Scanning tunnelling microscope. Scanning electron micrograph of the needle of a scanning tunnelling microscope (STM). The micromechanical STM's needle (at lower right) is just a few atoms wide at the tip and acts as an electrode. This STM can read data written as atomic-scale mounds or holes on a silicon chip. The STM works by bringing the needle to within a few ten millionths of a millimetre of the sample's surface. Electrons in the sample and tip interact and create a small tunnelling current. The needle is scanned across the sample, vertical movements maintaining tip-to-surface distance. These movements are processed to give a map of the surface. Magnification: x1520 at 6x7cm size. Magnification: x2300 at 10x12cm size.