The unit of mass - the kilogram |
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The kilogram is defined as the mass of a platinum-iridium cylinder, kept for the last 300 years in a bank vault in Sevrés near Paris.  The standard is kept very carefully. Copies are made and sent to standards laboratories in many countries. The original is taken out once a year by two people. One to carry it, and one to make sure he doesn't trip. |
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