Control unit for operating the Franck-Hertz Critical Potential Helium-filled and Neon-filled tubes. Equipped with an output for half-wave rectified sawtooth acceleration voltages, adjustable upper and lower limits of the acceleration voltage, and integrated picoammeter amplifier for anode-current measurement. Allows recording of the acceleration voltage as a function of the anode current. A slow sawtooth voltage (approx. 6 Hz) is available with an interface or XY-recorder, and a repetition rate of 20 Hz is available for oscilloscopic observations. The anode current and acceleration voltage, as well as their upper and lower limits, can be represented directly in conjunction with the digital display with an adjustable threshold unit (U18620).