This replica of a kilogram is on display at the National Institute of Standards and Technology, which is part of Department of Commerce. The domes are to protect it from environments that might alter it.
First, the kilogram vote passed on Friday! We now have a scientifically defined kilogram. Second, I was interviewed by Vox News for their daily podcast on Friday, November 16, 2018. I’m in the second half of the interview.
The metric system (or SI as it is known around the world) was first implemented in France back in 1795. Since then, almost every country in the world has adopted this set of measures with the United States being one of the few holdouts from full adoption. (The others are Liberia and Burma/Myanmar.)
Back In 1799 the meter was defined by a prototype meter bar. Later, a scientific standard for the meter was defined in 1960, and was redefined in 1983. It is currently the length of the path that light travels in a vacuum in 1/299,792,458 of a second.
If you have the precision equipment to make that measurement, all those “meters” around the world are exactly the same.
In contrast
To this day, the definition of the kilogram is a carefully protected platinum-iridium prototype that is the kilogram. It is held by the International Bureau of Weights and Measures (or the Bureau International Poids et Mesures) outside of Paris. Should anything happen to that physical standard it could theoretically change what the kilogram is around the world.
Having a physical standard/prototype has inherent problems. There are additional physical standards or “artifacts” that are stored around the world which are periodically compared to the one in France to make sure they all have the same mass. However, over time, the duplicate kilograms have “drifted” away from that the one in France. (Several of these prototypes are held by our own National Institute of Standards and Technology in Maryland.) That’s a problem when things like oils from people’s skin or even dust could impact its mass if it is not perfectly protected. And perfect, in this sense, is impossible.
As a result, a scientifically defined standard has been sought—until recently—without success. Much of this quandary was captured in Robert P. Crease’s book, World in the Balance: The Historic Quest for an Absolute System of Measurement, which was published in 2012.
Crease relates that two different technologies were being applied to solve the problem of creating the kilogram in the laboratory. One was the “Avogadro method” that “…realizes the mass unit using a certain number of atoms…”
(I’m not going to go into a lot of detail here because I’ve yet to understand it myself.)
Crease also relates…
The “watt balance” approach, on the other hand, ties the mass unit to the Planck constant, via a special device that exploits the equality of SI units of mechanical and electrical power. p. 255
(Again, very complicated.)
You can read a Vox News story that explores more of the science here.
Today, on November 16, the International Bureau (of which the U.S. is a member) will vote to determine if the scientific standard for the kilogram will be based on the “watt balance” method.
Should that occur (and it is expected to pass) the new standard, will go into effect on May 20, 2019.
Why May 20 next year? Because May 20 is the 144th anniversary of the signing of the 1875 Treaty of the Meter. That document gave the General Conference on Weights and Measures “…the international authority that ensures wide dissemination of the SI and modifies the SI as necessary to reflect the latest advances in science and technology.”
This is a developing story so stay tuned to this page for further developments.
Linda
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