Editing Thomas Edison (section)

===Carbon telephone transmitter===
In 1876, Edison began work to improve the [[microphone]] for telephones (at that time called a "transmitter") by developing a [[carbon microphone]], which consists of two metal plates separated by granules of carbon that would change resistance with the pressure of sound waves. A steady direct current is passed between the plates through the granules and the varying resistance results in a modulation of the current, creating a varying electric current that reproduces the varying pressure of the sound wave.

Up to that point, microphones, such as the ones developed by [[Johann Philipp Reis]] and [[Alexander Graham Bell]], worked by generating a weak current. The [[carbon microphone]] works by modulating a direct current and, subsequently, using a transformer to transfer the signal so generated to the telephone line. Edison was one of many inventors working on the problem of creating a usable microphone for telephony by having it modulate an electric current passed through it.<ref name="Adrian Hope 1102, page 378">Hope, Adrian (May 11, 1978), "100 Years of Microphone", ''New Scientist'', Vol. 78, No. 1102, p. 378. {{ISSN|0262-4079}}.</ref> His work was concurrent with [[Emile Berliner]]'s loose-contact carbon transmitter (who lost a later patent case against Edison over the carbon transmitter's invention<ref name="IEEE">''IEEE Global History Network: Carbon Transmitter''. New Brunswick, NJ: IEEE History Center {{cite web|url=http://www.ieeeghn.org/wiki/index.php/Carbon_Transmitter |title=Carbon Transmitter |access-date=January 14, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100318043500/http://www.ieeeghn.org/wiki/index.php/Carbon_Transmitter |archive-date=March 18, 2010 }}</ref>) and [[David Edward Hughes]]' study and published paper on the physics of loose-contact carbon transmitters (work that Hughes did not bother to patent).<ref name="Adrian Hope 1102, page 378"/><ref>Worrall, Dan M. (2007), {{Cite web|url=http://www.angloconcertina.org/files/HughesforWebsite.pdf |access-date=December 17, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20160913132543/http://www.angloconcertina.org/files/HughesforWebsite.pdf |title=David Edward Hughes: Concertinist and Inventor |archive-date=September 13, 2016 }}</ref>

Edison used the carbon microphone concept in 1877 to create an improved telephone for [[Western Union]].<ref name="IEEE"/> In 1886, Edison found a way to improve a [[Bell Telephone Company|Bell Telephone]] microphone, one that used loose-contact ground carbon, with his discovery that it worked far better if the carbon was [[roasted]]. This type was put in use in 1890<ref name="IEEE"/> and was used in all telephones along with the Bell receiver until the 1980s.