The official kilogram faces constant demand


What’s a kilogram to you? A litre of water, perhaps, or maybe a block of cheese.  Officially, it’s a lump of metal kept in a vault in Paris.  But that’s about to change.

After more than 100 years, the ultimate reference kilogram is set to be redefined.

Currently, the weight is the only unit of measurement that is still defined using a man-made object. The official kilogram, known as the International Prototype Kilogram, is a cylinder fashioned from an oxidation-resistant alloy made of platinum and iridium. This means that it doesn’t react with particles in the air, allowing its mass to remain unchanged over time – or so it was thought.

The proto-kilo was forged in 1879, together with several replicas. Every four decades, the replicas are brought together with the official one and weighed to record any changes.

Since the International Prototype Kilogram, by definition, cannot change mass, measurements are made of the replicas relative to the original. During the most recent weigh-in, in 1988, the kilogram was found to be on average 50 micrograms lighter than its doppelgängers. While this small change won’t have an effect on our everyday life, it can be a problem in
areas of scientific research that require precise measurements. A change in the kilogram is important because it is used to define other units, such as those used for measuring force and energy.

Experts aren’t sure exactly why this change in weight occurred, but it is clear that the problem could be solved if the kilogram was defined using a physical constant – a number found in nature that remains the same in any situation.  

After years of debate, metrologists – scientists who study measurement – have decided to redefine the kilo using the Planck constant – a fundamental quantity central to calculations in quantum physics. The constant describes the relationship between the amount of energy carried by a single photon and the frequency of its electromagnetic wave.

The most important part of redefining the kilogram, however, depends on accurately calculating the value of Planck’s constant in a reproducible manner.

Two main methods are used to accomplish this – watt balance experiments and the “Avogadro” method. 

The watt balance is an experimental device that works similarly to a traditional scale.

The traditional scale measures the weight of an object by balancing it with another known weight. In a watt balance, instead of balancing one weight
with another, the weight of an object is balanced with an electromagnetic
force. Because the electrical power used to create this force can be measured precisely, the US National Institute for Standards and Technology can now record the Planck constant with the degree of uncertainty reduced to just 34 parts per billion

The “Avogadro” method uses a sphere of pure silicon to accurately calculate the the number of carbon-12 atoms in 12 grams of carbon-12 – known as the Avogadro number, and named after 19th Century Italian scientist Amedeo Avogadro. 

Using the Avogadro number along with a second already precisely measured value, scientists can calculate Planck’s constant.

Measurements of the constant have already been made using both these methods, but the international metrological community has decided that a few conditions must be met for the 2018 redefinition to go forward. At least three experiments must produce consistent values of Planck’s constant with an uncertainty of less than 50 parts per billion, and at least one must have an uncertainty of under 20 parts per billion. Finally, values of the constant calculated using the “Avogadro” method and watt balance experiments must agree with each other.

While there is reason to be optimistic about the redefinition of the kilogram next year, there is still some work to be done before the International Prototype Kilogram and its replicas can finally be placed in a museum.

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