A substantial lowering of the work function of a tungsten Langmuir probe immersed in an argon plasma in a discharge tube is observed. The mechanism is thought to be similar to the Schottky effect. The electron temperature measurement is lowered about 30%, in a particular discharge condition, as the probe goes from room temperature to 2000°C. The charge number density measurement, by saturation electron current appears to be higher by 15%, and that by ion saturation current appears to be higher by almost one order of magnitude. This phenomenon can also account for the discrepancy of the ratio of ion and electron saturation current obtained by previous authors and this author.