The kilogram, with the symbol "kg", is a unit of mass in the International System of Units (SI). The mass of the international prototype of kilogram is defined as one kilogram. It is a cylindrical body made of a Pt-10Ir alloy (90% platinum and 10% iridium), whose diameter and height are both 39.17 mm. In the SI system, the kilogram is the only basic unit defined by an artifact.
The international highest standard of mass is the Pt-10Ir-alloyed prototype of the kilogram, which is kept by the International Bureau of Weights and Measures (BIPM) located in Sevres, France. The members of the Metre Convention are entitled to a replica of the prototype. Countries other than the members can purchase the replica to establish their highest standard of mass. BIPM began to distribute numbered replicas of the kilogram prototype to its members, and Taiwan purchased one replica with the number 78 from BIPM in 1995. Since then, the replica has been checked for calibration with BIPM every ten years to ensure that our mass standard is in line with the global standard. Accordingly, we have established our self-calibrating institution of mass standard.
