Aug 202009
 

According to an interesting report on the NPR, the object that establishes the official and exact weight of a kilogram may be losing weight.

The international prototype of the kilogram is inside three nested bell jars at the Bureau International des Poids et Mesures in Paris.

The international prototype of the kilogram is inside three nested bell jars at the Bureau International des Poids et Mesures in Paris.

More than 100 years ago a salt shaker sized object made of platinum and iridium was forged in London. This was then shipped to Paris where it was shaped and polished and carefully weighed until it was exactly one kilogram (about 2.2 pounds). By international treaty, this then became the international for the exact weight of a kilogram. The problem is, the mass of the cylinder may be changing.

According to the NPR report:

As it stands, the entire world’s system of measurement hinges on the cylinder. If it is dropped, scratched or otherwise defaced, it would cause a global problem. “If somebody sneezed on that kilogram standard, all the weights in the world would be instantly wrong,” says Richard Steiner, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

For that reason, the official kilogram is kept locked inside a secured vault at the International Bureau of Weights and Measures near Paris. Scientists are so paranoid that they’ve only taken it out on three occasions: in 1889, 1946 and 1989. Each time, they’ve compared it to a set of copies. In 1889, the copies and the kilogram weighed the same, but by 1989, they had drifted apart. Based on the data, the kilogram appears to weigh slightly less than the copies.

The issue is, scientists aren’t sure if the official cylinder has gotten lighter, or if the copies have maybe absorbed water molecules and gotten heavier, but this is, for scientists a very big issue.

Physicist Richard Steiner adjusts the watt balance. This extremely sensitive scale can detect changes as small as ten-billionths of a kilogram.

Physicist Richard Steiner adjusts the watt balance. This extremely sensitive scale can detect changes as small as ten-billionths of a kilogram.

The solution is to try to develop a constant for the measure. As an example, consider how the meter is measured. Originally the meter was equal to the length of another standard  piece of metal kept alongside the kilogram, but in 1983 it was redefined as the distance light travels in a vacuum over 1/299,792,458 of a second. Because the speed of light is constant, this new definition means that the meter will never change.

Scientists are now trying to use a watt balance to establish a constant number for weight. I’ll let you read the whole story at NPR.

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