In the study of the phenomena of electrical breakdown and of the motions of electrons and ions in gases, it has been almost universal to record the experimental data as functions of the parameter E/p, the ratio of electric field strength to gas pressure. This practice was introduced by J. S. Townsend, who first pointed out that the energy distribution of the electrons and ions in the gas is in most cases predominantly determined by the ratio of electric field strength to the gas number density N. The parameter E/p was satisfactory while all experiments were done at room temperature and while the variation in the constant of proportionality between N and p, resulting from the use of different (often unquoted) ambient temperatures, was small compared with experimental error.