An arc converter is a device that accepts electrical energy and generates audible or radio frequency energy. It makes use of an electrical arc between two conductors that are part of an electrical circuit, and requires an initial high voltage to generate the arc and a steady current to sustain the arc. The generated arc contains a wide spectrum of frequencies and can be tuned to a specific frequency using an electrical tuned circuit. It is also referred to as Poulsen arc after its inventor, and also known as an arc transmitter.
The electric arc is made up of highly ionized high-temperature ions crossing through a gas such as air, which is mostly inert nitrogen. It takes about 70,000 volts (V) to arc across a 1-inch (2.54 cm) gap. Once the arc is started and there is temperature buildup at the ends of the conductors, there is sustained arcing. Consideration is needed to ensure the ends of the conductors are not molten because the temperatures of these ends can reach the melting temperature of conductive metals, such as copper.
Radio frequency alternating current (AC) was first generated using an electric arc and a resonant circuit. The generation of radio frequency from an arc converter relies on the presence of the arc of a tuned circuit across the arc. This tuned circuit is a conductor with both inductive and capacitive characteristics, which are distributed and result in a parallel resonance frequency wherein the reactive characteristics cancel each other, causing a sharp decrease in net current through the tuned circuit. The result in parallel resonance state is a state seemingly disconnected from the arc at a certain frequency. The only arc energy that may exist to an arc converter is the energy at the parallel resonant frequency of the tuned circuit.
The resonant or tuned circuit in the radio frequency range is usually less than a single turn of wire with a diameter of about 12 inches (30.5 cm). This wire becomes a loop antenna, which has distributed capacitive and inductive characteristics. A higher-output frequency requires a smaller loop diameter, while a lower-output frequency requires a bigger loop diameter. While the capacitive characteristics are brought by the formation of capacitance due to the close proximity of two conductors separated by an insulator that can host a significant amount of electric field intensity, the inductive characteristics are brought by the buildup of magnetic field around a conductor that tends to oppose further change in current flowing through the conductor. In early wireless communications, a direct current (DC) power supply with an arc converter was the only radio transmitter available.