A load is finally off the kilogram

Scientists say the revisions will not visibly alter anything for the general public

By G.S. Mudur in New Delhi
  • Published 21.05.19, 3:32 AM
  • Updated 21.05.19, 12:50 PM
  • a min read
The kilogram will now be defined through the Planck constant (iStock)

The world adopted on Monday revised definitions of four units of measurements, including the kilogram, delinked from physical artefacts and relying instead on constants of nature.

The kilogram (the standard unit of mass), ampere (current), kelvin (temperature) and mole (amount of substance) will be now defined through constants of nature under revisions announced by the General Conference on Weights and Measures in France in November 2018.

“The kilogram had been defined by a lump of platinum-iridium, a physical artefact, that changes now,” said Dinesh Aswal, director of the National Physical Laboratory (NPL), New Delhi, a government lab that maintains standards for measurements. “The kilogram will now be defined through the Planck constant,” he said.

The Planck constant relates energy in one photon of electromagnetic radiation to the frequency of the radiation and is constant across the universe. Scientists say physical artefacts have potential drawbacks of susceptibility to damage.

The revised definitions are being implemented from May 20, World Metrology Day.

In preparation for the revised definitions, the NPL has recommended that the changes be incorporated by the National Council of Educational Research and Training in high school textbooks and by the All India Council for Technical Education in engineering courses.

Scientists say the revisions will not visibly alter anything for the general public but are expected to have long-term impacts on computing, electronics and aerospace among other areas critically dependent on high-precision measurements.

Under the changes, the ampere will be defined through the charge on the electron, kelvin through the Boltzmann constant that connects the energy in gas to its temperature and mole through the Avogadro constant, the number of atoms or molecules in a unit quantity of a substance.

Two other standard units — the second for time and the metre for distance — had been similarly linked to constants of nature earlier: the second to the ticking frequency of the caesium atom in 1960, and the metre to the speed of light in 1983.

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