An electrical discharge in a gas in which the electrons that arise on ionisation themselves produce further ionisation. According to the theory of the avalanche discharge (the English physicist J. S. Townsend, 1901), each electron produces α ionisation events per unit length of its path to the anode (α is the first Townsend coefficient). Ionisation by secondary electrons leads to an exponential growth in the number of electrons reaching the anode. Because positive ions reproduce new electrons, a non-self-sustained discharge passes into a self-sustained one. The theory was later refined to take account of space charge and of the diffusion of the charge carriers, but its main features have been kept for the description of steady avalanche discharges at low pressure (the glow and the arc discharge). At pressures close to atmospheric and higher, the avalanche mechanism is responsible for the phenomena of electrical breakdown. Discharges of this type are explained by the theory of streamers.