Historical context of the SI
At the end of the 18th century, a kilogram was the mass
of a cubic decimeter of water. In 1889, the 1st CGPM sanctioned
the international prototype of the kilogram, made of platinum-iridium,
and declared: This prototype shall henceforth be considered to be the unit of
mass. The picture at the right shows the platinum-iridium international
prototype, as kept at the International Bureau of Weights and
Measures under conditions specified by the 1st CGPM in 1889.
The 3d CGPM (1901), in a declaration intended to end the
ambiguity in popular usage concerning the word "weight," confirmed
that:
The kilogram is the unit of mass; it is equal to the mass of the
international prototype of the kilogram.
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