Sound-powered telephone

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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^[1] for both routine and emergency communication on ships to allow communication between key locations on a vessel if power is unavailable.^[2] A sound-powered phone circuit can have two or more stations on the same circuit. The circuit is always live, thus a user begins speaking rather than dialing another station. Sound-powered telephones are not normally connected to a telephone exchange.