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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