The use of commercial tubes permits a considerable amount of experimentation without the need for vacuum apparatus. In this note I won't go into the details of the experiments but will only outline the experiments that are described. Detailed explanations of the concepts may be found in almost any text on plasma physics. The second experiment is of particular interest because of the importance of the Langmuir probe in plasma diagnostics. A Langmuir probe is nothing more than a wire that is inserted into a plasma to measure its potential.
Early experimenters let the probe float and measured the voltage with a high impedance meter.
That gave totally erroneous measurements because the floating probe would permit charges to accumulate. Langmuir's technique involved connecting the probe to a source of variable potential. The probe voltage is swept and the resulting current vs. voltage characteristic, will yield the electron and ion currents to the probe. A central electrode, sheathed in a glass tube for most of its length, serves as the Langmuir probe. In the experiment, a discharge is struck between the anode and cathode.