The Raether limit is the physical limiting value of the multiplication factor (M) or gas gain in an ionization avalanche process (Townsend avalanche). Even though, theoretically, it seems as if M can increase without limit (exponentially), physically, it is limited to about M < 10⁸ or αx < 20 (where α is the first Townsend coefficient and x is the length of the path of ionization, starting from the point of the primary ionization. The multiplication factor or gas gain is of fundamental importance for the development of proportional counters.