Reasons why the Franck Hertz experiment might fail

[sunrah]

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?

 

[AbigailM]

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?