# What is a kilogram?

The International Prototype Kilogram (IPK) is kept at the International Bureau of Weights and Measures in Paris, and the metallic cylinder has been used to define the kilogram since 1889. Previously the unit had been defined as the mass of a litre of water.

Forty replicas of the IPK were made and distributed throughout the world, and now each varies in mass by a few micrograms. The increase is thought to be due to having accumulated impurities on the surface, and may be reversible by “washing” the block with ozone and UV light.

Since the kilogram is defined as being the mass of the IPK, then if that block gets heavier the kilogram simply becomes a larger mass unit than it previously was. But the problem with this is that the kilogram is one of the seven “base units” of the SI system, from which other units are derived.

For example the unit of force, the Newton, is defined as that required to accelerate a mass of one kilogram at a rate of one metre per second squared. In turn the unit of work or energy, the Joule, is a force of one Newton exerted over a distance of one metre.

The kilogram getting heavier therefore creates a domino effect in which many other units of measurement also change – and the instruments used to measure them have to be recalibrated.

The kilogram is unique in that it’s the only one of the base units currently defined via a physical thing rather than from a natural phenomenon – it’s the joker in the deck used to build this particular house of cards.

For example the metre, once defined as a proportion of the distance from the north pole to the equator, is now the distance travelled by light in one 299,792,458th of a second. And the second, once considered to a division of a 24-hour day, is “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”.

It’s a goal of the science of metrology to similarly define the kilogram in terms of natural physical constants. One proposal is to define it by fixing the value of Planck’s constant, which can be expressed in units that include the kilogram.

Others include defining it as a certain number of atoms of Carbon-12, or to employ a Watt balance – a type of scale that uses electrical current and voltage to measure the weight of a test mass very precisely.

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