Scitech | What’s in a kilogram?

The last SI unit to be defined in a way that can be universally replicated

Observations depend on units. Without them numbers would be absolutely meaningless – simply stating that the height of a building is 100 is unhelpful, while stating that it is 100 metres is a lot more useful. It is unsurprising, then, that there exists an International Committee of Weights of Measures, headquartered in Paris, responsible for monitoring the use and definitions of these units. Most of the world now uses the successor to the metric system: SI units (abbreviated from the french Système international d’unités), formally established by  the committee in 1960. The system is based on seven units, from which all the other units can be derived.

How to define the base units? Since units are nothing more than human-made magnitudes of given physical quantities, there needs to be a standard that scientists, researchers, and fruit-sellers alike can use to find out what the true quantity is.  In the beginning, this was achieved with physical objects. A cylinder of platinum iridium was created with a weight of exactly one kilogram, and all other kilograms were based on this cylinder. Similarly, a rod of the same material was used to standardize a metre. However, this method has an important flaw – despite the relative stability of the platinum-iridium alloy, it does degrade over time. In a world where we increasingly rely on very precise measurements – particularly in fields such as nanotechnology – we need an unerring and infinitely reproducible standard. The result of this need is a rush to attempt to redefine the standard SI units in terms of fundamental constants of nature. The metre, for instance, is now defined in terms of the velocity of light. Similarly the other SI units were redefined, rendering their physical standards obsolete. The exception to this was the kilogram, which has retained its original definition.

This is in the process of changing: the National Physics Laboratory in the UK has developed a piece of equipment called the watt balance, which can be used to make an accurate measurement of Planck’s constant–the number that relates the energy of a photon to its frequency. With the watt balance calibrated using the improved Planck’s constant, it would be used to weigh the platinum-iridium lump, and thus define the kilogram in terms of the energy. The watt balance uses a loudspeaker coil and a magnet to determine the mass of the system independent of its properties. Measurements using this apparatus have been accurate to an uncertainty of just over two parts in a hundred million, which is what is needed to reliably construct a new definition.

The research is currently being continued in the National Research Council in Ottawa (who purchased the equipment from the National Physics Laboratory), with even lower uncertainties. In fact, according the Alan Steele, director of the projects, the results are so precise that gravitational changes from the first to the second floor, or even the presence of a large delivery truck nearby, can severely skew the results.

From the evidence, it appears  that a certain lump of platinum-iridium may soon be consigned to an antique vault.


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