Initiation of discharges at electrical contacts

The conditions in which electrical discharges can occur at low-power contacts are surveyed, special attention being paid to the cases of make and break at medium voltage. The mechanism of production of the electrons which can initiate such discharges has been investigated experimentally by measuring the electron emission from the cathode under a succession of voltage pulses. Results showed that the emission was a cold-field process determined by the nature of the surface tarnish layers on the metal, the mechanism for thin (¿10¿7 cm) layers being different from that for thicker surface layers, which are usually oxide. The influence of the nature of the electrode surface and of the gas atmosphere was examined, and it was concluded that an insulating surface-tarnish layer was essential for the comparatively high rate of production of the initiatory electrons under the low electric fields (104 ¿ volts/cm) usually set up at small contact gaps. A theory of the mechanism is outlined.