7B30.12 - Franck-Hertz : Xenon or Neon Gas

Xenon App.
Neon App.
Code Number:
Demo Title:
Franck-Hertz : Xenon or Neon Gas
Critical Potential in a Gas
Area of Study:
Modern Physics
Franck - Hertz Xenon Tube Unit, 0 to 20 VDC Power Supply, 2 - Variacs, 2 - 6 volt Transformers, Volt and Amp Meters, Oscilloscope.

This version of the Franck - Hertz experiment is built around the 2D21 electron tube that has an appreciable amount of xenon gas present.  Since we like to show our peak on an oscilloscope instead of plotting it out, our hookup is a little different than that described in the instruction booklet in the files. We impress a 60 cycle, 3 to 3.4 volt ripple on the 0 to 12 VDC side of the circuit so we can get a signal that can be seen on the oscilloscope.  Put a new 9 volt battery in the holder on the under side of the yellow Franck-Hertz board.  Step 1, plug the small 6.3 volt transformer into a variac and then the secondary lead from the transformer into the "heater" sockets on the board.  Set the voltage here at 6.3 volts.  Step 2, to produce the ripple on the DC input, take the + lead from the DC power supply and connect it to one side of another 6.3 volt transformer and the lead from the other side of the transformer to the red 12V / 24V. DC terminal on the board.   Connect the GRD terminal of the DC power supply to the G1-K terminal on the board.  Monitor the voltage coming out of the transformer producing the ripple with a volt meter.  Connect the Channel 1 oscillocope leads to the "mA Meter" inputs on the board.  Run the DC power supply at about 1.5 to 3 volts on the meter.  When the tube is warmed up you will need to adjust the power supply and ripple voltage settings to get the best oscilloscope graph display.  A 3.2 volt ripple and 1.5 volt DC power supply settings seem to work well. Usually the ripple can be held to around 3.2 volts when everything stabilizes. The meter amplifier switch must be to Experiment 1 on the Franck - Hertz unit. Some tweaking of the power supply voltages and the control knobs on the Franck - Hertz unit will probably be necessary to obtain the best trace. Only one peak will be obtained with this unit.  

The Neon Franck-Hertz unit is very easy to use.  Make the connections as shown and then turn all the knobs on the power supply to the off position.  The ramping voltage starts when the power supply is turned on.  After that, just turn up the "Filament", "Grid",  "Acceleration", and "Reverse Bias" knobs to the approximate settings and values marked on the power supply.  After that, just adjust as necessary to get the desired trace on the oscilloscope.  Three peaks should be clearly visible as shown in picture 4.

  • Harry F. Meiners, "Ionization Potential of Xenon", AJP, Vol. 33, # 5, May 1965, p.17.
  • W. Bolton, "Energy Levels", Book 3 - Atomic Physics, Physics Experiments and Projects, 1968, p. 27-28. 

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