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In a vault in suburban Paris sits a little lump of metal that is the official kilo, the sanctioned yardstick, the 124-year-old standard from which numerous other measurements are derived.

The problem is, it's getting heavier.

Even in the carefully controlled and protected environment, contamination has built up on the surface of the object. The increase in mass is tiny – measured in micrograms, which are millionths of a gram – but the effect on science is real.

Unfortunately, the layman's solution of simply washing the cylindrical item isn't so easily done. So a pair of United Kingdom scientists has come up with an elaborate 30-hour procedure, a combination of rinsing, ultraviolet light and heating that promises to return the kilo to its true mass.

Technically speaking, this problem can't exist. The mass of the "international prototype of the kilogram" is, by definition, the accepted standard. But comparing the original in Paris with copies made in the late 19th century and sent around the world show that their relative masses have been changing.

"Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original," research lead Peter Cumpson, Newcastle University Professor of MicroElectroMechanical Systems (MEMS), said in a statement.

The IPK was created in 1879 and formally ratified as the official kilogram a decade later. The cylinder has a diameter of 39.17 millimetres and is as tall as it is wide. It is an alloy of 90 per cent platinum and 10 per cent iridium. It sits in protective custody in Sèvres and is handled as little as possible.

The relevance of the IPK cascades through science. For example, a newton is defined as the force required to accelerate a kilogram at a rate of one metre per second squared. If the IPK changes, so must the newton. And the newton is the basis for the pascal, the ampere and the joule, which is used in turn to determine the watt.

"We're only talking about a very small change … but mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge," Mr. Cumpson said. "There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for."

In a recently published paper available online to those with an account, Dr. Cumpson and co-author Naoko Sano explain the downsides of previous cleaning methods. Scrubbing the item introduces a human element that would be hard to reproduce exactly when cleaning the replicas, possibly resulting in prototypes that are cleaner but still differ in mass. And they seem to pick up dirt faster after being cleaned.

Any work also must be done very delicately, removing the surface contamination but not even the thinnest layer of the material itself. The paper notes that ultraviolet/ozone treatment is used to clean semi-conductors, where it is "quite acceptable" to achieve a pristine surface at the cost of losing a little material.

"This would clearly be disastrous in the treatment of platinum-iridium or other noble metal mass standards, where a single layer of atoms on the surface typically represents a mass of 30 [micrograms], and must not be sacrificed," the authors write, noting that 30 micrograms is "at least ten times the accuracy with which mass standards can be compared."

This may not be a problem much longer. The IPK is the last unit to be defined by a physical object and scientists voted in 2010 to change to a definition based on fundamental constants of nature. The change, which has not be fully fleshed out, is expected to come later this decade.

Editor's note: An earlier version of this article incorrectly stated that the kilo is increasing in weight. In fact, the kilo has increased in mass. This online version has been corrected.

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