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World's roundest object

This video shows the roundest object in the world – one kilogram sphere of silicon.  In 1799, the definition of a kilogram was the mass of one litre of water, at 4 degrees Celsius.  As of 2016, the kilogram is defined by the mass of a platinum-iridium sphere, located in Paris, called the International Prototype Kilogram.  In a perfect crystal sphere of silicon, the number of atoms in the sphere can be calculated using a laser to measure the diameter.  The number of atoms in the sphere could then be used to define Avogadro’s number, which in turn could define the kilogram.

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