The instrument measures current
Electrons accelerated through mercury vapor reach a collector only if they retain enough energy to overcome a retarding region. The historical observation is a current–voltage record, not a direct image of an energy level. Franck’s Nobel lecture connects a characteristic energy loss near 4.9 electronvolts with ultraviolet emission near 253.7 nanometres.
The causal reading is this: after an inelastic collision, an electron may lack the remaining kinetic energy required for collection. A current decrease can therefore indicate energy transfer to the atom. It does not mean electrons vanish, nor that every collision is inelastic. The location of collisions and the collection threshold matter to the measured signal.
Read the evidence: J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms
Spacing is more useful than one voltage
At higher accelerating voltages, electrons can regain energy after one excitation and excite another atom before collection. Repeated current structure can thus carry a characteristic energy spacing. For interpreting the teaching model, the approximate relation below connects the positive elementary charge, the voltage interval between corresponding features, and an excitation energy.
This is an approximate energy-accounting relation, not a fit to digitized historical data. Contact potentials can shift an apparent voltage origin; energy spread, vapor density, elastic scattering and the retarding field can alter a curve. One first dip alone should not be treated as an absolute excitation-energy measurement. Spacing must also be checked for consistency across several features.
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
Hertz’s 1968 recollection describes the early interpretation in terms of ionization and its later correction. An atom can absorb energy and remain neutral. Removing an electron is a different process. The historical distinction matters: recording a threshold was not the same event as correctly understanding it. Hertz also notes a lower metastable state whose weaker excitation probability made it inconspicuous under their conditions. The prominent spacing is not the smallest possible mercury excitation.
Franck’s lecture compares the collision energy with a particular ultraviolet line; Hertz’s lecture relates electron-impact results to Bohr’s stationary states. Their 1925 award concerns the laws governing electron impact on an atom. The lectures were delivered in 1926. Keep the experiment, the later interpretation, the award year and the lecture date separate.
Read the evidence: G. Hertz (1968), Memories of James Franck and the electron scattering experiments — Lindau recording and transcriptJ. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atomsG. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atomsNobel Prize in Physics 1925 — official award record
What the evidence rules out
A reproducible excitation scale challenges a description in which atomic internal energy accepts arbitrary small amounts without preferred thresholds. Agreement of a collision scale with radiation provides a cross-check between electrical and optical measurements. The inference is stronger than simply noticing that a graph has a dip.
It does not establish literal electron orbits, a complete mercury level scheme, or the absence of all classical effects in the apparatus. Those are different claims. An adequate comparison must let a proposed alternative predict the spacing and optical relation, while accounting for instrumental effects. Historical success is a constrained argument, not a universal proof from one curve.
Read the evidence: J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atomsG. Hertz, Nobel lecture (1926): The results of the electron-impact tests in the light of Bohr’s theory of atoms
Read the existing Labs as models
In the excitation Lab, scan accelerating voltage, retain the collection record, and compare several successive valleys. Change the available apparatus controls and ask whether the inferred spacing is more stable than the absolute first-valley position. Label the record as simulated; it is not a digitization of the 1914 experiment.
The hydrogen Lab supplies a separate spectrum comparison. The photoemission analyzer reverses the bookkeeping question: incoming light liberates an electron from a material, with a work function entering the balance. Neither its work function nor its displayed kinetic energy should be substituted for a mercury excitation threshold. The three pages connect methods without pretending they share one specimen or one historical dataset.
Read the evidence: N. Bohr (1913), On the Constitution of Atoms and Molecules, Part I, §§1–3J. Franck, Nobel lecture (1926): Transformations of kinetic energy of free electrons into excitation energy of atoms