THE kilogram, the scientific unit of mass, is defined by a cylindrical lump of platinum and iridium, made in 1879 and stored in Paris. There are also around 40 copies of it in scientific establishments around the world. When these copies are compared, however, as they are from time to time, it is apparent that they are changing weight relative to each other, and most of the copies seem to be getting gradually heavier relative to the original (the mass of which is, by definition, fixed). The amount is tiny: just tens of micrograms, which are millionths of a gram, over more than a century. Why is this happening, and what can be done about it?

The root of the problem is that other units, such as the second and the metre, are defined in terms of physical phenomena, and the kilogram is the last remaining scientific unit that is not. The second is defined as the duration of 9,192,631,770 vibrations by a caesium atom under certain conditions. A metre is then defined as the distance light travels in a vacuum in 1/299,792,458 of a second. These definitions allow scientists anywhere to reconstitute the second and the metre if necessary. But specifying a given mass is much harder. It could be done, for example, by defining the a kilogram to be the mass of 3 million billion billion gold atoms. But you then need to be able to count atoms with perfect accuracy. Another approach uses a supersensitive electromechanical scale called a “watt balance” to measure how much electrical energy is needed to counterbalance the force of gravity on a test mass, and would allow the definition of a kilogram to be linked to a fundamental constant called Planck's constant. But the uncertainty of this method is still more than one part in 100m, which is not accurate enough, as far as scientists are concerned. So instead they rely on lumps of metal.

Yet these lumps of metal may lose mass through wear and tear, or gain it by absorbing chemicals from the air, even though they are stored in special airtight containers. This causes their masses to vary. So too does cleaning them, which is done using steam, chamois leather and a mixture of ethanol and ether. Researchers at the University of Newcastle have recently proposed a new way to clean the weights, using a combination of ozone and ultraviolet light. This has the advantage that the cleaning process can be precisely standardised.

But ultimately scientists need a better definition. If the accuracy of watt balances continues to improve, they may provide the solution, though they are complex and expensive pieces of equipment. Scientists will vote on whether to adopt a definition of the kilogram based on the watt-balance method in 2014. Another proposal is to use very carefully polished spheres of crystalline silicon as a standard. And some scientists are still pursuing atom-counting approaches. But for the time being, scientists are stuck with lumps of metal—the masses of which are not, alas, entirely stable.