Comparison of the classical Thomson (1912) cross section for ionization of H, O, Mg and Fe atoms by electron impact with experimental results of Shah et al. (1987); Thompson et al. (1995); McCallion et al. (1992); Shah et al. (1993), respectively. The plot shows the ratio of the experimental values with respect to the theoretical cross section, plotted against the collision energy E scaled by the ionization energy I. The errors in the experimental values are in all cases typically 5% or less, with an additional uncertainty of around 7–14% in the absolute scale.