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STM(1) (Synchronous Transport Module) The European designation of SONET channel speeds. See STS and SONET.
(2) (Scanning Tunneling Microscope) A microscope that can image down to the atomic level. An STM uses a piezoelectric tube with a tiny sharp tip at the end that is moved within nanometers of the object being sampled. As it gets close, the voltage applied to the tip causes a "tunneling" current to flow to the object. As the tip is moved and bumps into an obstruction, the current changes. In order to maintain a certain constant current, the tip is raised and lowered accordingly, and this movement pattern is magnified and displayed on screen. See AFM, microscopy and nanotechnology.
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