WEIGHING VIRUSES. A "nanobalance," a vibrating carbon nanotube that can directly weigh microscopic organisms and particles piggybacked onto it, has been demonstrated by researchers at Georgia Tech (Walt de Heer, 404-894-7879). First discovered in 1991, a carbon nanotube is essentially a sheet of carbon atoms (arranged in a hexagonal pattern) rolled up into a nanometer-diameter tube that is capped at both ends with carbon hemispheres. In the present experiment, the researchers viewed their nanotubes, protruding from a carbon fiber, with an electron microscope. The fiber in turn was attached to a gold wire, mounted on an insulator. This setup allowed the researchers to send an electrical current through the nanotubes. Putting the whole assembly on a special sample holder put it only 5-20 microns away from an oppositely charged electrode. Applying an oscillating electrical voltage to the wire, opposed to this electrode, caused the tubes to vibrate. Tuning the oscillation frequency to the natural "resonance frequency" of a given nanotube would cause it to vibrate by a maximum amount. Attaching a particle to such a nanotube would change this resonance frequency, enabling researchers to deduce the mass of the particle. With this technique, the researchers measured a graphite particle to be 22 femtograms (22 quadrillionths of a gram). In general, this technique can determine the mass of particles with similar dimensions in the range of femtograms to picograms (trillionths of a gram) . This includes viruses. (Poncharal et al., Science, 5 March 1999; see figure at Physics News Graphics.)