English: A rotary spark gap from 1919 for use in a spark gap radio transmitter. It consists of a wheel spun by a motor with 8 electrodes around the rim, electrically connected. The two stationary electrodes are positioned so the moving electrodes pass by the stationary ones simultaneously. When two moving electrodes approach the stationary ones, sparks jump simultaneously between the two stationary and moving electrodes, allowing current to pass from one stationary electrode to the adjacent moving one, then through the wheel to the other moving electrode and through the second spark to the other stationary electrode

This was used in the tuned circuit of low power spark transmitters, connected between the capacitor and the coil (inductor). The capacitor was charged to a high voltage of 3-20 kilovolts by a supply transformer. When a rotating electrode approached the stationary electrode a spark started, discharging the capacitor through the coil. Because of the inductance of the coil the discharge was oscillatory, an oscillating current passed back and forth through the spark gap charging the capacitor plates alternately positive and negative. The coil was connected to the antenna, so the energy in this oscillating current was radiated as radio waves. When the rotating electrodes moved far enough away from the stationary electrodes the spark stopped (quenched) and the remaining energy in the coil was radiated by the antenna. The spinning wheel created sparks at a rapid rate in the audio frequency range, from 50 to 1000 sparks per second. The transmitter radiated one pulse of damped radio waves per spark, so in a radio receiver the signal from the transmitter sounded like a tone or buzz. The operator transmitted information by tapping on a switch called a telegraph key, turning the transmitter rapidly on and off, transmitting pulses of radio waves which spelled out text messages in Morse code.