千克原器是测量质量的标准，它是一个铂铱合金的圆柱体砝码，但经年累月，这个砝码的质量轻了50微克，虽然对人们的日常生活几乎无影响，但对科学界来说却是个引人深思的问题。

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.

But that may soon change. The kilogram is the only unit of measurement still based on a man-made artifact. A second of time, for instance, is now defined in terms of an electron transition of the cesium atom. And the meter is tied to the speed of light. Those standards are universal and unchanging—unlike the official 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.

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 keep pulling its weight. I mean, mass.

--John Matson