Why is the kilogram getting heavier, and what can be done about it?

By T.S.

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?