When is a kilogram not a kilogram?

The standard platinum kilogram at the Musee Des Arts And Metiers in Paris. Photo: Matthew Phillips/Flickr

The standard platinum kilogram at the Musee Des Arts And Metiers in Paris. Photo: Matthew Phillips/Flickr

The standard kilogram is not simply a number. It’s an object; a 130-year-old cylinder of platinum and iridium that weighs exactly one kilogram, has 40 replicas, and is stored in a vault just outside Paris.

But in the 1980’s, scientists found some discrepancy between the first standard kilogram and its clones. The original weighted about 50 micrograms less than the replicas, and for a long time researchers couldn’t figure out how to fix the problem.

Researchers believe they’ve found the reason for the difference. The replicas are handled more frequently than the original, and dust, dirt and other contaminants are gathering on the kilograms’ surfaces, adding extra weight. It’s taken 20 years, but scientists finally think they have the proper cleaning regiment to solve the problem. Newcastle metrologist Peter Cumpson helped develop the system, which involves exposing the cylinders to UV rays and ozone, breaking up hydrocarbon particles stuck to the surface. Then the cylinder is rinsed in purified water to get rid of dust. The technique still hasn’t been used on the standard kilograms, but the equipment is being installed to start the process.

But even the new cleaning method isn’t long for this world. Researchers are working on introducing a natural constant to determine a kilogram, either Avogadro’s or Planck’s, and are crafting pure silicon spheres that adhere to those constants to represent the unit of weight. Sounds like a lot of math…we wish them luck with that.

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