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  • PHY 350 Franck-Hertz Experiment

    Significance of the experiment

    In their 1914 experiment, James Franck and Gustav Hertz demonstrated that in collisions between accelerated electrons and gaseous atoms, energy was only transferred from the electrons to the atoms if the electron had gained an energy before the collision equal to or greater than the energy required to excite the atom from its ground state to its next lowest energy level. If it had more energy than needed, the electron only transferred the amount corresponding to the energy difference between the two atomic energy levels. That is, atoms have discrete, quantized energy states, providing early evidence for the validity of quantum theory. Franck and Hertz won the 1925 Nobel Prize in recognition of this result.

    Conceptual Introduction

    Review the introduction to the Franck-Hertz effect provided by the excellent HyperPhysics website. There are 5 key panels on this website that you should read:

    1. introduction, apparatus and sample data

    2. interpretation of data

    3. how the apparatus works

    4. sample data for mercury (Hg)

    5. sample data for neon (Ne)

    In addition, familiarize yourself with the more detailed description in (Melissinos 2003) on pages 10 - 19.

    Questions for first lab class

    A few short questions about this experiment and the apparatus are listed below. Review the sketch of the Franck-Hertz apparatus — particularly the functions of the cathode, anode, and grid.1 Write short answers to these questions in your lab book; bring the lab book to class.

    1. The gas of atoms is contained inside a glass chamber. Electrons are emitted from a metal surface called the cathode. How are electrons emitted from the cathode?

    2. The electrons gain kinetic energy (between collisions) by accelerating electrons towards a grid.How are the electrons accelerated towards the grid(s)?

    3. Are the atoms comprising the gas also accelerated? If so, when, and under what conditions?

    4. Some electrons accelerated towards the grid actually pass through it, and are then collected at the anode. A device — typically an electrometer or current preamp — is used to measure the current carried by these electrons. Why does the transfer of energy from an electron to an atom result in a dip in the electron beam current?

    5. What is the purpose of the negative voltage (counter voltage) between the anode and the grid?


    1. Sometimes there are two grids, one to initially accelerate the electrons away from where they are emitted towards the gas and one to accelerate the electrons towards the anode after their collisions.

    Activities for first lab class

    Your lab instructor will introduce you to apparatus used to observe the Franck-Hertz effect for collisions of electrons with Ne atoms. Discuss your answers to the questions above (which you answered in preparation for this class) and use the answers to explain how the apparatus works.

    Note that for the neon and argon apparatuses, there are two grids: \(G_1\) and \(G_2\).1 An example of this two-grid style apparatus and the circuit used to control it are shown in Figure 1.7 of (Melissinos 2003) on page 15; an example of just the apparatus tube is shown below in Fig. \ref{fig:TwoGridFHtube}.


    1. In the mercury apparatus, there is only one grid. The \(G_1\) (control) grid is missing; the one remaining grid plays the role of \(G_2\)