A crystal radio receiver is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna. It gets its name from its most important component, known as a crystal detector, originally made with a piece of crystalline mineral such as galena. This component is now called a diode.
Crystal radios are the simplest type of radio receiver and can be handmade with a few inexpensive parts, like an antenna wire, tuning coil of copper wire, capacitor, crystal detector and earphones. They are distinct from ordinary radios because they are passive receivers, while other radios use a separate source of electric power such as a battery or the mains power to amplify the weak radio signal from the antenna so it is louder. Thus crystal sets produce rather weak sound and must be listened to with sensitive earphones, and can only pick up stations within a limited range.
Crystal detectors were developed and applied to radio receivers in 1904 by Jagadish Chandra Bose, G. W. Pickard and others. Crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy era. Sold and homemade by the millions, the inexpensive and reliable crystal radio was a major driving force in the introduction of radio to the public, contributing to the development of radio as an entertainment medium around 1920.
After about 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tubes (Audions), and became obsolete for commercial use. However they continued to be built by hobbyists, youth groups and the Boy Scouts as a way of learning about the technology of radio. Today they are still sold as educational devices, and there are groups of enthusiasts devoted to their construction who hold competitions comparing the performance of their home-built designs.
Crystal radios can be designed to receive almost any radio frequency band, but most receive the AM broadcast band. By the nature of their operation, crystal radios can only demodulate amplitude modulation (AM) signals, and not frequency modulation (FM) or digital signals.
See also: http://en.wikipedia.org/wiki/Crystal_radio
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