In his proof of the existence of radio waves Hertz used the setup depicted in the following figure:
A sparc is created at regular intervals when the induction coil (2) creates a potential difference high enough to cause the sparc.
Wikipedia describes the process as follows:
Hertz's first radio transmitter: a dipole resonator consisting of a pair of one meter copper wires ending in 30 cm zinc spheres. When an induction coil applied a high voltage between the two sides, sparks across the center spark gap created standing waves of radio frequency current in the wires, which radiated radio waves. The frequency of the waves was roughly 50 MHz, about that used in modern television transmitters.
How does the process work by which radio waves are created when a sparc passes the gap?