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Full circle

How the kilogram has come to be defined, once again, in terms of universal constants

Source: https://www.thehindu.com/opinion/editorial/full-circle/article27239907.ece

Why in news?

  • The international system of measurements has been overhauled with new definitions for the kilogramme and other key units.
  • In their vote, countries also unanimously approved updates to three other key units- the kelvin for temperature, the ampere for electrical current and the mole for the amount of a substance.


  • 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.
    • IPK would put on a little extra mass when tiny dust particles settled on it; when cleaned, it would shed some of its original mass.
  • Scientists have long stressed that the fundamental units should be defined in terms of natural constants.
    • 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, the kilogram was redefined, and the date for the new definition was fixed for May 20, 2019.

Measure for measure

  • The new definition for kilogram fits in with the modern definitions for the units of time (second) and distance (metre).
    • Now, a more abstract definition of the kilogram has been adopted in terms of fundamental constants, namely, the Planck’s constant h, and the metre and second which already have been defined in terms of universal constants such as the speed of light.
  • Second: The 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: Once the second was defined, the metre fell into place.
    • By its modern definition, a metre is the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined).
  • Planck constant: This is where the Planck constant comes in.
    • It has been measured precisely at 6.626069… × 10-34 kilograms per second per square metre.
  • With the second and the metre already defined, a very precise definition for the kilogram follows.
  • Along with the units of time and distance, the unit of luminous intensity (candela) is already defined in terms of a natural constant.
  • Along with the kilogram, the units of current (ampere), temperature(kelvin ), and amount of substance (mole) too took on new definitions. That covers all seven fundamental units.


  • The modern definition of the second has already helped ease communication across the world via technologies like GPS and the Internet.
  • Scientists have often been quoted as saying the change in the kilogram’s definition will be better for technology, retail and health. With this redefinition, the range of universality of the measurement has been extended in an unprecedented way.
  • Earlier, if a mass had to be verified to match with a standard kilogram, it would be placed on one of the pans of a common balance, while the prototype would have to be placed in the other pan — and mass would be measured against mass.
  • Now, by using a Kibble balance, which balances mass against electromagnetic force, to measure the mass of an unknown piece, the very methodology of verification has been altered. The constants involved are known precisely and are universal numbers. Hence, whether the mass is measured on earth or, say, on the moon, it can be determined with precision.
  • This is the culmination of a series of historical changes, which are also described by Richard S. Davis et al in their 2016 article in the journal Metrologia.
  • Originally the definition of mass was in terms of what was then thought of as a universal physical constant.
    • In 1791, 1 kg was defined as the mass of one litre of distilled water at its melting point. Thus, the density of water was the physical constant on which this definition hinged.
    • In 1799, the kilogram came to be defined using a cylinder of platinum – the first time an artefact was used for this purpose. But it was also defined as equivalent to the mass of one litre of distilled water at atmospheric pressure and at about 4 degrees Celsius, the temperature at which water has the maximum density.
    • This was done away with in 1889 when the community adopted the International Prototype of the Kilogram — a cylinder made of an alloy that’s 90% platinum and 10% iridium. The reference to the ‘physical constant’, i.e. mass of one litre of water, was abandoned.
  • Now, as a culmination of this historical process, we come back full circle and find that the kilogram is defined again in terms of a fundamental physical constant — the Planck’s constant. Planck’s constant is a robust number to match.
  • Not until the art of travelling at relativistic speeds, close to the speed of light, is mastered, will we have to redefine these abstract definitions. Until then, it looks like metrologists are on a stable berth.

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