Nottingham effect of a superconducting metal

The emission of electrons from a metal by a strong electric field is connected with the thermoelectric Nottingham effect, which at low temperatures heats the cathode. We have studied this effect in the state of superconductivity (niobium) where other thermal effects disappear and more precise measurements become possible. The heat produced by the emission of one electron, 〈ε〉, is measured by a microcalorimetric method with use of a field-emitter tip. 〈ε〉 is determined by a comparison of the Nottingham heat flux with a reference heat flux produced by a calibrated electron bombardment. The experiments show that 〈ε〉 increases with field strength (3×${10}^7$ <F<7×${10}^7$ V/cm) from 0.1 to 0.3 eV per electron. The results are in reasonable agreement with theoretical predictions. Under certain conditions the Nottingham effect destroys the superconductivity.