Kilogram


Kilogram
The kilogram is a metric unit that describes mass. The official kilogram equals the mass of a particular piece of platinum-iridium metal kept in Paris. It is the only SI unit left that is defined such that it needs to be compared to some object. There are now attempts to define the kilogram in a different way, for example by specifying a number of atoms of a certain substance (at a certain temperature).
One kilogram is a little more than 2.2 pounds. One tonne is one thousand kilograms.
History.
In 1879 the piece of metal was made. It was officially chosen to be the kilogram in 1889. It was made of 90% platinum and 10% iridium because both do not rust or corrode the way most metals do. From 1795 to 1799, the unit of mass was not called "kilogram" but was named "grave".
Problems.
There are various copies of the original Kilogram. The problem is now that some of these copies have gained or lost some mass. They no longer weigh the same.


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