Heinrich Hertz

A spark excites a resonant electrical oscillation.

Hertz used an AC RLC system whose resistance, inductance and capacitance set a resonant frequency. High voltage across a small gap broke down the air and created a spark. The resulting rapid oscillation of charge in the loop produced changing electric and magnetic fields.

Those fields did not remain confined to the wire. Part of the energy radiated outward as electromagnetic waves, exactly as Maxwell's theory predicted.

A tuned loop detects energy without a wire.

Across the laboratory Hertz placed a second loop connected to another resonant circuit. When tuned to the transmitter's frequency, the receiving loop developed an oscillating voltage. A tiny spark appeared across its own gap.

The receiver had no metallic connection to the transmitter. The spark was therefore direct evidence that an electromagnetic disturbance had carried energy through space.

The waves behaved like light.

Hertz's experiments verified propagation, reflection, interference and a speed consistent with the speed of light. The result confirmed Maxwell's central prediction and supplied the experimental basis for radio communication, although practical transmitters, detectors and antennas still had to be developed.