= Hertz spark transmission and detection experiment
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In his 1888 reflection experiments, <Heinrich Hertz> excited a straight conductor with a central <spark gap> driven by an <induction coil>. The receiver was a wire circle of $35\,\mathrm{cm}$ <radius> with an adjustable small <spark gap>; tiny sparks were observed in darkness. There was no connecting transmission wire. <Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity through Space by Heinrich Hertz>[Hertz, Electric Waves (1893 translation), VII, pp. 108–109; VIII, pp. 125–127]{ref}
He removed hanging metal fixtures, installed a $4\,\mathrm m$ by $2\,\mathrm m$ zinc reflecting sheet, and moved and rotated the receiver. Spark strength alternated with position and gap orientation, supporting <specular reflection> and <wave interference>. He checked observations from farther away because the observer's body perturbed the response. Changing oscillator dimensions changed the spatial pattern. <Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity through Space by Heinrich Hertz>[Hertz, Electric Waves, VIII, pp. 125–127 and 135–136: preparations and controls]{ref}
These tests supported propagating <electromagnetic waves>, rather than mere detection of nearby induction. They did not establish unlimited reception distance. His wire-versus-air <wave interference> experiment addressed finite <velocity>; he later judged its quantitative velocity comparison doubtful because of room effects. <Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity through Space by Heinrich Hertz>[Hertz, Electric Waves, VII, pp. 107–108; Introduction, pp. 9–12: retrospective limitations]{ref}
The physical inference combines transmission with a spatially structured response to a reflected disturbance. In an ideal monochromatic incident-and-reflected model, <standing wave> nodes are separated by $\lambda/2$; combining a reliable <wavelength> with independently known <frequency> gives <phase velocity> $v=\nu\lambda$. A spark is a threshold event, however: its presence or length is not a calibrated <wave intensity> or a direct arrival-time measurement. The actual loop also responds to its orientation and finite size, so the simple standing-wave model is an explanation of the test's logic, rather than a reconstruction of Hertz's measured points. <Poynting theorem> explains how source <energy> can reach and drive the receiver's discharge.
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