How Much Does a Kilogram Weigh?

As a recent New York Times article observes, the kilogram is officially defined as “a unit of mass equal to the mass of the international prototype of the kilogram.” Well, it turns out that the prototype, a chunk of platinum and iridium housed at the International Bureau of Weights and Measures in France, has lost a bit of weight since it was made in the 1880s. The builders of the prototype did their best to design for the long-term, choosing a 90% platinum / 10% iridium alloy for its corrosion resistance and good thermal properties, sheltering it with bell jars and a vault, and minimizing its surface area. Time, however, has proven their efforts insufficient. The New York Times points out that the method for standardizing the kilogram has been going out of style:

The kilogram is the last base unit of measurement to be expressed in terms of a manufactured artifact. (Its cousin, the international prototype of the meter, was retired from active duty in 1960, when scientists redefined the meter. They redefined it again in 1983; a meter is now officially “the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second,” for those who would like to try it at home.)

Scientists now have similarly bold plans for the kilogram, and indeed for several other base units of measure. A draft resolution to be considered at the General Conference of Weights and Measures in October includes new and improved definitions for the ampere, the mole and the candela.

“This would be the biggest change in metrology since the metric system was introduced during the French Revolution,” Dr. Quinn said.

Which is all very exciting and very revolutionary. But it is easier said than done.

Indeed, we all take these standards for granted, but they are one of the things that allow us to build on the past and conceive into the future.  Their definition may seem esoteric, but one only has to go to a gas station in a country without standards enforcement to see the potential pit falls of a lack of them.  Moving into the future with standards not defined by physical items, the Bureau of Weights and Measures discusses some of the difficulties they face, such as the degree of uncertainty in Planck’s constant, here on its website.

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