Scientists Struggling to Make the Kilogram Right Again (NY Times)

キログラム原器が軽くなりつつある。原因は不明。

The kilogram is defined by a platinum-iridium cylinder, cast in England in 1889. No one knows why it is shedding weight, at least in comparison with other reference weights, but the change has spurred an international search for a more stable definition.

国際キログラム原器はパリ郊外のお城で厳重に守られ、1年に1回だけ金庫の鍵を持つ3人によってその重さが量られるそうなのだけど、そのイメージはまるで「キログラム教」の司祭みたいで笑ってしまった。で、新たな原器はどうやって作るか？

To update the kilogram, Germany is working with scientists from countries including Australia, Italy and Japan to produce a perfectly round one-kilogram silicon crystal. The idea is that by knowing exactly what atoms are in the crystal, how far apart they are and the size of the ball, the number of atoms in the ball can be calculated. That number then becomes the definition of a kilogram.

To separate the three isotopes of silicon, Dr. Becker and his team are turning to old nuclear weapons factories from the Soviet Union, where centrifuges once used to produce highly enriched uranium are able to produce the required purity of silicon...or 99.99 percent pure silicon 28.

というわけで純粋なシリコン28の結晶の球を使ってそこに含まれる原子数を決定するわけだけど、もちろん原子を1つ1つ数えるわけじゃあない。

the challenge of calculating the precise number of atoms in a silicon crystal is too imprecise with today's technology so they are refining a technique to calculate the kilogram using voltage...

...The idea of the watt balance is to measure the electromagnetic force needed to balance a reference kilogram. As long as the gravitational field is precisely known for the location of the experiment, the mass on the scale can be related to power...The definition of the kilogram would then be a measurement of that power or in terms of something that could be derived from it, like the mass of an electron.

つーことでワット天秤なるもので電気的に測定するらしい。