When is a kilogram not a kilogram? When it starts to weigh less. It came into existence more than two centuries ago and has become the standard unit of weight around the world, from the shopping malls of Europe to the souks of the Middle East, but scientists believe that the reign of the kilo as we know it is about to come to an end.
A group of experts meeting in London today want to redefine the kilogram so that it is no longer based on the mass of a solid cylinder of platinum-iridium alloy that sits beneath three layers of protective glass sealed in a locked vault in Sèvres, France.
This metal block, known as the International Prototype Kilogram, has been used since it was first registered with the International Bureau of Weights and Measures (BIPM) in 1889 as the definitive unit of mass against which all other kilograms are measured.
In the past 122 years, it has been brought out of storage just three times to calibrate the national prototype kilograms used by countries around the world. However, scientists now believe it is time to redefine the kilogram because there is evidence that the precise mass of the international prototype in Sèvres is not as constant as it should be.
"We think it is losing weight, and we don't know why," says the BIPM's Michael Stock, who is due to attend the meeting at the Royal Society in London that will look again at the kilogram. "From the three times we have had it out to make calibrations, we have had indications that it is not perfectly stable. It seems to have lost about 50 micrograms and there is no real explanation."
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