When the accelerating voltage reaches 4.9 volts, the current sharply drops, which is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms.  
The electrons at the center of this experiment are accelerated in its namesake apparatus and the collected current rises with accelerated voltage.  When the accelerating voltage reaches 4.9 volts, the current sharply drops, which is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms.  This suggests that mercury electrons cannot accept energy until they reach the threshold for elevating them into an excited state.  FTP, identity this experiment named after its two conductors which proved the Bohr hypothesis of energy quantization.
The electrons at the center of this experiment are accelerated in its namesake apparatus and the collected current rises with accelerated voltage.  
Franck-Hertz experiment