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New definition of kilogram by 2018

New definition of kilogram by 2018
Recent studies have shown that the kilogram may need to go on a diet because it has gained tens of micrograms of mass from surface contamination.
LONDON: Scientists are now a step closer to a new kilogram. Of all the standard units currently in use around the world, the kilogram — the official unit of mass in the  International System of Units (SI) — is the only one that still relies on a physical object for its definition.
But revising this outdated definition will require precise vacuum-based measurements that researchers are not yet able to make.
Patrick Abbott of  the National Institute of Standards and Technology (NIST) in Gaithersburg will unveil to the world on October 27, the development of a novel system to allow a direct comparison of an object being weighed in a vacuum to one outside a vacuum.
Using a vacuum ensures that there is no contamination from particles in the air and reduces uncertainty.
So far, researchers are on track to having a new definition of the kilogram by 2018, Abbott says.
Recent studies have shown that the kilogram may need to go on a diet because it has gained tens of micrograms of mass from surface contamination.
As a result, each country now has a slightly different definition of the kilogram, which could throw off scientific experiments that require very precise weight measurements for  international trade in highly restricted items that are restricted by weight, such as radioactive materials.
The original kilogram — known as the International Prototype Kilogram or the IPK — is the standard against which all other measurements of mass are set. Stored in the International Bureau of Weights and Measures in Paris, forty official replicas of the IPK were made in 1884 and distributed around the world in order to standardize mass. Over 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.
No one knows why.  “It’s not really clear if the IPK is getting lighter or the national prototypes are getting heavier,” Abbott said.
Loss of mass due to wear is unlikely because the IPK is hardly ever taken out of its vault.
To address these discrepancies, an international assembly of metrologists — researchers who study the science of measurement — decided in 2007 to wean itself off of the prototype and redefine the kilogram using something more reliable: a constant of nature.
The metrologists eventually chose Planck’s constant, which describes the relationship between the energy of a photon and the frequency of light it emits.
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.
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 one kilogram. The other, called the watt balance, measures the constant by an indirect or virtual comparison of mechanical power to electromagnetic power.
Researchers using the watt balance experiment at national measurement institutes around the world are working to find the materials best-suited to measuring Planck’s constant with this method. Efforts are also underway to find a level of vacuum that is good enough to get results without being too difficult to build or maintain.
“Ours is the only project of its type in the world and we believe that it will be critical in the accurate dissemination of the redefined kilogram,” Abbott said.
While many teams around the world work to improve measurements of Planck’s constant, Abbott’s group is looking beyond redefinition and toward making these measurements practical.
“Whenever redefinition occurs, a robust method will be required to disseminate the kilogram realized in a vacuum to a world that works in air,” Abbott said. His group is creating a system that will bridge the vacuum-air interface using a magnetic suspension technique. The set-up will allow a direct comparison between the mass of a standard kilogram in a vacuum and the mass of a specimen in the  atmosphere of a normal room.
The international system of units (SI) is the most widely used system of measurement for commerce and science. Ideally, the seven base units within the SI (metre, kilogram, second, kelvin, ampere, mole and candela) should be stable over time and universally reproducible, which requires definitions based on fundamental constants of nature.
The kilogram is the only unit still defined by a physical artefact.  Planck’s constant is a fundamental constant of nature which relates the frequency (colour) of a particle of light (a photon) to its energy.
The General Conference on Weights and Measures (CGPM) agrees that the kilogram should be redefined in terms of Planck’s constant.
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