English: A quenched or series spark gap, a type of spark gap invented in 1906 by German physicist Max Wien and used in spark gap radio transmitters into the early 1920s. It consisted of a number of metal disks (in this example 9) separated by thin mica ring spacers, creating a number of microscopic spark gaps in series. The wide outer surfaces of the rings serve as a heat sink and in operation are cooled by a fan. The handle at right attached to a screw putting pressure on the stack was used to control the width of the gaps, which were approximately 0.2 mm.

Spark gap transmitters, the first type of radio transmitter, generated rado waves by discharging a capacitor through a spark gap in a resonant circuit, generating oscillating electric currents. In an inductively coupled transmitter, the spark gap excites oscillations in the primary winding of a resonant transformer, which are coupled into the secondary winding which is attached to the antenna which radiates the energy. In the inductively coupled circuit with an ordinary spark gap, the oscillating energy shifts back and forth between the primary and secondary circuit, so much of the energy is dissipated as heat in the spark. The quenched gap was invented to reduce the damping of the inductively-coupled transmitter, to reduce its bandwidth so it did not interfere with transmitters on nearby frequencies. In the quenched gap transmitter, after the energy transfers to the secondary circuit and the oscillating current in the primary circuit momentarily goes to zero, the large surface areas of the spark electrodes cool the spark and absorb the ions, terminating (quenching) the spark and thus the primary currents. This allows the current in the secondary circuit and antenna to oscillate freely without additional energy loss, producing long "ringing" waves that were closer to continuous waves.