India on Monday adopted a global resolution to redefine four of the seven base units – kilogram, kelvin, mole and ampere – that was accepted by representatives from 60 countries at the General Conference on Weights and Measures in Versailles, near Paris, on November 16. The International Bureau of Weights and Measures had organised the conference.

More than 100 countries have adopted the metric system of measurements, also known as the International System of Units, which has been in practice since 1889, PTI reported. The system was formalised in 1960 and has been updated several times to account for development in measurement technology.

Seven main units of measurement are in use at present, including the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance, and the candela for luminous intensity.

“The fundamental constants are invariants of time and space and have successfully replaced the artifact based units, and aptly opened up the new era for quantum world by linking all seven base units to fundamental constants or quantum standards,” K Aswal, the director of the National Physical Laboratory, told PTI. The laboratory, which is one of the oldest scientific institutions in the country, takes care of setting meteorology standards in the country.

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How will the kilogram be measured?

For more than a century, the kilogram was defined by a metal cylinder, made of platinum-iridium alloy, kept in a vault in Paris called the International Prototype of the Kilogram. However, there was always a possibility of the cylinder losing mass or gaining some because of extraneous factors, thereby altering the definition. Scientists indeed found by comparing the cylinder with its copies distributed across the world that despite their efforts to protect the cylinders, their mass changed.

Now, a kilogram will be defined using the Planck constant, which relates a photon’s energy to its frequency. The Planck constant describes the behaviour of particles and waves on the atomic scale. It depends on three units: metre, kilogram and second. As second and metre are measured and defined using the speed of light, they can be used with the fixed Planck constant to define a kilogram. Although the value of the kilogram will not change, the redefinition will ensure its reliability, and enable far more accurate mass measurements.

The National Physical Laboratory has sent its recommendations to the National Council of Educational Research and Training, requesting that the proposed changes be incorporated in textbooks, Aswal said. The laboratory has also sent its proposals to the All India Council for Technical Education, the Indian Institutes of Technology, the National Institutes of Technology, and other academic institutions.