventually chose Planck`s constant, which describes the relationship between the energy of a photon and the frequency of light it emits.
Its value has been measured with a relative uncertainty of between 30x10-9 and 35x10-9. However, to assure agreement between the current IPK system and the Planck-defined kilogram, researchers will need to improve their measurements to a relative uncertainty of 20x10-9. And to get better measurements, they will need the ability to perform state-of-the-art metrology in a vacuum.
Currently, researchers use two types of experiment to measure Planck`s constant, and both require vacuums. One method involves determining the number of atoms in a high-purity silicon sphere with a nominal mass of 1 kilogram. The other, called the watt balance, measures the constant by an indirect or "virtual" comparison of mechanical power to electromagnetic power. Using a vacuum ensures that there is no contamination from particles in the air and reduces uncertainty in some of the measurements that are conducted with laser interferometry.