Franck-Hertz Experiment,interpretation of graph

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Homework Statement

As it is shown in the attached file,there is multiple maximums of the function.
i understand that the first one (about 4,9V) corresponds to resonant line of the atom.
What I don't understand is why the next one is higher than the first one, what does it mean?

Homework Equations

The Attempt at a Solution

I m speculating,that the next maximums could correspond to other energy levels.
If I m not mistaken,then the values of next maximums is multiples of the first maximum value,so
if the next maximums corresponds to the same energy level,why they are multiples no just continues function with equally placed maximums?

 

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Im sorry if the attachment isn t there.
Then there is some kind of problem.

But any Franck-Hertz Experiment graphical result will do ,to understand the question.