Abstract: To explain non-monotone variation trend of anode current with the increasing of the control grid voltage, the mechanism of the control grid voltage on the anode current is discussed with a series of IA-UG2K experimental curves based on Franck-Hertz experiment. Besides the positive effect of control grid voltage, whose essence is to restrain the space charge limited currents （SCLCs） , this paper bases on Ramsauer-Townsend effect, the Kinetic-Molecular theory and the collision space and probability theory, focus on giving the negative effects in which the increase of the control grid voltage can cause the diminishing of the cathode background current and the rise of the inelastic collision probability. Depending on the above mentioned theory and the cathode emission theory, we introduce the changing of the dominant role of these dual effects in different control grid voltage intervals, which eventually makes the ＂dual contrary effects＂ action mechanism complete.

Key words: Franck-Hertz experiment, control grid voltage, Ramsauer-Townsend effect, collision probability, space charge limited current