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Published online 24 August 2009 | 460, 1070 (2009) | doi:10.1038/4601070a
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Canada assumes weighty mantle
Instrument to help redefine the kilogram makes a transatlantic move.
In the mission to define the kilogram more sensibly, only two of the instruments known as 'watt balances' have proven good enough to tackle the job. And one of them is currently in pieces, having been sold and shipped from the United Kingdom — the birthplace of this type of device — to Canada.
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I'm confused..the Watt balance is supposed to be used for defining the kilogram, yet the article states that it has as one of its parts a 'precisely known mass'; against what has this mass been determined? Presumably the standard kg in Paris, ultimately. Surely the point of a new definition of the kg is that we no longer need to refer to the cylinder in Paris?
Yes, that was not explained very well...
At present the idea is to measure Planck's constant (units m^2 kg/s) using a kilogram mass which is derived from the Paris standard. If different experiments agree then the value of Planck's constant will be defined to be a particular number with no uncertainty. Then the metal cylinder kilogram standard can be discarded, and the kilogram is redefined as the mass that gives the specified value of Planck's constant when put into a watt balance.
Thank you! I understand now...