Scientists want to bring the kilogram into 21st century

Lumps of metal are so last season

By Lucy Sherriff • Get more from this author

Posted in Science, 26th April 2005 12:32 GMT

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The National Institute of Standards and Technology (NIST) in the United States, is calling for the kilo to be redefined in terms of a natural phenomenon, such as the number of atoms in a silicon crystal, or the amount of magnetic force required to levitate an object in a watt balance.

For 115 years, the kilogram has been defined by a lump of platinum-iridium alloy, kept just outside Paris. But scientists argue that relying on an actual physical artifact leaves the kilogram vulnerable to misinterpretation, and means other nations have to travel to France to verify their own standard masses, LiveScience reports.

The kilo is the only international standard measurement still defined by an actual thing. The metre, for example, was originally defined as a ten-millionth of the distance from the north pole to the equator. It is now agreed to be the distance light travels in one 299,792,458th of a second.

Peter Mohr from NIST in the US argues that a physical artifact can change over time - even if it is a very stable substance, like platinum-iridium alloy. Mohr told LiveScience that changes of as much as 50 parts per billion have been seen over a hundred years. This is probably not a serious problem for banana salesmen, but could impact subatomic physicists, for example.

He also pointed out that since 1889, when the Parisian lump was accepted as the international standard, US scientists have had to travel to France three times to verify their own standard masses.

The advantage of the watt balance, or counting atoms, is that any country can recreate them, Mohr argues. The new standards could also be as much as 50 times more precise, and Mohr argues that these factors "tip the scales in favour of the redefinition". Such wit.

But these are not the only arguments in favour of a change. "A meteor could strike Paris - destroying the prototype," Mohr says. "The watt balance can always be recreated."

In the aftermath of a meteor strike, it would indeed be good to know exactly how many kilos of rubble have fallen on your house, for example, or exactly how heavy was the thing that has wiped Paris from the face of the Earth.

Further, the news that the kilo is in such imminent danger of extinction will no doubt be seen as a vindication by Brits reluctant to sell their fruit and veg in metric units. ®