International Prototype of the Kilogram (IPK)

A kilogram (symbol kg) is the fundamental unit of weight. It has been defined to correspond approximately to the weight of 1 liter of water. According to SI system, kilogram is defined as the mass of the international prototype of the kilogram deposited at the International Bureau of Weights and Measures in Sevres (France).

International prototype of kilogram is a cylinder-shaped body with a height and a diameter of approximately 39 mm made of an alloy comprising of 90% platinum and 10% iridium. On this basis, copies were made as similar as possible and stored in countries around the world. The material for the fabrication of the prototype was chosen according to the following criteria: corrosion resistance, high density (reduce the influence of buoyancy when measured in air), good electrical conductivity (elimination of static electricity), low magnetic conductivity ‒ diamagnetism (to limit undesirable magnetic effects ‒ e.g. attraction) thermal stability and hardness (abrasion resistance).



**Note**: It is no longer taken as a prototype of kilogram as there has been a unanimous vote of representatives from more than 50 countries to base it rather on some unalterable features of the universe – such as the speed of light, time and Planck's constant, a number that helps scientists figure out the energy of a photon of light, given its wavelength. (The approximate numerical value of Planck's constant is 6.626 x 10-34 joule-second.)