Image: Hughes carbon rod microphone 1878

Description: Crude carbon rod microphone invented by David Edwards Hughes in 1878. It consists of a carbon rod (A) suspended by its pointed ends vertically between two metal electrodes (C). Sound vibrations transmitted from the wooden back (B) to the carbon-steel contact causes varying pressure on the carbon granules, causing variation in the carbon's resistance. Edwards found that the device was very sensitive, he was able hear the sound of a fly walking on it, and named it "microphone". 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 rod microphone 1878
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. 11 on Google Books
Author: Thomas Gray
Usage Terms: Public domain
License: Public domain
Attribution Required?: No