Electron number density measurement devices working under the conditions of non-steady-state high-pressure Thomson discharge are discussed. The necessary conditions for evaluating the electron number density and the production rate of the external ionization using Ohm's law are qualitatively stated. In addition, an algorithm for the space charge influence on the electrostatic field applied is derived. The theoretical estimations and some preliminary experimental results show the predictions of this algorithm to be sufficiently good. Some advantages and possibilities of use of the method are emphasized and discussed. Using the results obtained and/or summarized in this work, an easy interpretation of other results obtained using Ohm's law, neglecting the influence of avalanche ionization, is possible.