section_path: Menlo Park laboratory (1876–1886)/Carbon telephone transmitter
section_name: Carbon telephone transmitter
paragraph_text: 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. 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) and David Edward Hughes ’ study and published paper on the physics of loose-contact carbon transmitters (work that Hughes did not bother to patent).
  
  Edison used the carbon microphone concept in 1877 to create an improved telephone for Western Union. In 1886, Edison found a way to improve a 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 and was used in all telephones along with the Bell receiver until the 1980s.