Written by Talent KAS

Related Topic in KAS Prelims Syllabus:

Science and Technology [Paper-II]: Nature and scope of Science and Technology


  • The prototype of one kilogram (NPK-57) is now available in India and placed at the National Physical Laboratory, New Delhi.
  • The new definition of kilogram has come into effect from May 2019. Few countries have developed the system of realization of unit of mass ‘kg’.

Earlier definition of Kilogram

  • The global standards for measurement are set by the International Bureau of Weights and Measures (BIPM), of which India became a member in 1957.
  • At BIPM in Sèvres, near Paris, stands a cylinder of platinum-iridium locked in a jar.
  • Since 1889, the kilogram has been defined as the mass of this cylinder, called Le Grand K, or International Prototype Kilogram (IPK).
  • In India, NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with IPK.
  • The IPK was the last physical artifact used to define any of the fundamental units.

New definition

  • Scientists have long stressed that the fundamental units should be defined in terms of natural constants. They wanted to create a measurement system that is based entirely on unchanging fundamental properties of nature.
  • On November 16, 2018, following a vote at BIPM, representatives of 60 countries agreed that the kilogram should be defined in terms of the Planck constant.
  • The Planck constant is a quantity that relates a light particle’s energy to its frequency.
  • Using a machine called a Kibble balance, in which the weight of a test mass is offset by an electromagnetic force, the value of the Planck constant was fixed and the kilogram was redefined.

  • The new definition for kilogram is in line with the modern definitions for the units of time (second) and distance (metre).
  • Second’ is defined as the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133.
  • ‘Metre’ is defined as the distance travelled by light in vacuum in 1/299,792,458 of a second.
  • Along with the kilogram, the units of current (ampere), temperature (kelvin), and amount of substance (mole) too took on new definitions. These are also now tied to constants in nature. But unlike the kilogram, their definitions were not based on a physical object.
  • The new SI System will be helpful in bringing in accuracy while dealing with international trade, biotechnology, high-tech manufacturing and human health and safety.

[Source: Indian Express, nplindia.in]

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