The kilogram is getting an update

The Associated Press
John Leicester

SEVRES, France–The kilogram is getting an update.
No, your bathroom scales won’t suddenly become kinder and a kilo of fruit still will weigh a kilo. But the way scientists define the exact mass of a kilogram is about to change.
Until now, its mass has been defined by the granddaddy of all kilos: a golf ball-sized metal cylinder locked in a vault in France. For more than a century, it has been the one true kilogram upon which all others were based.
No longer.
Gathering in Versailles, west of Paris, governments are expected tomorrow to approve plans to instead use a scientific formulation to define the exact mass of a kilo.
The change is expected to have practical applications in industries and sciences that require ultra-precise measurements of mass.
And it will mean redundancy for the so-called “Grand K,” the kilo that has towered above them all since 1889.
Made of a corrosion-resistant alloy of 90 percent platinum and 10 percent iridium , the international prototype kilo rarely has seen the light of day. Yet its role has been crucial, as the foundation for the globally-accepted system for measuring mass upon which things like international trade depend.
Three different keys, kept in separate locations, are required to unlock the vault where the “Grand K” and six official copies collectively known as “the heir and the spares” are entombed together under glass bell-jars at the International Bureau of Weights and Measures in Sevres, on the western outskirts of Paris.
Founded by 17 nations in 1875 and known by its French initials, the BIPM is the guardian of the seven main units humanity uses to measure its world: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance, and the candela for luminous intensity.
Of the seven, the kilo is the last still based on a physical artefact, the “Grand K.” The metre, for example, used to be a metre-long metal bar but now is defined as the length that light travels in a vacuum in 1/299,792,458th of a second.
“This, if you like, is a moment of celebration because it’s like the last standard remaining from 1875 that will finally be replaced by new innovation,” Martin Milton, the BIPM director, said in an Associated Press interview.
“Everything else has been recycled and replaced and improved,” he noted. “This is the last improvement that dates back to the original conception in 1875.
“So that’s a tribute to what was done in 1875, that it’s lasted this long.”
Only exceedingly rarely, and exceedingly carefully, have the BIPM’s master kilos been gingerly taken out so that other kilos sent back to Sevres from around the world could be compared against them, to be sure they still were properly calibrated–give or take the mass of a dust particle or two.
The U.S. officially is a kilo country, too: it was one of the original 17 founders of the BIPM in 1875.
Its primary kilo is called K20 and was assigned to the country in 1889 by the BIPM, along with another, K4.
One kilo is equivalent to 2.2 pounds.
The U.S. also has six other platinum-iridium kilos: K79, 85, 92, 102, 104, and 105. They all are looked after by the U.S. National Institute of Standards and Technology, a branch of the U.S. Commerce Department.
To verify their mass, K20 and other kilos from around three dozen other countries were measured in Sevres against the BIPM’s master kilos in a painstaking calibration exercise from 1988-92.
Suffice to say that the update should, in time, spare nations the need to occasionally send their kilos back to Sevres for calibration against the “Grand K.”
Scientists instead should be able to accurately calculate an exact kilo, without having to measure one precious lump of metal against another.

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