The thyratron valve is essentially a triode which contains a small quantity of some inert gas, usually mercury vapour. If a suitable voltage be applied between anode and filament, with sufficient negative grid bias, the valve behaves like an ordinary vacuum tube; but, as the grid is made more positive, at a certain critical voltage an arc strikes between the anode and the filament, with enormous increase in the anode current. The grid then exercises no further control over the anode current, which must be limited by external resistance so as not to exceed the saturation emission current from the filament. If the voltage across the tube exceeds the “disintegration” potential, of some 20–25 volts, the cathode will be disintegrated by positive ion bombardment. Limitation of the anode current to the emission of the filament ensures that the voltage, while higher than the ionization potential of mercury, is considerably below the disintegration potential.