The emission of electrons from cold metals by electric fields. In order to build up sufficiently large electric fields, the metal is usually shaped to a sharp needle point. Field emission is an example of the tunnel effect in quantum mechanics, with an electron in the metal being in a potential barrier. In field emission the probability of tunnelling, which can be calculated using the semiclassical approximation, is related to the work function of the metal. An important application of field emission is the field-emission microscope.