A loudspeaker (or loud-speaker or speaker) is an electroacoustic transducer; a device which converts an electrical audio signal into a corresponding sound. The first crude loudspeakers were invented during the development of telephone systems in the late 1800s, but electronic amplification by vacuum tube beginning around 1912 made loudspeakers truly practical. By the 1920s they were used in radios, phonographs, public address systems and theatre sound systems for talking motion pictures. The most widely-used type of speaker today is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal input is applied through the voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move rapidly back and forth due to Faraday’s law of induction, which causes a diaphragm (usually conically shaped) attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound. Speakers are typically housed in an enclosure which is often a rectangular or square box made of wood or sometimes plastic. Where high fidelity reproduction of sound is required, multiple loudspeakers may be mounted in the same enclosure, each reproducing a part of the audible frequency range (picture at right). In this case the individual speakers are referred to as “drivers” and the entire unit is called a loudspeaker. Miniature loudspeakers are found in devices such as radio and TV receivers, and many forms of music players. Larger loudspeaker systems are used for music, sound reinforcement in theatres and concerts, and in public address systems.