The original definition of the kilogram was the mass of one liter of
distilled water at a temperature of 4 degrees Celsius; a liter was
defined as 1,000 cubic centimeters of water.
The current definition of the kilogram is the mass of
the one kg prototype made out of a platinum-iridium alloy.
This prototype weight is stored in a vacuum. Technicians that handle
the prototype weight must be cleaner than a surgeon during surgery.
The one kg prototype weight is stored in a
vault at the Bureau International des Poids et
Mesures near Paris, France.
Many countries have copies of the prototype weight. Official
comparisons are made every forty years.
