The kilogram (kg) is the base unit of mass in the SI system. 1 kg = 1,000 g = 2.20462
lb. The kilogram is the only SI unit that is defined in terms of an artifact instead of an experiment, this artifact being a cylindrical platinum-iridium slug kept by the International Bureau of Weights and Measures at Sèvres,
near Paris. It is copied by all the bureaus of standards and laboratories
in the world which adhere to the SI. For example, a replica of the metal
cylinder at Sèvres is kept at the National Institute of Standards
and Technology in Gaithersburg, Maryland, and serves as the SI standard
of mass for the United States.
|The US national prototype kilogram kept at NIST,
Gaithersburg, MD. Credit: NIST
The kilogram used to be defined as the mass of 1 liter of water.
This mass served as the archetype for the artifact at Sèvres.
[Thanks to Charles Watson for contributions to this entry.]