Definition of the Kilogram
- 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.
- Here is a picture of the
U.S. National Kilogram Prototype.
- Some proposals to redefine the kilogram:
The mass of 6.0221415 ×
1023 / 0.012 carbon-12 atoms
The mass of 1,097,769,238,499,215,084,016,780,676,223 electrons
- You may recognize 6.0221415 ×
1023 from chemistry as Avogadro's Number.
- The second definition is consistant with the currently accepted electron
mass of 9.01093826 × 10-31 kg.