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Kilogram conundrum on the road to resolution

Posted on behalf of Hannah Hoag.

A longstanding effort to redefine the kilogram could be but a day’s drive away from its goal, researchers attending the annual meeting of the American Association for the Advancement of Science say.

The International Bureau of Weights and Measures has been trying to move away from the present definition of a kilogram — the mass of a 125-year-old platinum-iridium cylinder locked in a vault outside of Paris — towards a constant of nature, which, in this case, is Planck’s constant, h. It has proven to be one tough piece of science to crack.

Here’s where the trouble lies: in order to change the definition of the kilogram, two complementary — but different — approaches to measuring Planck’s constant must provide the same values.

The chemistry approach determines the mass of a single atom of highly enriched silicon. The physics approach uses a complex scale called a watt balance to measure the kilogram using electric and magnetic fields. Until recently, the values have been sufficiently divergent to prevent consensus.

But the National Research Council (NRC) Institute for Standards and Technology in Ottawa, Canada, has scored a bit of a coup. Three years after purchasing one of the world’s two working watt balances from the United Kingdom, the group has used both approaches, and their results agree to within 12 parts per billion, which pulls the value of h within the target range of uncertainty.

Dave Inglis, the head of the electric kilogram project at NRC, says that he had hoped the results would be enough to sway consensus so that the new definition could go forward. (The goal is to roll it out by 2015.) But the uncertainty remains too large. Also, the other watt balance (pictured) — housed at the National Institutes of Standards and Technology (NIST) outside Washington DC — yields results that differ by a small, but important, amount.

“For the first time, the two watt balances are a one day’s drive apart,” says Inglis. He is working with the team at NIST to find out “what’s going on,” scrutinizing the components of the system to find the factor that may be providing the offset. Ultimately, they will take the same standard kilograms and drive them between the two locations for cross-border weigh-ins.

Photo credit: NIST

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