Hertz's 1888 finite-velocity tests: sparks, wire nodes and qualitative interference

Primary source and reading limits. Heinrich Hertz, 'On the finite velocity of propagation of electromagnetic actions' (Berlin Academy, 2 February 1888; Wiedemann's Annalen 34 p. 661), in the authorized English Electric Waves translation (D. E. Jones, Macmillan 1893), Chapter VII, printed pp. 107-123; the complete chapter narrative was read in the Internet Archive OCR of electricwavesbe00hertgoog, whose tables are visibly scrambled and have not been reconstructed or recomputed, and the rest of the volume was not read. This is a primary-report translation with an OCR limitation, not a modern replication or a measured-$c$ precision source. The account below is the report's apparatus descriptions, procedures and reported observation indicators, structured by the record contract of Report-backed operational evidence with uncertainty.

A spark-excited primary conductor with brass end plates and a central spark gap drove rapid oscillations; circular or square secondary conductors with micrometer-adjusted spark gaps served as qualitative detectors; the comparison used waves on a long wire versus direct action through air, with tuned secondary orientations and adjustable additional wire length; the room was darkened, and reflections and nearby fixtures were apparatus concerns (chapter VII narrative, printed pp. 107-123).

Locating nodal points of the wire standing wave by moving the tuned secondary along the wire and marking their positions with paper riders; comparing spark strength and suppression at different positions along a defined baseline; lengthening the added wire from 100 cm to 600 cm in 50 cm increments and repeating the comparison (printed pp. 117-118); changing the distance between the primary and secondary circuits; and comparing the direct air-path action with the wire-wave action (chapter VII narrative).

Orientation-dependent spark responses of the secondary detector; approximately recurring and roughly equally spaced nodal points along the wire; and changing signs of the interference indicator as the path length or the distance is varied. These are the reported observed indicators; the finite phase delay and the propagation speed of the order of light speed are the author's model-dependent inferences, requiring his assumed oscillator, detector, wire-wave and air-path phase interpretation.

The report expressly treats its inferred absolute period from circuit theory as doubtful beyond order of magnitude; it supplies no modern calibrated spark-intensity transfer function, no independent absolute period calibration, no trial-level likelihood or sampling model, no confidence interval and no synchronized-clock travel-time measurement; the numerical tabular signs cannot be faithfully recovered from the OCR used, and no quantitative fit, exact speed or uncertainty interval is claimed here. The author's rough order-of-magnitude remarks are not confidence intervals.

The described apparatus observations qualitatively support finite-propagation and wave-interference models under the stated auxiliary assumptions; they do not deductively prove the Maxwell equations, do not exactly establish the value of $c$, and do not eliminate every alternative, since the comparison involves the assumed oscillator, detector and wire-wave models. The historical decomposition into separately propagating electrostatic and electromagnetic components is not promoted to a modern Lorentz-covariant theorem. The source limitations travel into any downstream historical or empirical conclusion, and the support is a nonlogical relation to the wave predictions.