The 26th meeting of the General Conference on Weights and Measures (CGPM) was held during November 13-16 2018 at Palais des Congrés, Versailles, France. CGPM is the highest international body of the world for accurate and precise measurements. The 26th CGPM meeting was very special and historic as the members have voted for the redefinition of 130 years old “Le grand K – the SI unit of kg” in terms of the fundamental Planck’s constant(h). The new definitions will come into force on 20 May 2019.
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.
Le Grand K has been at the forefront of the international system of measuring weights since 1889. Several close 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 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.
How wrong is Le Grand K?
The fluctuation is about 50 parts in a billion, less than the weight of a single eyelash. But although it is tiny, the change can have important consequences.
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.
By measuring the current running through the electromagnet to incredible precision, the researchers are able to calculate h to an accuracy of 0.000001%. This breakthrough has paved the way for Le Grand K to be deposed by “die kleine h”.
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