James Franck and Gustav Ludwig Hertz measured the transfer of kinetic energy from free electrons into atomic excitation. In Mercury vapor, the collected electric current changes repeatedly as the accelerating voltage rises. Below an excitation threshold, an electron cannot excite the relevant atomic quantum state; above it, an inelastic collision can remove a characteristic energy. Repeated acceleration and energy loss produce the successive features, linking the Franck-Hertz experiment to quantized atomic structure.^[ref][ref]

Physics.^[ref]

Gustav Ludwig Hertz and James Franck accelerated electrons through Mercury vapor and measured the collected electric current as the accelerating voltage changed. Repeated features in that curve indicate a preferred loss of kinetic energy in inelastic collisions. An electron must first acquire enough energy to excite the relevant atomic transition; further acceleration can enable another such collision. The Franck-Hertz experiment therefore connects an electrical current–voltage measurement with discrete atomic excitation energies.^[ref]

Physics.^[ref]

Louis de Broglie proposed associating a wave with material particles, relating its wavelength to the particle momentum by $\lambda =h/p$. This connects a particle quantity with an experimentally testable wave scale: increasing the momentum decreases the expected diffraction wavelength. His account of matter waves helped motivate a description in which standing-wave conditions select allowed atomic motions, and his Nobel lecture discusses the relation between this proposal and later electron-diffraction experiments.^[ref]

Theoretical physics.^[ref]

George Paget Thomson demonstrated wave behavior of electrons by passing an electron beam through thin material and recording diffraction patterns. Many crystal orientations in a thin polycrystalline film produce rings, whose geometry can be compared with the wavelength associated with the electron momentum. The experiment connects a photographic pattern with the de Broglie wavelength, making electron interference observable at energies and in a geometry different from a reflected crystal beam.^[ref]

Physics.^[ref]

An experimental atomic physicist who demonstrated a thermal atomic beam of sodium in 1911. Vapor escaped through narrow slits into an evacuated vessel and condensed on a wall cooled with liquid air. The deposit matched the geometry predicted for straight-line trajectories, giving a visible test of molecular motion between collisions and a foundation for later molecular beam experiments.^[ref]

Harvey Fletcher worked with Robert Millikan on the development of the oil drop experiment. His recollection describes replacing rapidly evaporating droplets with oil and observing individual charged drops for longer times. The measurement combines motion under gravity and Stokes drag law with the effect of an applied electric field. Estimating the drop's size and comparing its motion before and after charge changes makes small increments of electric charge measurable. His account therefore connects the experimental choice of droplet material with the precision of the charge measurement.^[ref]

Physics and acoustics.^[ref]

An experimental atomic physicist who developed ultraviolet spectroscopy at wavelengths strongly absorbed by air. Evacuated optical apparatus made this region experimentally accessible, and the Lyman series of hydrogen became a fundamental example of atomic line structure. The apparatus work mattered as much as the wavelength measurement: removing absorption along the optical path was necessary to observe the radiation at all.^[ref]

Paul Horowitz works on experimental electrical circuits and astronomical instrumentation, including radio and optical searches for extraterrestrial signals. His institutional account describes designs that combine optical collection with fast photodetectors.^[ref]

Such searches turn a candidate pulse into a measurement problem involving timing, noise, detector response and independent checks. Instrument sensitivity and interference rejection determine which signals a null result can exclude; absence of a detection alone does not specify every possible source model.

An experimental physicist who studied electron diffraction by sending cathode rays through thin celluloid films. His contemporary scientific obituary identifies this research and the definitive paper that followed the preliminary report. A transmitted electron beam developing an angular diffraction pattern tests the matter wave description of electrons, connecting a particle beam produced in a vacuum apparatus with phenomena familiar from wave optics.^[ref]

Hugo Atterling's scientific memoir of Manne Siegbahn explains how apparatus and source composition shaped early X-ray spectroscopy. One example traces two unexpected lines in a 1914 spectrum to a silver layer beneath a platinum coating, observed in first and second diffraction orders. The account connects identification of a chemical element with the interpretation of the instrument, rather than treating each observed line as a new property of the nominal target. His memoir also documents the development of precision and vacuum spectrometers.^[ref]

Physics.^[ref]

Werner Heisenberg's 1925 reformulation of quantum mechanics replaced unobservable classical electron orbits with quantities attached to transitions between stationary quantum states. Transition frequencies combine as ${\nu }_{nm}+{\nu }_{mk}={\nu }_{nk}$, suggesting the indexed product rule that became matrix multiplication. The resulting physical quantities need not commute. This approach made measured spectral lines and transition strengths the starting data for a new dynamics rather than requiring a miniature classical planetary model of the atom.^[ref]

A physicist.^[ref]

Enrico Fermi developed neutron bombardment as a way to create and investigate radioactive isotopes. Unlike a charged projectile, a neutron does not face the same repulsive potential barrier when approaching a positive atomic nucleus. Induced radioactivity supplies a delayed readout of reactions that occurred during irradiation. His Nobel lecture describes how comparing target materials, projectile conditions and the subsequent decay products connected experimental activation with the structure and transformations of atomic nuclei.^[ref]

A physicist.^[ref]

Edoardo Amaldi coauthored the 1934 investigation of hydrogenous materials and neutron-induced radioactivity. The enhanced activation observed with paraffin wax showed that the material between source and target could improve the production of radioactive nuclei. Neutron moderation supplies the physical connection: collisions change the neutron energy distribution and hence the chance of a later nuclear reaction.^[ref]

A physicist.^[ref]

An experimental nuclear physicist who coauthored the study of hydrogenous substances in neutron-induced radioactivity. Placing paraffin wax around a source and target could enhance activation rather than merely absorb the radiation. Collisions with light nuclei slow the neutrons, connecting neutron moderation to the probability of subsequent nuclear reactions.^[ref]

Emilio Segrè coauthored experiments showing that hydrogen-rich material can enhance neutron-induced radioactivity. Placing paraffin wax around the irradiation arrangement changed activation rather than merely shielding the target. Collisions with light nuclei slow the neutrons, altering their subsequent nuclear-reaction probabilities. This made the surrounding material part of the experiment and helped establish neutron moderation as a method for controlling induced radioactivity.^[ref]

A physicist.^[ref]

Bruno Pontecorvo helped uncover the importance of slowing neutrons in induced radioactivity. While investigating the activation of silver, he noticed that the supporting material changed the resulting activity. Comparing a wooden table with other surroundings led to tests of hydrogen-rich materials and the enhancement caused by slower neutrons. His later neutron well-logging work used returning radiation to probe geological layers, connecting the same moderation physics with an applied measurement.^[ref]

A physicist.^[ref]

An experimental atomic physicist who coauthored the molecular-beam method for measuring nuclear magnetic moments. Instead of determining a moment solely from a beam's static deflection, the experiment found the Larmor frequency in a known magnetic field. A resonance-induced loss of focused beam intensity made this frequency measurable with high precision.^[ref]

An experimental atomic physicist who coauthored the Rabi molecular beam magnetic resonance experiment. Two nonuniform-field magnets focused a beam onto a detector, while an oscillating field in the intervening uniform region drove magnetic transitions. At resonance, changed magnetic states disrupted the focusing and reduced the detected intensity, allowing nuclear magnetic properties to be measured through a frequency.^[ref]

Polykarp Kusch and Henry Michael Foley measured atomic magnetic resonance frequencies to determine the electron magnetic moment. Different quantum states weight orbital angular momentum and spin contributions differently, allowing ratios of magnetic responses to isolate the spin contribution. The inferred electron spin g-factor differs slightly from two. The Kusch-Foley magnetic moment experiment thus probes a microscopic interaction through accurately counted frequencies, rather than a mechanical measurement of an isolated electron.^[ref]

A physicist.^[ref]

Felix Bloch developed the nuclear-induction description of nuclear magnetic resonance. A precessing transverse magnetization generates an oscillating magnetic flux and therefore a voltage in a surrounding coil. The signal provides direct access to the collective motion of nuclear magnetic moments. His equations include both precession in the applied magnetic field and relaxation toward equilibrium, connecting the coherent nuclear motion with the decay and recovery measured in an experiment.^[ref]

A physicist.^[ref]