- World
- Nov 16
Scientists vote to redefine kilogram
For nearly 130 years, the kilogram was defined by a block of metal kept at a vault in Paris. Now, scientists have voted for change. Following a historic vote on 16 November at the General Conference on Weights and Measures (CGPM), the unit will no longer be defined by the international prototype kilogram dating back to 1889, but by Planck’s constant, a number deeply rooted in the quantum world. The new system defines the unit of mass through the electrical force needed to counteract the weight of a kilogram on a machine called a Kibble balance.
Who represented India at the meeting?
The CGPM comprises 60 countries, including India, and 42 associate members. India was represented by Department of Consumer Affairs secretary Avinash Srivastava, National Physical Laboratory (NPL) director DK Aswal and NPL Planning, Monitoring & Evaluation head TD Senguttuvan.
What else has changed?
Along with the kilogram, three other base units will be redefined. The units for electric current (ampere), temperature (kelvin) and amount of substance (mole) all become linked to constants of nature, namely the electric charge, the Boltzmann constant and the Avogadro constant, respectively. In all, the International System of Units has seven base units, including the unit of time (second), the unit of length (metre) and the unit of luminosity (candela). The definitions of these will remain unchanged.
Where do the advantages lie?
The advantages of the Kibble balance is that the international prototype kilogram need not to be sent to the International Bureau of Weights and Measures (BIPM) for calibrations and the accuracy and stability of the Kibble balance is high.
When will the new definitions come into force?
The new definitions will come into force on 20 May 2019.
Why kill off the Le Grand K?
The international prototype kilogram (also called Le Grand K) has been at the forefront of the international system of measuring weights since 1889. Several replicas were made and distributed around the globe. But the master kilogram and its copies were seen to change - ever so slightly - as they deteriorated. In a world where accurate measurement is critical in many areas, such as in drug development, nanotechnology and precision engineering, those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.
How does the new system work?
The Kibble balance works by measuring the electric current needed to produce an electromagnetic force equal to the gravitational force acting on a mass. The pull of the electromagnet is directly related to the amount of electrical current going through its coils. Therefore, there is a direct relationship between electricity and weight. So, in principle, scientists can define a kilogram, or any other weight, in terms of the amount of electricity needed to counteract the weight.
did you know?The base unit of time is the second, which was once defined as 1⁄86,400 of an average day, is now defined as being the time taken for a caesium atom to vibrate 9,192,631,770 times.