Reasons why the Franck Hertz experiment might fail

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The Franck Hertz experiment using mercury vapor yielded excitation potential intervals of 5.75 and 6.40 V, which deviate from the expected 4.9 V. Possible causes for the discrepancies include a faulty tube, air mixture issues, or the need for oscilloscope calibration. Additionally, the quality of cables and solder joints should be checked, as well as grounding connections. The capacitance of glass could also affect results, emphasizing the importance of not touching the tube during the experiment. Ensuring the filament is adequately warmed up is also crucial for accurate measurements.
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We conducted the Franck Hertz experiment with mercury vapour and got intervals for the excitation potential of 5.75 and 6.40 V. I know that the intervals should be 4.9 V.
The potentials were measured using an oscilloscope at the most convenient scales.

I just can't think what may have gone wrong other than faulty tube/air in mixture (although we should still see some intervals at 4.9 V I think?) or oscilloscope needs callibrating. The apparatus was allowed to heat up a good ten minutes before we started.

What else could have affected things?
 
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Check your cables. Sometimes the solder joints to the plugs at the end of the cables break.

Check your grounding.

Glass has capacitance and can accumulate charge but any good company will usually coat their tubes will an extremely thin metallic film to dissipate it.

Because of the capacitance, don't let anyone touch the tube during the experiment.

Did you give the filament a little time to warm up?
 

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