It’s Time To Redefine The Kilogram


The alternative method to measure the kilogram is through Avogadro’s constant, which represents the number of atoms in a certain mass of a certain substance. That is achieved by constructing a perfect kilogram sphere made of a particular type of silicon and measuring its diameter 500,000 times in slightly different positions. Knowing the volume and the properties of silicon, scientists can simply calculate the number of atoms.

That allows for a precise estimation of Avogadro’s number. The Avogadro system has two advantages: It requires one to simply count the number of atoms and it's defined in terms of the Planck constant.

Although the measurement is still linked with an artifact, it is not dependent on a specific one – it is how the artifact is created that matters and not about the object itself. Still, the silicon solution has its drawbacks.

The artifact is spherical and so it is not easy to handle. In addition, it has a large volume, so you have to weigh it in a vacuum to determine its mass (the mass of the air would interfere). It also has a huge surface area, so it will get 10 times as dirty as the fundamental kilogram, making the artifact inaccurate very quickly.

The two experiments are getting closer and closer to the required precision the CIPM ask for, though, so hopefully the kilogram – along with the kelvin, the ampere, and the mole (whose definitions are also up for revision) – will be redefined at the 26th General Conference on Weights and Measures in 2018.

This silicon sphere weighs an exact kilogram. Julian Stratenschulte/dpa 

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