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Experimental investigations show that Langmuir probe measurements can give quite erroneous results, due to work function changes of the probe, insulating layers on the probe surface, or different work functions at different places of the probe. The deposition of evaporated cathode compounds, the adsorption of oxygen released from oxide cathodes, and the sputtering and evaporation from the probe may change the work function of a pure tungsten probe in time scales of seconds. In oxide cathode tubes especially, it is necessary to measure immediately after the probe has been thoroughly cleaned. An X ‐ Y plotter, which recorded the whole probe characteristic in seconds, was found to be most suitable. No deviation from the Maxwellian velocity distribution of the plasma electrons could be found over a range of nearly four orders of magnitude in probe current, even in oxide cathode tubes under conditions where such deviations have been found by other authors.