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 Mass  
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♣  The Evolution of the Definition of the Kilogram
    The unit of mass, the kilogram (kg), was originally defined as the mass of distilled water at its maximum density in a cube measuring 10 cm per side. In 1889, it was replaced by the mass of Prototype Kilogram, a cylinder measuring about 39 mm in both diameter and height made of an ally of 90% platinum and 10% iridium.
     Until now, the Japanese mass standard has been established by the Japanese copy of Prototype Kilogram, which is stored at NMIJ. Made in 1890, this Japanese prototype was produced concurrently with the establishment of Prototype Kilogram and is of the same shape and material.

♣  Prototype Kilogram
    The mass of the Japanese copy of Prototype Kilogram is calibrated against Prototype Kilogram kept at the International Bureau of Weights and Measures near Paris, France, at intervals of about 30 years, and its equivalency to Prototype Kilogram is confirmed. The most recent measurement of the Japanese copy, taken in 1993, showed that it had changed by only 7 µg over a period of 100 years.


Japanese Prototype of Kilogram

♣  Road to Higher Accuracy
    Since Prototype Kilogram and its copies are defined to be used in the atmosphere, the effects of buoyancy and surface adsorbates cannot be ignored. Thus, a balance that incorporates sinkers of equal mass with different volume or surface area was developed to effect accurate correction of air buoyancy and molecular adsorption on the sinkers' surfaces based on the apparent mass difference between the sinkers. The correction was successfully made for the first time in the world. Today, using these measuring technologies, the mass standard to an accuracy order of 10-8 (one part in 100 million) is established in Japan.


1 kg sinkers for correction of air buoyancy and surface adsorption

The definition of the kilogram: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. (the 3rd CGPM, 1901)
Note: 1. The word "weight" denotes a quantity of the same nature as a "force".
2. The standard acceleration due to gravity is 980.665 cm/s2. (the 3rd CGPM, 1901)

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