By Prarthana Mitra
The official definition of kilogram, the standard international (SI) unit of mass, is about to change as the yardstick for it — a block of metal which has determined the mass of all kilograms since 1889 — was recently deemed inaccurate and unreliable. Until now, the unit of mass was the only SI unit of measurement that was still based on a physical object. The kilogram is the last of the seven base units to be still linked to a physical artefact.
What is the IPK and why is it being replaced?
The IPK, or the International Prototype Kilogram, is a small cylinder of platinum-iridium alloy, about 39 mm (or an inch and a half) in both height and diameter that has been locked in an underground airtight vault in International Bureau of Weights and Measures in Paris for some 129 years. It has lost atoms and mass due to microscopic contaminants over time and due to periodic cleaning.
This means that the official value of a kkilogram has been necessarily and constantly recalibrated to match the new mass multiple times, even if the weight lost every year was equivalent to that of an eyelash. This doesn’t seem to suggest dire consequences when you are buying flour or weighing your baggage at the airport, but the repercussions of this inaccurate and variable definition could be disastrous in chemistry laboratories, or when it comes to medicine dosage.
To complicate this further, around 40 allegedly exact copies of the IPK were distributed across the world to calibrate all existing scales, weights, and balances at laboratories, supermarkets, bakeries and pharmacies. With their mass undergoing changes at different and indeterminable rates, the unit’s definitions are currently spinning out of sync.
A move towards standardisation and hyper-accuracy
After much discussion, the International Bureau of Weights and Measures, which regulates the metric system, will vote on November 16 to remove the IPK and replace it with the Planck constant.
Based on a fundamental law of nature — something that doesn’t change — the Planck constant aims to solve problems caused by the lack of a dependable universal standard. It bases itself on the fact that the behaviour of particles and waves depends on the metre, the second (both determined by the speed of light), and the kilogram.
From now onwards, the fixed Planck constant, using the Kibble balance, will start defining a kilogram by equating mechanical energy exerted by the mass of an object with an equivalent amount of electrical energy.
The Planck constant’s precise value after years of experimentation gives the unit an unchanging quality, and eliminates the inaccuracies involved in measuring mass against a mutable block of metal. The instrument, invented by british physicist Bryan Kibble in 1975, will help ascribe a more stable value to the SI unit.
Three other base SI units, apart from the kilogram — the ampere, the Kelvin, and the mole — will also undergo changes. The ampere, for instance, will be redefined by the charge carried by a single proton, while the value of Kelvin will be now be based on the Boltzmann constant.
If scientists vote in favour of the Planck constant at this week’s General Conference on Weights and Measures, laboratories may have newly calibrated instruments for measuring mass as early as next year, perhaps on May 20 (World Metrology Day).
Dr J.T. Janssen, director of research at the National Physical Laboratory (UK) told New Atlas, “Once implemented, all the SI units will be based on fundamental constants of nature whose value will be fixed forever. This will pave the way for far more accurate measurements and lays a more stable foundation for science.” The redefinition of the kilogram will be of immense use, particularly in technology, science, health and industrial sectors.
Prarthana Mitra is a staff writer at Qrius.
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