Why is the first peak in the Franck-Hertz experiment longer than the others?

[Terrycho]

Homework Statement

In Franck-Hertz Experiment, why is the spacing to the first peak different than the spacing between successive peaks?

Relevant Equations

λ=hc/E

In the experiment, I know that the spacing between successive valleys gives the excitation energy to be somewhere around 4.9eV. However, when you look at the plot, you can see that the spacing from zero to the first peak is much longer than any other spacings between two successive peaks. I was just wondering why that one is so much longer.

 

[DrClaude]

I don't understand what you mean. The first peak is at 4.9 V and the peaks are separated by 4.9 V.
https://en.wikipedia.org/wiki/Franck–Hertz_experiment#/media/File:Franck-Hertz_en.svg

 

[Terrycho]

It does look like the first peak is at 4.9V. However, when I did the experiment, the first peak was not 4.9V. It was larger than that. It seems like this plot also has the first peak is larger than 4.9V.
https://foothill.edu/psme/marasco/4dlabs/4dlab8.html

 

[Terrycho]

I don't understand what you mean. The first peak is at 4.9 V and the peaks are separated by 4.9 V.
https://en.wikipedia.org/wiki/Franck–Hertz_experiment#/media/File:Franck-Hertz_en.svg

I found this explanation,

The contact potential is the difference between the work functions of the cathode and anote, since they are oppositely directed in the electric field, that is, the electric field has to work against the cathode potential but is helped in the case of the anode. Thus we should expect that the voltage to the first peak will be greater than the average peak to peak voltage, due to the con- tact potential. The contact potential can be calculated as the average peak to peak voltage sub- tracted from the first peak voltage.

But does not quite make sense to me.

http://instructor.physics.lsa.umich.edu/adv-labs/Franck_Hertz/franck-hertz.pdf