Massive News: Kilogram Redefinition Heads Into Crucial Test
This will be interesting, it’s not surprising that the tests will take place in the same locale, since local gravity varies (in very minute ways,) by local density and latitude.
The world’s metrologists aim to change this state of affairs in 2018 by fixing the kilogram to the Planck constant, a fundamental physical constant. That shift would, at least in principle, allow any laboratory to “realize” the kilogram from scratch with a series of experiments and specialized equipment. But for that scheme to work, the kilogram derived by one laboratory must be the same as those derived by others. [For details of how this is done, see “The Kilogram Reinvented,” IEEE Spectrum, May 2012.]
“The question is, How would these independently realized kilograms agree?” says BIPM’s Michael Stock, who is leading the pilot study. Stock expects roughly half a dozen groups to weigh in. Two will hail from the national metrology institutes of Germany and Japan, which maintain ultrapure silicon spheres whose characteristics were recently measured with great precision—one possible way of determining the kilogram. The remaining groups will represent institutes that are working on watt balances, which measure masses against the electrical force needed to resist their weight.
There are a number of watt balances and similar electromechanical balances under construction around the world. But to participate in this pilot study, a group must demonstrate that it can measure a kilogram with an uncertainty of 0.00002 percent or less, meaning it can’t be off by more than 200 micrograms.