Over the course of its century-plus lifetime, the IPK has emerged only three times to serve “campaigns” of active duty, most recently in 1988–1992, when it participated in a formal verification of all kilogram prototypes belonging to the 51 Meter Convention member states. On that occasion, however, the IPK itself was found wanting. Despite all the protective protocols and delicate procedures, it had mysteriously changed. No one can say whether the IPK has lost weight (perhaps by the gradual escape of gases trapped inside it from the start) or if most of the prototypes have gained (possibly by accumulating atmospheric contaminants). The difference is approximately 30 micrograms —30 billionths of a kilogram—in a hundred years. (Imagine 30 cents out of a $10 million stack of pennies.)
This alarming show of instability is driving global efforts to redefine the kilogram, so that mass need not depend on the safety or stability of some manufactured item stored in a safe. In fact, more than mass hangs in the balance, for the kilogram is tied to three other base units of the International System of Units (SI), namely the ampere, the mole, and the candela. Several more quantities—including density, force, and pressure—are in turn derived from the kilogram.
I blame global warming.