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→Mass
=== Mass ===
=== Mass ===
The base unit of mass is the ''gram''. Originally a gram was the mass of one cubic centimeter (see above), then called a ''milliliter'', of [[water]]. The Lagrange committee fashioned an artifact exactly one thousand times as heavy as this--the volume, in short, of one liter of water--and stored this permanently at Sèvres.
The base unit of mass is the ''gram''. Originally a gram was the mass of one cubic centimeter (see above), then called a ''milliliter'', of [[water]]. The Lagrange committee fashioned an artifact exactly one thousand times as heavy as this--the volume, in short, of one liter of water--and stored this permanently at Sèvres.
[[Joseph Lagrange|Lagrange]]'s committee also proposed a unit of mass caled the ''grave'', to be equal to one thousand grams. But the [[French Revolution|Revolutionary]] government rejected this name as sounding too much like the [[German]] word ''graf'', translated as ''comte'' or [[count]] in French. Therefore, the artifact at Sèvres would be called a kilogram instead--and that artifact has that name to this day.
Mass never changes. But in addition to the prototype meter changing its length over time, scientists later realized that water has different densities at different [[temperature|temperatures]].
Mass never changes. But in addition to the prototype meter changing its length over time, scientists later realized that water has different densities at different [[temperature|temperatures]].
Today, the standard [[kilogram]] is kept at the International Bureau of Standards headquarters in Sevres. It remains today the only generally accepted unit of measurement based on an artifact. But the definition of the liter has now changed, and is no longer dependent on the length of the meter. Today, one liter is defined as the volume of one kilogram of water at four degrees [[Celsius]]. One milliliter is simply one-thousandth of this, and one milliliter of water at this temperature weighs one gram. But to say that one liter is "one thousand cubic centimeters" is no longer correct, simply because the definition of length has changed, while that of mass has not.
Today, the standard [[kilogram]] is kept at the International Bureau of Standards headquarters in Sèvres. It remains today the only generally accepted unit of measurement based on an artifact. But the definition of the liter has now changed, and is no longer dependent on the length of the meter. Today, one liter is defined as the volume of one kilogram of water at four degrees [[Celsius]]. One milliliter is simply one-thousandth of this, and one milliliter of water at this temperature weighs one gram. But to say that one liter is "one thousand cubic centimeters" is no longer correct, simply because the definition of length has changed, while that of mass has not.
== Relation to other systems of measurement ==
== Relation to other systems of measurement ==