IEEE Spectrum - North American - March 2016 - 17

top: national rESEarch council of canada; Bottom: SomEya laBoratory

standards at Canada's National Research
Council. As IEEE Spectrum went to press,
he and his colleague Richard Green were
working through a series of measurements that would calibrate K50 against
masses measured in the group's watt balance. The apparatus recently achieved
an uncertainty of just 18 micrograms,
currently the lowest published and well
below BIPM's requirement.
But a critical test will be how well other
measurements agree. Previous experiments, which effectively flip the wattbalance measurement around so that
masses are used to measure the Planck
constant, have not been in perfect accord.
BIPM hopes to receive all the test
masses by the end of March, although
the list of participants and their expected
arrival times was still being finalized
at press time. The comparison measurements will take some months, and
MaSSeS In MotIon: Canada will send a
cousin of this kilogram mass to Paris for a dry
the results are not likely to be available
run of the unit's redefinition. It'll be joined by
before next year.
masses from around the world.
The outcome of the test is uncerunder construction around the world. tain. So, too, is the fate of the kilogram
But to participate in this pilot study, a in 2018. "If everything goes well, [the
group must demonstrate that it can mea- comparisons] would confirm that the
sure a kilogram with an uncertainty of future system would work. If it's not
0.00002 percent or less, meaning it can't satisfactory, it could be an obstacle
[to the redefinition]," Stock says. "So
be off by more than 200 micrograms.
"We are ready to go," says Carlos it's definitely going to be interesting."
Sanchez, discipline leader for electrical -r achel courtl a nd

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.
"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
You are soft. It's nothing to be ashamed of. So
with great precision-one
sensors designed to interact with you should
possible way of determining
be soft, too. One big problem is that a pressure senthe kilogram. The remainsor's accuracy falters when the sensor is bent. And
ing groups will represent
bending is very important, because you bend. So
institutes that are working
engineers in Japan, led by Takao Someya, came
on watt balances, which
up with nanofiber-based sensors that change their
measure masses against
resistance under pressure. The fibers reorient themthe electrical force needed
selves when bent, so sensors stay accurate even
to resist their weight.
when curved around an 80-micrometer diameter.
There are a number of
watt balances and similar
For more, see http://spectrum.ieee.org/nanoclast0116
electromechanical balances

fEEl thE forcE

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MaR 2016

http://spectrum.ieee.org/nanoclast0116 http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2016