File:60 kW Poulsen arc transmitter labeled.png

English: Drawing of US Navy 60 kW Poulsen arc transmitter, manufactured by Federal Telegraph Co., with parts labeled. The Poulsen arc was an early radio transmitter invented in 1903 by Valdamar Poulsen that was one of the first that could generate continuous waves. It was used until the early 1920s when it was replaced by vacuum tube transmitters. The Poulsen arc transmitter consisted of an electric arc of about 500 volts supplied by a DC power supply (not shown) between two electrodes which were connected to a resonant circuit made of a capacitor and inductor (not shown), which was in turn connected to the antenna. The negative resistance of the arc excited radio frequency oscillations in the tuned circuit. The arc burned in a closed chamber (center) containing a hydrocarbon vapor such as alcohol (contained in cup at top), in between the poles of a powerful electromagnet (visible above and below chamber). One of the electrodes (entering at left side) was copper, water-cooled, and the other (entering at right side) was made of carbon and rotated continuously.