Looking for a wave no eye could see
Maxwell’s equations predicted electromagnetic waves, but a prediction on paper was not yet a laboratory object. Heinrich Hertz built a transmitter with a spark gap and a simple receiving loop with another tiny gap.
When the transmitter sparked, a much smaller spark appeared in the receiver across the room. Hertz varied the geometry, reflected the radiation, and formed standing-wave patterns.
Diagram description
Animated physics diagram: hertz_radio_wave; showing time.
Maxwell’s wave entered the room
The new radiation reflected, refracted, interfered, and polarized like light. Its speed was consistent with the electromagnetic prediction. Light had acquired an invisible family.
Hertz did not foresee radio broadcasting and reportedly doubted practical applications. His apparatus answered a narrower question beautifully: can a changing electric current launch a field disturbance that travels through space?
The answer crossed the room as an invisible wave and announced itself with a spark almost too small to see.