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The world’s smallest set of scales could weigh viruses, say researchers at
the Georgia Institute of Technology. By attaching tiny objects to the end of
carbon nanotubes, they have measured the mass of objects that weigh just a
thousandth of a picogram (Science, vol 283, p 1513). In the macro
world, a calibrated spring can measure the mass of an object if you know the
properties of the spring. A similar principle applies in the nano world: a
transmission electron microscope can measure resonant oscillations in the
nanotube when a small voltage is applied to it.