When the accelerating voltage reaches 4.9eV (lowest energy required by the mercury atom for excitation), the current drops sharply. This drop is due to inelastic collisions between the accelerated electrons and electrons in the mercury atoms. The sudden onset suggests that the mercury electrons cannot accept energy until it reaches the threshold to elevate them to an excited state. Collected current drops at multiples of 4.9eV. At 9.8 V, each electron gets sufficient energy to participate in two inelastic collisions. They excite two mercury atoms, and no energy will be left. This process will repeat, for each interval of 4.9eV.