Inside this vault stands one of the six official copy of the International Prototype Kilogram in Paris, the original artifact whose mass defines at present the SI unit of mass. The kilogram is the only base unit that is still defined directly by a physical object rather than a fundamental physical property that can be reproduced in a laboratory. The IPK is of cylindrical form, with diameter and height of about 39mm and is made of an alloy of 90% platinum and 10% iridium. Air and manipulation can cause micro-material losses despite its constitution and is kept under conditions specified by the 1st CGPM in 01889. In fact, every year the standard kilogram changes weight. Access to the IPK and its copies is under the strict supervision of the International Committee for Weights and Measures. In 02018, the standard kilogram could be replaced by the Planck constant.
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