The constants A and B interpolate the first Townsend coefficient α α = A p e − − B p / E . They are determined experimentally and found to be roughly constant over a restricted range of E / p for any given gas. For example, air with an E / p in the range of 450 to 7500 V/(kPa·cm), A = 112.50 (kPa·cm) and B = 2737.50 V/(kPa·cm).