For physicists, measuring temperature takes more than reading a column of mercury. They would like to define it in terms of a physical constant, just as length is described with respect to the speed of light (one meter isthe distance traveled by light in an absolute vacuum in 1/299,792,458 of one second). Currently the basic unit of temperature, the kelvin, is awkwardly defined as 1/273.16 the difference between absolute zero and the triple point of water—that is, when water's gas, liquid and solid phases may coexist at a certain pressure. One kelvin spans the same increment as one degree Celsius.

Now physicists have invented an electronic thermometer that ties temperature directly to a fundamental number—namely, the Boltzmann constant, a value related to the kinetic energy of molecules. (The constant is typically abbreviated in high school chemistry as k or kB.) The device centers on the fact that in an array of tunnel junctions—thin, insulating layers sandwiched between electrodes—the electrical conductance can change in a manner directly proportional to the Boltzmann constant multiplied by the temperature.

Although such Coulomb blockade thermometry, as the technique is called, already appears in some specialized devices, fluctuations in the electronic properties of existing versions make them unreliable at very low temperatures. The new thermometer, made by scientists at the Helsinki University of Technology, works down to 150 millikelvins. Moreover, the Finnish physicists say it can be mass-produced using standard semiconductor manufacturing methods. They describe their work in the November 14 Physical Review Letters.