Instant Guide

Fixing the kilogram's weight problem

Forty platinum cylinders are used to standardize measurement around the world. But they've grown heavier over time

Tucked deep in a meticulously monitored, pressure-controlled vault in Paris is a metal cylinder of profound importance to the world: The international standard for the kilogram. Created in London in 1884, the cylinder, weighing 2.2 pounds, is the very definition of a kilogram. Some 40 identical replicas of the glittering ur-kilo — or at least as close to identical as humanly possible — were forged out of platinum and shipped to different corners of the globe to standardize measurements of mass. 

Naturally, maintaining an incredibly precise weight for a three-dimensional object is an arduous affair, especially when it's at the mercy of dust and other unseen contaminants. Although gatekeepers store the kilo's twins in laboratories where filtered air is kept at a constant temperature and pressure, debris and other particles still manage to accumulate. Thus, they've packed on some weight

It's gotten so problematic, in fact, that in the late 1980s scientists discovered that the original kilo was on average a full 50 micrograms lighter than its clones around the globe — despite the fact that the kilos are all periodically rubbed with delicate, chamois leather and dipped in alcohol.

The problem with hand-cleaning is that the procedure is incredibly difficult to standardize. Each cylinder is, as a result, slightly unique. That's why for the past few decades, scientists have been researching new hands-off ways to clean the kilos without damaging them. 

A new technique described in the journal Metrologia entails cleaning the kilograms with ozone and ultraviolet light, followed by a pure water rinse to remove any dust particles. Better yet, the whole procedure only needs to be done once per decade.

While the procedure is not entirely foolproof, scientists describe it as "a much more repeatable, controllable, reproducible method of cleaning these kilograms." And it looks like it will have to suffice until a more exacting standard for the kilogram is adopted. Right now, scientists are using Avogadro's constant to build a replacement kilo — a silicon sphere in which every atom is painstakingly accounted for.

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