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Updated: January 24, 2011 23:58 IST

Kilogramme set for the scrap heap

Alok Jha
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An undated picture of the prototype of a kilogramme.
AFP/BIPM
An undated picture of the prototype of a kilogramme.

Scientists look at alternatives to the mass of platinum used as international standard measure, which has lost 50 micrograms.

For more than a century, all measurements of weight have been defined in relation to a lump of metal sitting in Paris. The “international prototype” kilogramme has been at the heart of trade and scientific experiment since 1889, but now experts want to get rid of it.

On Monday, scientists will meet at the Royal Society in London to discuss how to bring the kilogramme into the 21st century, by defining this basic unit of measurement in terms of the fundamental constants of nature, rather than a physical object.

“The kilogramme is still defined as the mass of a piece of platinum which, when I was director of the International Bureau of Weights and Measures, I had in a safe in my lab,” said Terry Quinn, an organiser of Monday's meeting. “It's a cylinder of platinum-iridium about 39mm high, 39mm in diameter, cast by Johnson Matthey in Hatton Garden in 1879.”

One problem with using a lump of metal to define such a basic quantity as the kilogramme is that it is liable to change over time. Measurements over the past century have shown that the international prototype has lost around 50 micrograms, around the weight of a grain of sand.

Instead, experts want to link the kilogramme to a fundamental unit of measurement in quantum physics, the Planck constant.

This redefinition would bring the kilogramme into line with the six other base units that make up the International System of Units (SI) — the metre, the second, the ampere, the kelvin, the mole and the candela.

None of these are now based on a physical reference object. — © Guardian Newspapers Limited, 2011

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