Charles Susskind, "Observations of Electromagnetic-Wave Radiation before Hertz", Isis vol. 55 no. 1 (March 1964) pp. 32-42. https://www.jstor.org/stable/227753 — opened in Chrome; only the first-page preview is visible without login (no login attempted). Google snippets [web] say: Thomson, in 1875, "remembered an observation that he had made four years earlier, in 1871. He had used a Rühmkorff coil ..."; "between the vibrator and an overhead gas pipe turned the entire pipe into ..." (Edison); "Thomson drew sparks from the doorknob, from the eyepiece of the telescope"; "...son's tin still, Edison's overhead pipes, Dolbear's gilt kite and tin roof"; "... Thomson's 1871 experiments while his own paper was in proof"; "Thomson and Houston were science instructors in a secondary school." The paper is based in part on a paper read in Philadelphia, 1 Sept 1962 (first-page footnote, screenshot).
Karl T. Compton, "Elihu Thomson the Scientist" (1939). https://www.jstor.org/stable/1664186 — snippet only: "It was found that tiny sparks could be obtained from metal objects wherever they were, in the cases or outside, from the door-knobs or from apparatus". Not opened (time limit).
Bearing: priority 2/3 — a historian of technology (Susskind) places the 1871 Central High observations in the pre-Hertz record of radiated electromagnetic waves; this is his characterization. Houston's own 1871 and 1876 texts do not speak of waves.
Result: partial (paywall; preview and snippets only). Recheck: yes (obtain full text through a library).