In a laboratory vault outside Paris is a small cylinder of platinum–iridium alloy that ___1___ the standard for all mass measurements worldwide. By an 1889 international accord, the mass of this metal cylinder defines the kilogram.

But that may soon change. The kilogram is the only unit of measurement still based on a ___2___. A second of time, for instance, is now defined in terms of an electron transition of the cesium atom. And ___3___. Those standards are universal and unchanging—unlike the official kilogram. The reference cylinder's mass has ___4___ through the years—not enough to throw off your bathroom scale, but enough to bother measurement scientists.

Some of them are meeting January 24th at the Royal Society in London to discuss future improvements to the measurement units. The plan is to eventually relate the kilogram to a universal number known as Planck's constant. But the technology needed to do that is not yet fully developed. So, for the time being, that little metal cylinder outside Paris will just have to ___5___. I mean, mass.
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