Why the kilogram is overweight

Just like the rest of us, the kilogram has put on a bit of weight – and a little dose of sunshine may be the way to get it back down to size.

Using a Theta-probe XPS machine – the only one of its kind in the world – experts at Newcastle University in the UK have shown that the original kilogram is probably tens of micrograms heavier than it was when the first standard was set in 1875.

“Statute decrees the IPK is the kilogram,” says Professor Peter Cumpson. “It doesn’t really matter what it weighs as long as we are all working to the same exact standard – the problem is there are slight differences. Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.”

The International Prototype Kilogram, or IPK, is the standard against which all other measurements of mass are set. Forty official replicas were made in 1884 and distributed around the world in order to standardise mass.

Since then, contaminants have built up on the surface, adding to the IPK’s mass by a few dozen micrograms – a comparatively tiny amount.

“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,” says Cumpson. “There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for.”

The team’s tests show that a little wash with ultraviolet light and ozone can remove contamination without damaging the platinum surface.

“What we have done at Newcastle is effectively give these surfaces a suntan,” says Cumpson. “By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight.”

The kilogram is the only one of the seven SI base units from which all other units can be derived to be measured against a physical object. All others are standardised against known constants. Work is underway internationally in several National Measurement Institutes to find an alternative.