English: Schematic diagram of the Poulsen arc converter, an early radio transmitter invented by Valdemar Poulsen in 1903, from his 1904 paper. It was one of the first continuous wave radio transmitters capable of transmitting sound (AM), and was used in many of the first radio stations until the early 1920s, when it was replaced by vacuum tube transmitters. It consists of a continuous electric arc across a spark gap (SG) supplied with a DC voltage (V_DC) of about 500 volts. A resonant circuit consisting of a capacitor (C) and an inductor (L) is connected across it. The negative resistance of the arc cancels the inherent positive resistance of the tuned circuit, exciting sinusoidal oscillations in the tuned circuit.

The arc is inside a chamber of hydrogen gas, which allows higher frequency oscillations. The oscillations are encouraged by a magnetic "blowout", an electromagnet (M) that generates a magnetic field at right angles across the arc. At the peaks of the current the magnetic field exerts a a transverse force on the moving ions, "blowing" them out of the arc, cooling it, causing the current to decrease quickly. The radio frequency choke (RFC) prevents the oscillation current from passing through the power supply.

Alterations to image: added labels (red) to the parts