How does the Franck-Hertz experiment demonstrate energy quantization in atoms?

[Pushoam]

In the experiment, it is observed that the minimum value of anode current occurs periodically and its period multiplied with e is known as excitation potential energy.

I don't see the link between this periodicity and quantisation of energy levels of atom.
Could anyone please make this clear?

 

[Mentz114]

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In the experiment, it is observed that the minimum value of anode current occurs periodically and its period multiplied with e is known as excitation potential energy.

I don't see the link between this periodicity and quantisation of energy levels of atom.
Could anyone please make this clear?

From the wiki article.
It seems the periodicity is a resonance.When the KE is near a multiple of the 4.9eV energy then absorption is more likely.

They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away.[4] This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero.[5] A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.