By Daily Mail Reporter
Last updated at 4:06 AM on 24th January 2011

It is news to gladden the heart of any metric martyr.

Scientists have discovered that the kilogram is struggling with a weight problem.

Measurements taken over the past 100 years show that the international prototype – the metal object on which the scientific unit of mass is based – has got lighter.

Weight off: The cylinder of platinum-iridium alloy may have shed around 50 micrograms, the equivalent to the mass of a small grain of sand

Weight off: The cylinder of platinum-iridium alloy may have shed around 50 micrograms, the equivalent to the mass of a small grain of sand

For reasons that remain a mystery, the cylinder of platinum-iridium alloy, which is just under four centimetres tall and wide, may have shed around 50 micrograms, the equivalent to the mass of a small grain of sand.

Scientists are now looking for a way to define the unit without referring to a physical object.

If they are successful, the 'gold standard' kilogram, which since 1889 has been triple-locked under two bell jars at the International Bureau of Weights and Measures in Paris, will follow the prototype metre into history.

Dr Michael Stock, who works at the bureau, presented the latest research on the matter to the Royal Society in London yesterday.

The experiments aim to define the kilogram based on a link between mass and the Planck constant – the fundamental unit of measurement in quantum physics.

Dr Stock said there were 'reasonable chances' that the new definition will be introduced in 2015, when the General Conference on Weights and Measures is due to meet in Paris.

He said: 'The international prototype has fulfilled its role quite well during the last century.

'However, measurements get more and more precise, and precise measurements require well- defined measurement units to express their results.

'Another disadvantage is that the international prototype exists only at one place, in Paris.

'A material artefact could also be damaged, with obvious negative consequences.'

Brussels directives state that the kilogram should be the basis for all sales by weight in the EU.

Vendors have been prosecuted for selling goods priced only in imperial units, earning them the nickname 'metric martyrs'.