The kilogram may finally get a break from its yo-yo diet. An international team of scientists is closer to redefining the unit of mass based on fundamental constants, instead of a piece of metal in France that loses weight only to put it back on again.
Since 1889, the international standard for the kilogram has been a cylinder of platinum, tucked under a glass jar inside another glass jar, stored in a vault outside Paris. But despite exceedingly stringent storage conditions, the cylinder (and six exact copies of it) gains weight from dust in the atmosphere, necessitating regular steam baths to remove the crud. On top of that, the seven cylinders change mass relative to each other by micrograms per century, for reasons no one can fully explain. So scientists want to redefine the basic metric unit of mass based on something that’s truly constant, just as the meter is defined as the distance light travels in one three-hundred-millionth of a second.
Several large teams have been attempting to define the kilogram in terms of the Avogadro constant, well-known to chemistry students as the number of atoms or molecules in one “mole” (about 6.022 times 10
23
).
A team of scientists based in Germany measured the constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28. The researchers chose silicon because its atoms tend to line up in crystal formation, eight atoms to a cube. Engineers ground and polished the spheres for two years to near perfection — if the spheres were enlarged to the size of the Earth, the tallest hill would be 9 feet tall, according to says Arnold Nicolaus, a physicist at the Physikalisch-Technische Bundesanstalt in Germany.
In principle, counting the atoms is like estimating how many Coca-Cola cans are in a giant mound of 12-pack cartons, says Nicolaus. Simply measure the mound’s volume and calculate how many cartons will fit inside, taking into account how the cartons are spaced. The researchers did that for the silicon sphere by measuring the distance between its atoms using X-ray interferometry.
The researchers found the Avogadro constant to be 6.02214084 times 10
23
, with an uncertainty of only 30 parts per billion, they write in a paper published online October 12 at arXiv.org. The hunk of metal in Paris, for all its faults, is still a little more reliable than that. Its mass is uncertain to 20 parts per billion.
“We’re getting very close. It’s a big step,” said Edwin Williams, an emeritus physicist at the National Institute of Standards and Technology in Gaithersburg, Md.
But the problem now is that not everyone agrees on the true number for the Avogadro constant. American and English projects have tried measuring the constant a different way, by balancing gravity’s force on a mass with the electrical forces needed to suspend it in midair, but repeated trials gave inconsistent measures of Avogadro constant. Currently, the Avogadro number is set by a committee that averages the results from different experiments.
The beauty of a constant, however, is that it doesn’t matter which number you choose, Williams says. As long as the Avogadro constant stays constant, it will be useful for defining the kilogram.
