Editing Microphone (section)

=== Carbon ===
{{Main|Carbon microphone}}
[[File:Western Electric double button carbon microphone.jpg|thumb|upright|[[Western Electric]] double button carbon microphone]]

The carbon microphone was the earliest type of microphone. The carbon button microphone (also known as the Berliner or Edison microphone) uses a capsule or button containing carbon granules pressed between two metal plates. A voltage is applied across the metal plates, causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. The changes in resistance cause a [[Ohm's law|corresponding change in the current]] flowing through the microphone, producing the electrical signal. Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range but are very robust devices. The Boudet microphone, which used relatively large carbon balls, was similar to the granule carbon button microphones.<ref>{{cite web |title=Boudet's Microphone |url=http://www.machine-history.com/Boudet%20Microphone |website=Machine-History.com |archive-url=https://web.archive.org/web/20150822100052/http://www.machine-history.com/Boudet%20Microphone |archive-date=August 22, 2015 }}</ref>

Unlike other microphone types, the carbon microphone can also be used as a type of amplifier, using a small amount of sound energy to control a larger amount of electrical energy. Carbon microphones found use as early [[telephone repeater]]s, making long-distance phone calls possible in the era before vacuum tubes. Called a Brown's relay,<ref>{{cite web |title=Brown Type G Telephone Relay Owned by Edwin Howard Armstrong |url=https://americanhistory.si.edu/collections/search/object/nmah_890933 |website=National Museum of American History |access-date=June 15, 2022 }}</ref> these repeaters worked by mechanically coupling a magnetic telephone receiver to a carbon microphone: the faint signal from the receiver was transferred to the microphone, where it modulated a stronger electric current, producing a stronger electrical signal to send down the line.