Frank Hertz Experiment

James Franck and Gustav Hertz first studied the Franck Hertz experiment in 1914 and presented it to the German Physical Society. Furthermore, it was the first electrical measurement of its kind that clearly showed the quantum nature of atoms. Moreover, this experiment was crucial in transforming the understanding of the world.

Introduction to Frank Hertz Experiment                                                       

Franck Hertz experiment involves a vacuum tube whose design is such that it facilitates the study of the energetic electrons that flew via a thin vapour of mercury atoms. Moreover, the experiment led to the discovery that only a specific amount of the kinetic energy of an atom would lose as the collisions of the electrons take place with the mercury atoms. Also, this energy loss corresponds to decelerating an electron’s speed from about 1.3 million meters per second to zero.

An important point to remember here is that a faster electron does not completely decelerate once a collision takes place. However, the electron would lose precisely the same amount of its kinetic energy. Furthermore, slower electrons tend to bounce off mercury atoms without showing any significant loss of kinetic or speed energy.

franck hertz experiment

                                                                                                                       Franck Hertz Experiment

The Experiment


The aim of the Frank-Hertz Experiment procedure is to demonstrate the concept of quantisation of the energy levels in accordance with the Bohr’s model of an atom.

Materials required for this experiment

  1. A control unit for power supply
  2. A DC amplifier
  3. Oven
  4. Mercury filled Franck-Hertz tube
  5. Neon filled Franck-Hertz tube


The original experiment made use of a heated vacuum tube of temperature 115 °C. Furthermore, the drop of mercury was of the vapour pressure of 100 Pa. Also, the fitting of the tube takes place with three electrodes.

Three electrodes that are fitted include an electron-emitting hot cathode, a metal mesh grid, and an anode. Furthermore, to draw emitted electrons, the voltage of the gird is positive with respect to the cathode.

The measurement of the electric current in the experiment is the result of the movement of the electrons from the grid to the anode. Moreover, the electric potential of an anode is slightly negative in comparison to the grid so that the electrons have the same kinetic energy as in the grid. Also, the explanation of the experiment took place in terms of elastic and inelastic collisions between the electrons and the atoms of mercury.

The graph shows the dependence of the electric current that flows out of the anode and the electric potential present between the cathode and the grid.

The Observations

The graph shows the following observations:

  1. With the steady increase in the potential difference, there would be a steady increase of the current via the tube.
  2. The dropping of the current is almost to zero at 4.9 volts.
  3. Again, there is an increase in the current as the increase in the voltage takes place to 9.8 volts.
  4. Again, an observation of a similar drop takes place at 9.8 volts.

One can clearly see the energy absorption from electron collisions in case of Neon gas. Furthermore, when the accelerated electrons excite the electrons in neon to upper states, they de-excite in such a manner that there is the production of a visible glow in the gas region in which is occurring the excitation.

There are about ten peak electrons levels that lie in the range of 18.3 to 19.5 eV. Furthermore, their de-excitement takes place by dropping to lower states at 16.57 and 16.79 eV. Moreover, this energy difference is the reason why the light is in the visible range.

FAQs For Frank Hertz Experiment

Question 1: Explain the Frank-Hertz Experiment setup?

Answer 1: The Frank-Hertz experiment setup includes a control unit for power supply, DC amplifier, oven, mercury-filled Franck-Hertz tube, and neon-filled Franck-Hertz tube.

Question 2: Differentiate between elastic and inelastic collision?

Answer 2: An elastic collision refers to an encounter that takes place between two bodies in such a way that the total kinetic energy of the two bodies remains the same. In contrast, an inelastic collision is defined for the two bodies for whom conservation of the kinetic energies does not take place due to internal friction.

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One response to “Atomic Spectra”

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