The dynamic characteristics of a discharge can be determined by making square‐wave changes of arc current. This is done by putting the discharge tube in the plate circuit of a group of pentodes connected in parallel and supplied with a square‐wave signal superimposed on a d.c. bias.Probes operated at constant current are very useful in recording the characteristics of a modulated discharge. For measuring electron temperature (Te), two identical probes are centered about points having the same space potential. If the probes are operated at different fixed currents on the straight line part of the probe characteristic, the voltage between them is proportional toTe. Longitudinal voltage gradient or radial potential differences may be recorded by use of identical probes operated at the same current.A probe operated at a suitable current will follow roughly the variations in space potential of the adjacent region of the discharge. Another probe made sufficiently negative with respect to such a probe will collect only positive ions. A probe made 0–10 volts positive with respect to the constant‐current one gives records from which electron concentrations can be computed. In this case, a bias connection without current flow is obtained by use of a cathode‐follower with two stages of direct‐coupled amplification.The oscillograph amplifier used for dynamic probe records must be either a differential one or one with ungrounded power supply. In the latter case, the stray capacitance is many times larger; but fairly reliable records can be obtained if the arc current is 0.1 ampere or more, and if the time constant for the decay of the transient effects being measured is at least 10 microseconds. The oscilloscope amplifier and any preamplifier used for recording spectral intensities should be direct‐coupled.