How much does a kilogram weigh?

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, even with regular steam baths to remove the crud. Because the kilogram gains about 50 micrograms every century, 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 1023).

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.

Read more here Holy Moley – Science News.

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