COUNTING UP TO 100 MILLION. The science of measurement, metrology, has been moving away from standards based on artifacts such as a meter stick and toward the use of quantum phenomena to provide reliable, accurate and, if possible, portable calibrations that can be used by researchers in the field. Examples are resistance defined in terms of the quantum Hall effect (Update 205) and voltage in terms of the Josephson effect (Update 406). Consider capacitance, the measure of how well a tiny reservoir can store electrical charge. NIST already has the best capacitance standard, accurate to 0.02 parts per million (ppm). But this device is cumbersome and, more importantly, its accuracy is frequency dependent. For rendering the value of capacitance in circuits operating outside a certain frequency range, the standard is no better than 2 ppm. A promising new approach to capacitance (pioneered at NIST; contact Mark Keller, 303-497-5430) uses a single-electron transistor (SET), which contains at its heart a tiny refuge for electrons where the arriving charges can be counted one at a time, all the way up to 100 million or more. When combined with an accurate voltage measurement this becomes an accurate capacitance standard (C=Q/V). The SET approach has now achieved a measurement accuracy of about 2 ppm, and the NIST researchers hope soon to reach 0.1 ppm. The setup is relatively portable and its output is largely independent of frequency. (Keller et al., Science 10 Sept.)