---
title: "Franck–Hertz: what a drop in current can show"
description: "Why does a loss of collected electrons reveal an atomic excitation energy?"
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language: "en"
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last_modified: "2026-10-15"
---

[Quantum physics: reading the evidence](https://sandboxphysics.com/stories/quantum-evidence/)

Nobel A · Evidence and control

# Franck–Hertz: what a drop in current can show

Sandbox Physics · 2026-10-15

Why does a loss of collected electrons reveal an atomic excitation energy?

## The apparatus measures a current

Electrons accelerated through mercury vapor reach the collector only if they keep enough energy to cross a retarding region. The historical observation is a curve of current against voltage; the energy level is inferred from it. Franck’s Nobel lecture links a characteristic energy loss near 4.9 electronvolts to ultraviolet emission near 253.7 nanometres.

After an inelastic collision, an electron may have too little kinetic energy left to reach the collector, so a drop in current can signal energy handed to an atom. The electrons do not vanish, and not every collision is inelastic. Where the collisions happen and where the collection threshold sits both shape the measured signal.

Read the evidence: J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms

## The spacing between dips tells more than one voltage

At higher accelerating voltages, an electron can regain energy after one excitation and excite another atom before it is collected. The current structure then repeats, and its spacing can carry a characteristic energy. To read the teaching model, the approximate relation below links the positive elementary charge, the voltage interval between matching features and an excitation energy.

The relation is an approximate energy balance; it is not fitted to digitized historical data. Contact potentials can shift where the voltage scale appears to start, and energy spread, vapor density, elastic scattering and the retarding field can change the curve. The first dip alone does not give an absolute measurement of the excitation energy. Check that the spacing stays the same across several features.

$$
\Delta E\approx e\,\Delta V
$$

Read the evidence: G. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atoms

## Excitation is not ionization

In his 1968 recollection, Hertz describes how the effect was first read as ionization and later corrected. An atom can absorb energy and stay neutral; removing an electron is a different process. The history shows that recording a threshold and understanding it correctly were separate steps. Hertz also notes a lower metastable state that was less likely to be excited and so barely showed under their conditions. The prominent spacing is therefore not the smallest excitation energy of mercury.

Franck’s lecture compares the collision energy with a particular ultraviolet line; Hertz’s relates electron-impact results to Bohr’s stationary states. Their 1925 prize was for the laws governing the impact of an electron on an atom, and the lectures were given in 1926. Keep apart the experiment, its later interpretation, the prize year and the lecture date.

Read the evidence: G. Hertz (1968), Memories of James Franck and the electron scattering experiments — Lindau recording and transcript J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms G. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atoms Nobel Prize in Physics 1925 — official award record

## What the evidence rules out

A reproducible excitation energy argues against a picture in which an atom can take up any small amount of internal energy, with no preferred thresholds. The collision energy also matches the emitted radiation, so electrical and optical measurements check each other. That makes the argument stronger than noticing a dip in a graph.

The evidence does not establish literal electron orbits, a complete level scheme for mercury, or the absence of every classical effect in the apparatus. A fair test of an alternative asks it to predict both the spacing and its link to the optical line, after allowing for instrumental effects. The historical case rests on these specific checks; one curve is not a universal proof.

Read the evidence: J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms G. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atoms

## The Labs are simulations, not historical data

In the excitation Lab, scan the accelerating voltage, keep the collected-current curve and compare several successive valleys. Change the apparatus controls and see whether the inferred spacing stays steadier than the position of the first valley. The curve is simulated; it is not digitized from the 1914 experiment.

The hydrogen Lab offers a separate spectrum comparison. The photoemission analyzer turns the energy question around: incoming light frees an electron from a material, and the work function enters the energy balance. Its work function and displayed kinetic energy cannot stand in for a mercury excitation threshold. The three pages share methods, not a specimen or a historical dataset.

Read the evidence: N. Bohr (1913), On the Constitution of Atoms and Molecules, Part I, §§1–3 J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms

## Continue with the models

These teaching Labs produce simulated data, which are separate from the historical observations discussed above.

- [Scan electron-impact excitation](https://sandboxphysics.com/labs/franck-hertz-excitation/)
- [Compare atomic spectra](https://sandboxphysics.com/labs/hydrogen-orbitals-spectra/)
- [Compare photoemission energy accounting](https://sandboxphysics.com/labs/photoemission-analyzer/)

## Sources and reading notes

We read Bohr’s paper and the transcript of Hertz’s retrospective lecture as primary texts. The Nobel lecture PDFs are linked for further reading; downloading them directly returned HTTP 403 while this article was prepared. No historical curve is reproduced or digitized here.

1. [N. Bohr (1913), On the Constitution of Atoms and Molecules, Part I, §§1–3](https://www.gutenberg.org/cache/epub/72787/pg72787-images.html)
2. [J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms](https://www.nobelprize.org/uploads/2018/06/franck-lecture.pdf)
3. [G. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atoms](https://www.nobelprize.org/uploads/2018/06/hertz-lecture.pdf)
4. [G. Hertz (1968), Memories of James Franck and the electron scattering experiments — Lindau recording and transcript](https://mediatheque.lindau-nobel.org/recordings/31437/memories-of-james-franck-and-the-electron-scattering-experiments-german-presentation-1968)
5. [Nobel Prize in Physics 1925 — official award record](https://www.nobelprize.org/prizes/physics/1925/summary/)

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