In the late 19th century, cathode rays—lights emerging from the negative electrode inside near-vacuum glass tubes—attracted considerable interest. Lenard attached a very thin aluminum foil, later called the “Lenard window,” to the glass wall and managed to let the rays escape the tube. This allowed him to measure how cathode rays travelled several centimeters in air, made fluorescent screens glow, and charged objects electrically. By studying their deflection in electric and magnetic fields he showed that cathode rays were streams of negatively charged particles, providing quantitative support for earlier observations by Crookes. His findings encouraged J. J. Thomson’s discovery of the electron and set the stage for Einstein’s quantum explanation of the photoelectric effect. Within the broader development of modern physics, Lenard’s experimental techniques became indispensable for probing the micro-world of electrons and atoms. Discharge-tube technology subsequently evolved into X-ray tubes and cathode-ray tubes, with far-reaching applications in medical imaging and television. Thus, research on cathode rays profoundly influenced both science and society.

Lenard modified the conventional Crookes tube by attaching an aluminum foil only a few micrometres thick, thereby separating the vacuum inside from ambient air while allowing cathode rays to pass through. He discovered that the intensity of the rays decayed exponentially in air and that the absorption coefficient scaled with gas density, providing the first systematic study of energy loss of charged particles in matter. By bending the rays in electric and magnetic fields he determined their specific charge e/m to be about 1.3×10^11 C kg⁻¹, a value later confirmed by Thomson and consistent with a universal constituent he named the “Korpuskeln” and we call electrons. He also investigated the secondary electrons, fluorescence, and ultraviolet radiation produced when cathode rays strike glass or metal, opening quantitative discussions of energy conversion processes. The concept of the Lenard window was directly inherited by later beta- and alpha-particle counting cells and by the optical windows of photomultipliers. Moreover, Lenard observed that irradiating a metal surface with cathode rays caused the emission of electrons, a phenomenon equivalent to the internal photoelectric effect that Einstein cited in his 1905 paper. Improved measurement precision came from home-built electrometers and balance techniques, prototypes of today’s beam monitoring instrumentation. Lenard’s results influenced plasma physics, surface science, accelerator technology, and provided essential data for the industrial and medical use of electron beams. Although his later political activism is controversial, the evaluation of his scientific contributions must be separated from his ideological stance. There is no doubt that clarifying the nature of cathode rays was an indispensable step toward quantum theory and electronic engineering, and the 1905 Nobel Prize recognized this experimental breakthrough.

Lenard implemented a few-micron-thick Al window to extract cathode rays into air, allowing him to record range distributions and to provide the first empirical dE/dx curves for electrons. Although predating Bethe’s stopping-power formulation, his data already hinted at the proportionality of linear energy transfer to medium density and became reference points in radiation physics. For the e/m measurement he introduced compensating electrostatic fields that mitigated space-charge broadening, a technique described in detail in his laboratory reports. He quantified the extreme-UV radiation generated at the impact region with a rudimentary spectrometer and discussed the excitation mechanism of glass fluorescence. His “Kanalstrahlen” studies extended to the dynamics of negative ions, providing benchmarks for the nascent field of ion spectroscopy. The Lenard window concept was inherited by modern accelerator beamline extraction foils and cryogenic target cells, where Al or Be windows remain standard. In 1928–1931 he expanded his interfacial model of the photoelectric effect, introducing a “Lenard coefficient” for electron emission efficiency, though his estimates underestimated the temperature dependence of quantum yield. His series of papers in Zeitschrift für Physik and Annalen der Physik contain unusually detailed schematics of apparatus, compensation circuits, and derivations of deflection theory. While now classical, the work provides invaluable primary documentation for the historical traceability of solid-state electron emission models, radiation detector design, and electron beam transport theory.