Franck Hertz Experiment Simulation Using Neon Tube

Theoretical Background

• In 1913 Niels Bohr introduced his model of the hydrogen atom. One of the predictions was that the electrons occupied only certain energy levels. 
• This was in agreement with the observed spectrum, but physicists were eager to find another experiment that would also show this result.
• In 1914 James Franck and Gustav Hertz (nephew of Heinrich) performed an experiment on a vacuum tube with a small amount of mercury enclosed. 
• The tube was heated in an oven in order to vaporize the mercury, and then a series of voltages was applied to the tube. 
• A small voltage was used to heat a filament for use as an electron source. 
• Three more voltages were used to establish electric fields inside the tube.
• The first field is a small field, it is used in order to sweep the electrons away from the filament. 
• It is observed that when filaments eject electrons they become slightly positive, and the region around the filament becomes slightly negative due to the cloud of electrons. 
• If a small field isn't put in place to draw the electrons away from the filament, it becomes hard to draw out more electrons. 
• The second field is an accelerating field, this is what gives the electrons the bulk of their kinetic energy. 
• This is usually called the grid voltage because it is established by a grid that the electrons can penetrate. 
• Once the electrons go through the grid there is a reverse field that acts to retard electrons from the counter. 
• If there is only vacuum in the tube then the grid voltage will accelerate electrons to the counter, and if the retarding voltage is less than the grid voltage a current will be detected.

                                         Diagram of Franck-Hertz Tube

• In the Franck Hertz experiment simulation with neon, the low-pressure mercury vapor affects the detected current. 
• At low grid voltages the electrons gain kinetic energy. 
• They collide with mercury atoms, but these are elastic collisions, and since electrons have such a smaller mass than mercury the electrons retain most of their kinetic energy. As the voltage increases, so does the current. 
• However, once the electrons gain kinetic energy equal to the excitation energy of mercury they can have inelastic collisions - the kinetic energy of the electron exciting the mercury atom. 
• If an electron with exactly the excitation energy had an inelastic collision, it would have a final velocity of zero. 
• This is why the experiment also features a retarding field. Only electrons with a kinetic energy that can overcome the retarding field get counted in the current. 
• As the grid voltage is increased to the excitation voltage, the current will drop because many of the filament electrons have lost their kinetic energy to inelastic collisions and cannot overcome the retarding field. 
• If the observed current is plotted against voltage, there will be a series of peaks and valleys — this characteristic curve forms the basis of the Franck-Hertz experiment explanation.
• The peak-to-peak (or valley-to-valley) spacing will correspond to the excitation energy of the vapor (the multiple valleys are due to multiple inelastic collisions).

Current Measurements in the Franck Hertz Experiment Simulation

Principle of Work

• The Franck Hertz experiment simulation uses a tube consisting of an electron-emitting cathode, two mesh grids for accelerating electrons and an anode plate for collecting electrons. It is filled with a low pressure gas. 
• The mesh grids provide the accelerating potential to electrons and the anode is held at slightly negative potential to provide braking. 
• At low accelerating potential, the electrons gain kinetic energy and reach the anode plate with purely elastic collisions with the gas atoms. 
• As accelerating potential is increased the gas atoms absorb energy as they are excited due to in-elastic collision by the energized electrons. 
• Thus fewer electrons reach the anode plate and there is a dip in the plate current. 
• These dips in current are observed at approximately fixed intervals of accelerating potential which demonstrate the presence of discreet or quantum energy levels in the Bohr’s model of atom — the central finding of the Franck-Hertz experiment conclusion
• The Franck Hertz simulation further allows students to visually observe how varying the accelerating potential directly influences the measured current across the tube.