International prototype of the kilogram

Scientists vote on new way to measure a kilogram


Everything from kitchen scales to gym weights around the world have been manufactured to the standard set by the cylinder of platinum iridium, which has been kept in a high-security vault in the French capital since 1889.

Different countries have their own “prototype kilograms” that serve as national standards and are calibrated to the Paris artifact. But now, scientists want to update how the weight is defined.

The reason for a proposed change to the International System of Measurement (SI) units, which the international community will vote upon on Friday, is that over time the prototype has lost atoms and therefore mass because it is “susceptible to damage and environmental factors,” according to the National Physical Laboratory (NPL), which houses Kilo 18, Britain’s copy of Le Grand K — or the International Prototype Kilogram, (IPK) as it is officially known.

Le Grand K is compared to the various copies only once every 40 years, which makes the calibration potentially inaccurate. Though the change in mass is roughly equivalent to the weight of an eyelash, there could be wide repercussions.

“This is fine when it comes to measuring a bag of sugar, but is becoming unacceptable for more sophisticated science, such as when measuring doses in pharmaceuticals,” a statement from the NPL said.

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Friday’s vote at the General Conference on Weights and Measures in Versailles — which is widely expected to be approved — is set to permanently redefine the kilogram and send the IPK into retirement.

The new definition being proposed is based on the Planck constant — a constant observed in the natural world, which is inherently stable, according to the NPL.

Although the value of the kilogram will not change, the redefinition of the kilogram using a constant will ensure it remains reliable, and enable far more accurate mass measurements in the future.

The Planck constant describes the behavior of particles and waves on the atomic scale and depends on three units: the meter, kilogram and second. As the second and meter are measured and defined by the speed of light, they can be used with the fixed Planck constant to define a kilogram.

The Planck constant is in turn measured using an instrument known as the Kibble balance, first developed at NPL by the late physicist Bryan Kibble.

Scientist preparing  the  BIPM Kibble  balance for a  
measurement.

“The redefinition of the kilogram is a tremendous leap for the international measurement community and science as a whole,” said NPL fellow Ian Robinson, who worked on the device’s development with Kibble.

“By using a universal constant of nature to define the kilogram we have enabled the whole world to contribute to the topmost level of mass measurement and, in addition, paved the way for future innovations. Much like upgrading a building’s foundations, we’re building a stable base for future science and industry.”

If approved, the redefinition will come into effect on World Metrology Day on May 20. The conference is hosted by the International Bureau of Weights and Measures.

CNN’s Meera Senthilingam contributed to this report.



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