Image: Hughes carbon microphone

Description: Adjustable carbon rod microphone invented by David Edwards Hughes around 1878. It consists of a horizontal carbon rod (M) balanced on a fulcrum support (A) making contact lightly with a carbon block (C). Sound vibrations made the rod vibrate, putting varying pressure on the carbon granules at the contact point, causing variation in the resistance. When the two wires were connected in a circuit with a telephone receiver and a battery, the sounds could be heard from the receiver. The pressure at the contact point could be precisely adjusted by the thumbwheel and spring (S). Edwards did not invent the microphone but experimented with a wide range of "microphonic" electrical contacts. With another type of contact he discovered he could detect electromagnetic waves from a spark, thus inventing a type of coherer.
Title: Hughes carbon microphone
Credit: Retrieved April 8, 2015 from Thomas Gray, "Telephone", Encyclopaedia Britannica, 9th Edition, Charles Scribner's Sons, New York, 1888, Vol. 23, p. 132, fig. 12 on Google Books
Author: Thomas Gray
Usage Terms: Public domain
License: Public domain
Attribution Required?: No