Welcome, the new kilogram

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Future of weights

After nearly 130 years the measurement definition of kilogram was finally altered. In 1889 the kilogram was first defined by a lump of metal in a vault in Paris. It was described as the international prototype kilogram (IPK), a platinum alloy cylinder but now it will be defined by Planck’s constant, a number that is deeply rooted in the quantum world.

The old specification was sealed under a trio of nested glass bell jars, a gleaming metal cylinder placed in a temperature-controlled vault in the bowels of the International Bureau of Weights and Measures in Sevres, France. This lonely piece of platinum and iridium has defined mass around the globe for more than a century and held equal relevance for bathroom scales to medical laboratory balances.

But it changed when scientists recently agreed through a landmark vote at General Conference on Weights and Measures held at Versailles attended by representatives of 57 countries to adopt the new system that redefines the global measure of mass in terms of a fundamental constant of nature. The nature of vote portrayed the phenomenal extent of the metrology enveloping human endeavour.

The old definition was considered appropriate but scientists have long known that even though, by definition, it is precisely one kilogram, its weight changes over time. When in use, the tiniest wear makes it lighter while pollution in the air binds to the surface and over time makes it ever so slightly heavier. In order to rectify the inconsistencies the new process defines the unit of mass through the electrical force needed to counteract the weight of a kilogram on a machine called a Kibble balance. The electrical force itself is linked to the Planck constant through quantum electric effects described by two Nobel Prize winners, Brian Josephson and Klaus von Klitzing.

Balance for the new kilogram
International prototype of a kilogram

Along with altering the nature of kilogram, the change will also be applied to three other base units and they are: units for electric current (ampere), temperature (kelvin) and amount of substance (mole) all becoming linked to constants of nature, namely the electric charge, the Boltzmann constant and the Avogadro constant respectively. It may be borne in mind that the International System of Units has seven base units, including unit of time (second), unit of length (metre) and unit of luminosity (candela). These definitions will, however, remain unchanged.

The change in definition, however, will have no impact on the way fruit and vegetables are weighed but the change marks the culmination of decades of work to link the basic units that underpin metrology to constants woven into the fabric of the universe. The changes will come into effect on 20 May 2019 and for specialists of metrology it will be the most profound moment.

The system of measurement has an interesting history that can be traced back to the mid-18th century when increasing trade links made it necessary to design common weights and measures. In the late 1700s King Louis XVI commissioned scientists to find a more sensible approach and there emerged a system inspired by the natural world, one intended “for all times, for all people”.

The exercise resulted in defining a metre as one ten-millionth of the distance from the North Pole to the equator. A kilogram was the mass of a litre of water. To make the units more practical, each was enshrined in a physical object, a metal bar for the metre and a weight for the kilogram. Gradually the system evolved into an international system of units (SI) with seven base units.

The current system became acceptable and was finely in use for more than century. However experts doubted the accuracy of parts of the system because in storage, the platinum picked up pollutants from the air and the cylinder got slightly heavier. When it was cleaned, the kilogram lost weight as tiny amounts of alloy were removed. The net effect was hard to gauge but copies can put on tens of micrograms in a century. All scientists know for sure was that the kilogram that defined all others was not what it used to be.

It was enough to irk metrologists who complained that the entire accuracy of a crucial measure of weight was based on a lump of metal kept in a vault. They thought that the subtle variation in weight was eventually harmful from the galactic point of view. Though the definition has been altered but it is difficult to carry it out as it initially involves usage of a supremely sensitive piece of equipment known as Kibble balance to calculate Planck’s constant from a 1kg reference mass.

Kibble is similar to a pair of scales but instead of counteracting one weight with another, the object being weighed is balanced by electromagnetic forces. Planck’s constant is proportional to the energy needed to balance the mass. The method is considered to be able to provide precise value of Planck’s constant and by doing the reverse, it could be employed to measure unknown masses. Associating weights with Planck’s constant ensures that the dream of devising a measurement system for all times to come may finally come true as this constant never changes.

Izay Ayesha is into linguistics

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