The 26th General Conference on Weights & Measures (GCWM), which is comprised of 60 member countries, voted for the redefinition of the kilogram.The existing definition of the kg is over 130 years old.Currently, it is defined by the weight of a platinum-based ingot called “Le Grand K” which is locked away in a safe in Paris.

WHY DEFINITION CHANGED

                                  The master kilogram and its copies were seen to change – ever so slightly – as they deteriorated.The Kilogram is currently defined by a Lump of platinum-iridium, stored in a vault near Paris. Because objects can easily lose atom or absorb molecules from the air, using one to define an SI unit is problematic. Compare to the prototype, some official copies have gained atleast 50 micrograms over a century.

                                    In a world where accurate measurement is now 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.
                                      NEW DEFINITION
                       

The new SI system, which is defined in terms of the Planck’s constant, would be stable in the long term and practically realisable.The new definition of kg involves accurate weighing machines called ‘Kibble balance’. It uses the constant to measure the mass of an object using a precisely measured electromagnetic force.

Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, such as old cars. The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. There is, therefore, 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 (gravitational force acting on a mass).There is a quantity that relates weight to electrical current, called Planck’s constant – named after the German physicist Max Planck and denoted by the symbol h.

But h is an incredibly small number and to measure it, the research scientist Dr Bryan Kibble built a super-accurate set of scales. The Kibble balance, as it has become known, has an electromagnet that pulls down on one side of the scales and a weight – say, a kilogram – on the other.The electrical current going through the electromagnet is increased until the two sides are perfectly balanced.

                       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.It will be adopted on May 20, which is World Metrology Day.India will adopt a new definition of the kilogram (kg) from May next year.