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?