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Kilogram Redefined!

Probably everyone's heard about the second law of thermodynamics - which states that the entropy of any isolated system always increases.  In common language - chaos - something we're all familar with at some time in our lives....


But on Monday May the 20th 2019 - that was World Metrology Day - we all took a step away from chaos!


Since 1889 the official International Prototype Kilogram (IPK), a 90% platinum/10 % iridium alloy highly polished metallic cylinder with height of 39mm and diameter of 39mm has been housed at the International Bureau of Weights and Measures in France. Unfortunately, material objects lose or even gain tiny bits and pieces over time - that's the nature of physical reality (something else we're probably all familiar with!) There were various official copies here and there in the world. Interestingly, over time, it had been observed that the mass of these copies was moving away from that of the IPK, and nobody knew whether the copies were losing mass or the IPK was gaining mass - talk about chaos!  The problem was in micrograms - not good for precision measurement!


And no matter how much the Official Kilogram weighed - it was always the Official Kilogram - even if it had - or  maybe hadn't - changed a bit.


But - the new definition defines the kilogram in terms of the Planck constant - that's a concept in quantum mechanics and it's about how electrons release energy in discrete steps, called quanta, and it can never change.  The Planck constant is defined by the characteristics of atoms in the element cesium and it's taken years of hard work to figure the Planck constant out - measuring the very smallest thing there is to measure.


If you're interested to read about this, there's a great description here <>.


And not just the kilogram, but also the ampere, kelvin and mole have been redefined based on the fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k) and the Avogardo constant (NA), respectively.


So here's the new definition of the kilogram -


The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s -1 , where the meter and the second are defined in terms of c and ∆νCs.











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