Townsend put forward the hypothesis that positive ions also produce ion pairs, introducing a coefficient  expressing the number of ion pairs generated per unit length by a positive ion (cation) moving from anode to cathode. The following formula was found
since , in very good agreement with experiments.
The first Townsend coefficient ( α ), also known as first Townsend avalanche coefficient is a term used where secondary ionisation occurs because the primary ionisation electrons gain sufficient energy from the accelerating electric field, or from the original ionising particle. The coefficient gives the number of secondary electrons produced by primary electron per unit path length.