In a laboratory vault outside Paris is a small cylinder of platinum – iridium alloy that serves as the standard for all mass measurements worldwide. By an 1889 international accord, the mass of this metal cylinder defines the kilogram.
…
The reference cylinder’s mass has drifted slightly through the years — not enough to throw off your bathroom scale, but enough to bother measurement scientists.
Researchers plan to discuss relating the kilogram to Planck’s constant.