゛How much does a kilogram weigh these days?゛ may sound an odd question. Does a kilogram not weigh just that, a kilogram? In fact, a kilogram is the mass of a cylindrical lump of platinum-iridium alloy that was cast in 1879 in Hatton Garden, the jewellery district of London, and then dispatched to the International Bureau of Weights and Measures (BIPM) in Sevres, near Paris. It sits there still, nestled beneath three cheese-dome-like bell jars in a safe that can be unlocked (though it rarely is) only by turning a set of three keys, each entrusted to a high-ranking BIPM dignitary.
Despite such precautions, the international prototype does not rest entirely unperturbed. On those occasions when it has been compared with some of its official copies, discrepancies of up to 69 micrograms - slightly smaller than a grain of sand - have been observed. Since there is no reason to believe that ゛the゛ kilogram is inherently more stable than any of its cousins, it is either shedding weight through wear and tear or putting it on through chemical processes such as those involved in cleaning the thing.
In a world increasingly dependent on precise measurements, metrologists think such inconstancy no longer cuts the mustard. On January 24th, therefore, they gathered at the Royal Society in London, to discuss progress towards recasting the kilogram in terms of something less mercurial than a platinum-iridium alloy.
The kilogram is the last bit of the International System of Units (SI) to be tied explicitly to an artefact. Once, the metre was too. It was the length of another platinium-iriditm ingot stored in Sevres. But the metre has been redefined twice since that ingot was deposited in 1889： first, in 1960, in terms of the wavelength of a particular sort of light； then, in 1983, as the path travelled by light in a vacuum in 1/299,792,458 of a second. Which, of course, raises the question of what a second is. Not, as you might think, a sixtieth of a sixtieth of a twenty-fourth of the period of the Earth's rotation. No. A second is the duration of 9,192,631,770 periods of a phenomenon called microwave transition in an atom of caesium-133. The remaining four basic SI units, the ampere (electricity), kelvin (temperature), more (quantity of atoms, molecules or other particles) and candela (light), followed suit - though the ampere, candela and mole are linked to the kilogra
m, and so indirectly to the Sevres prototype.
