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Sound-powered telephone
A sound-powered telephone is a communication device that allows users to talk to each other with the use of a handset, similar to a conventional telephone, but without the use of external power. This technology has been used since at least 1944〔https://books.google.com/books?id=aiEDAAAAMBAJ&pg=PA124&hl=en#v=onepage&q&f=false〕 for both routine and emergency communication on ships to allow communication between key locations on a vessel even if power, including batteries, is no longer available. A sound-powered phone circuit can have two or more stations on the same circuit. The circuit is always live, thus a user simply begins speaking rather than dialing another station. Sound-powered telephones are not normally connected to a telephone exchange.
== Operation ==
The microphone transducer converts sound pressure from a user's voice into a minute electric current, which is then converted back to sound by a transducer at the other end. The most significant distinction between ordinary telephones and sound-powered telephones is in the operation of the microphone. Since the microphones used in most telephones are designed to modulate a supplied electric current they cannot be used in sound-powered transducers. Rather, most sound-powered telephones use a dynamic microphone. A common approach to transducer design is the balanced armature design because of its efficiency. The number of simultaneous listeners is limited because there is no amplification of the signal.
A sound-powered telephone circuit can be as simple as two handsets connected together with a pair of wires, which is defined as the "talk" portion of the circuit. Talk circuits can be realized over a pair of wires that are 50 km (30 miles) long. More complex circuits include magnetos, selector switches and bells to allow one user to select and call another, which is defined as the "calling" portion of the circuit. The voice communication ("talk") circuit is completely separate from the "call" circuit, allowing communication to take place without external power.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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