Maxwell's prediction
In 1865 James Clerk Maxwell published four equations that unified electricity and magnetism. A consequence: changing electric fields should radiate energy at the speed of light. Nobody had ever seen such waves. Maxwell died in 1879 without proof.
The spark-gap transmitter
Hertz built a high-voltage induction coil that arced across two brass spheres. Each spark pumped a burst of electromagnetic energy into the air. Across the room a thin wire loop with its own tiny gap caught a faint spark in sync. Invisible waves had crossed the room.
Measuring the wavelength
Hertz bounced the waves off a zinc sheet and mapped the standing-wave pattern with his detector loop. Distance between nulls gave him the wavelength — about 66 cm. Multiplied by frequency, he got the speed of light. Maxwell was vindicated.
'Of no use whatsoever'
Asked about applications, Hertz replied that his waves were 'of no use whatsoever — this is just an experiment that proves Maestro Maxwell was right.' He died of vasculitis in 1894 at age 36, never seeing Marconi's wireless telegraph or the radar that would carry his name as a unit of frequency.