Editing Telephone (section)

== Sound-powered telephones ==
[[Image:HMNZS Te Kaha sound-powered telephone 20080702.jpg|thumb|right|Modern [[emergency telephone]] powered by sound alone]]
{{Main|Sound-powered telephone}}
A sound-powered telephone is a telephone which transmits voice communication by wire, powered by the energy of the sound waves of the operator speaking.

=== Principle of operation ===
A moving-coil microphone converts the sound waves into an electrical signal, which is then converted back into sound waves at the receiver's end. Similar to early regular landline telephones, operators of sound-powered telephones generally alert the receiver of a call using a hand-cranked generator (magneto), which generates an electrical current which activates a buzzer at the receiver's end, sometimes known as a howler or growler. Some telephone systems can use external electrical power to operate ringers or amplifiers, but will revert to sound-powered communications in the event of failure of the external power supply.<ref name="NavElec">{{cite web |title=Products: Civil Vessels Telephone: Sound Powered Telephone |url=https://www.navelec.com/products/civil-vessels-telephone/sound-powered-telephone/ |website=NavElec |publisher=NavElec consortium |access-date=10 December 2024 |url-status=live |archive-date=23 April 2024 |archive-url=https://web.archive.org/web/20240423202316/https://navelec.com/products/civil-vessels-telephone/sound-powered-telephone/ }}</ref> Stations are usually connected via [[twisted pair]] wires to reduce electrical interference, and can be positioned at considerable distances from each other in the order of several kilometers. Using 1mm core diameter twisted-pair wiring, some sound-powered telephone systems can operate a pair of handsets positioned up to 48&nbsp;km (30 miles) apart.<ref name="Dynalec FAQ">{{cite web |title=Sound Powered Telephone: Frequently Asked Questions |url=http://soundpoweredtelephone.com/sound-powered-frequently-asked-questions.html |website=Sound Powered Telephone |publisher=Dynalec Corporation |access-date=10 December 2024 |archive-url=https://web.archive.org/web/20241210120847/http://soundpoweredtelephone.com/sound-powered-frequently-asked-questions.html |archive-date=2024-12-10 |date=2011 |url-status=live}}</ref>

=== Applications ===
[[Image:Sound powered telephone.jpg|thumb|A [[United States Navy|U.S. Navy]] [[petty officer]] uses a sound-powered telephone during a [[general quarters]] drill.]]
Because sound-powered telephones do not require external electrical power, they are used where reliable communications are vital even in event of loss of power. They are often used for communications in airports, railways and public utilities, mining, ski slopes, bridges, sporting arenas and shipyards. Because they operate at low voltages, they are suitable for use in situations where there is a risk of explosions or fire, such as chemical plants, oil and gas works, arsenals, mines and quarries.<ref name="Dynalec">{{cite web |title=How do sound powered telephones work? |url=http://soundpoweredtelephone.com/how-sound-powered-works.html |website=Sound Powered Telephone |publisher=Dynalec Corporation |access-date=10 December 2024 |archive-url=https://web.archive.org/web/20240519053547/http://soundpoweredtelephone.com/how-sound-powered-works.html |archive-date=2024-05-19 |date=2011 |url-status=live}}</ref> They are frequently used aboard ships, especially naval vessels, and in land military communications.

Aboard naval vessels, sound-powered telephones generally have auxiliary wiring circuits routed through the ship, to reduce the likelihood that all circuits will be rendered inoperable by battle damage.<ref name="USNI">{{cite web |last1=Cutler |first1=Thomas J. |title=The Reliability of Sound-Powered Phone Systems |url=https://www.usni.org/magazines/naval-history-magazine/2021/february/reliability-sound-powered-phone-systems |website=U.S. Naval Institute |date=February 2021 |access-date=10 December 2024 |archive-url=https://archive.today/20241210125546/https://www.usni.org/magazines/naval-history-magazine/2021/february/reliability-sound-powered-phone-systems |archive-date=2024-12-10 |url-status=live}}</ref>