Recent determinations of attachment and ionization coefficients have been carried out using a number of different techniques including (i) a method due in the first instance to Lozier using an electron beam (Craggs, Thorburn, and Tozer 1957; Tozer, Thorburn, and Craggs 1958), (ii) a development of Townsend's Driginal method of determining ionization coefficients by the measurement of ionization currents between parallel plane electrodes of variable separation (Geballe and Harrison 1952, 1953), and (iii) a modification of Doehring's method depending on measurements of time of flight (Chanin, Phelps, and Biondi 1959). In experiments of the second and third type it is not possible to estimate experimentally the mean energy of the electrons and, if the coefficients are to be expressed as functions of electron energy, it is necessary to use the results of independent experiments.