Cathode spot, a brightly glowing spot on the surface of the cathode. It arises in the transition from a glow discharge to an arc discharge, because the basic mechanism of electron generation changes: in the simplest case field emission gives way to thermionic emission, the emission region contracts from practically the whole surface of the cathode into a small cathode spot, and the temperature in the area of that spot rises sharply and reaches the melting or sublimation temperature. Depending on the material and geometry of the cathode and on the magnitude of the current, mechanisms other than thermionic emission are also possible in the transition from a glow discharge to an arc (for example explosive electron emission, or a plasma cathode).