Number 206 (Story #2), December 8, 1994 by Phillip F. Schewe and Ben Stein AN ELECTRONIC MICRO-REFRIGERATOR , a device that removes hot electrons from already-cold metal electrodes, has been built by scientists at the National Institute of Standards and Technology in Boulder, Colorado (contact John Martinis, 303-497-3597). In their experiment, the NIST researchers removed hot electrons flowing through a copper electrode. Attached to the copper electrode is the refrigerator, a "tunnel junction" consisting of a superconducting aluminum metal separated from the copper electrode by means of a thin barrier. Only electrons higher than a certain energy can travel through the barrier. In the NIST setup, electrons approach the tunnel junction, and the higher-energy ones leave the metal through the tunnel junction. The remaining electrons circulate through an electrical circuit and return to the copper electrode through a superconducting lead contact which allows electrical current to flow without the transmission of heat from the circuit. In the first demonstrations of this method, Martinis and colleagues lowered the temperature associated with the electrons from 100 mK to 85 mK. This technique, analogous to blowing the hot steam molecules from a cup of coffee to make the coffee cooler, has the potential to cool low-temperature electronic devices further so that they can make more sensitive, less noise-laden measurements. Such devices could include the instruments on the balloon-borne experiments that measure the cosmic background radiation. (M. Nahum et al., Applied Physics Letters, 12 December 1994.)