## Redefining the kilogram

We use kilograms to measure how much things weigh. But how much is a kilogram, anyway?

There is only one object in the world that weighs exactly one kilogram. It is a cylinder of metal locked away in a cabinet in France. Standards labs from around the world take copies of this kilogram, and use them to make sure that a kilogram in Australia is the same as a kilogram anywhere else.

But what if something happens to the kilogram? If the weight in France was lost, or damaged, we would no longer have a standard to compare to. We would be less certain that our measurements were the same. Mathematicians and scientists are trying to find a new way of defining a kilogram, so that anyone can make one.

cientists working at the National Measurement Institute in Sydney and CSIRO are part of a world wide effort called the Avogadro Project. They want to define the kilogram as a certain number of atoms of silicon. Because you can make an object composed only of silicon atoms that all weigh exactly the same, this would allow scientists to make their own kilograms without having to compare to the original.

However, there are around 2.15 x 1025 (21 500 000 000 000 000 000 000 000) atoms of silicon in one kilogram. This is far too many to count individually. So instead, the scientists made two spheres that were the roundest (most like a sphere) objects ever made. Because they were so round, they could work out the volume very accurately, and then they could calculate how many atoms were inside. Knowing the number of atoms, and how much one atom weighed, they could work out how much the whole sphere weighed.

The two spheres are now being monitored to see if they stay the same weight over time. Maybe someday soon, the meaning of a kilogram will change.

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