Lee Billings in Scientific American:
The most accurate thermometer in the known universe sits in a rather nondescript white building in Teddington, England, on the campus of the U.K.’s National Physical Laboratory (NPL). It looks nothing like a slender tube filled with mercury or colored alcohol. Instead, it’s a copper vessel about the size of a large cantaloupe, filled with dilute ultrapure argon gas and studded with microphones and microwave antennas, precisely shaped by a diamond-tipped lathe so that its radius varies with an uncertainty of only about 12 atomic layers of copper. The purpose of this thermometer is not really to measure temperature, however. Rather, new results from this and other similar devices could soon allow scientists to redefine temperature completely and bring it in line with the meter and other standard international units of measurement.
What the device actually measures is the relation between energy, as measured in joules, and temperature, as measured in the international standard unit, the kelvin. This relation is expressed as the Boltzmann constant and, in a perfect world, would be the kelvin’s ideal physical basis. That it’s not is purely a historical accident born of the fact that most of our planet’s surface is covered with liquid water, a substance which conveniently changes to ice or vapor at well-known thresholds of temperature.
More here.
