File:Poulsen's arc radiotelephone transmitter.jpg

English: First Poulsen arc radiotelephone transmitter built by Valdamar Poulsen around 1904. The Poulsen arc was an early radio transmitter technology that was used briefly until the mid 1920s when it was superseded by vacuum tube transmitters. It was the first transmitter which produced undamped sinusoidal waves and so could be amplitude modulated (AM) to transmit sound.

The photo shows Poulsen's first AM transmitter. The Poulsen arc is on the right. It consists of a rectangular arc chamber containing two electrodes to which a DC voltage of about 500 volts was applied, creating a continuous electric arc. A resonant circuit consisting of a capacitor (on desk to right of arc chamber) and inductor (behind arc chamber) was connected across the arc. The negative resistance of the arc canceled the positive resistance of the tuned circuit, exciting sinusoidal radio frequency oscillating currents in the circuit. The arc was between the poles of an electromagnet (cylindrical objects attached to each side of the chamber) which created a magnetic field across the arc. The arc chamber was filled with alcohol vapor from alcohol dripping from the cup on top. These two features enabled the arc to oscillate at a high enough frequency for radio transmission. The tuned circuit was attached through the transmission line on the wall to an outdoor wire antenna which radiated the energy as radio waves. The carbon microphone (center) was connected directly in the antenna wire. The sound waves striking the microphone's diaphragm put varying pressure on carbon granules, varying its resistance, thus varying the current allowed through to the antenna. To handle the high power of the transmitter the microphone consists of three separate units attached to a single mouthpiece.