A coherer is a type of radio wave detector and was one of the earliest such devices developed. It first came into use in the late 19th century and is believed to have been invented by French scientist Edouard Branly around 1890. Its development was an important step in wireless telegraphy and radio communications. A typical coherer was a small glass tube, sometimes with the air removed, partially filled with shavings or irregular granules of conductive material, often nickel, silver, or a combination of the two.
The principle of the operation of a coherer involves the conductivity of the material within the tube under differing conditions. Under normal conditions, the resistance of the metal granules or shavings in the tube is fairly high due to the imperfect contact between the various small bits of material. When exposed to radio waves, however, the conductivity of these particles, as a whole, goes up, which means the resistance is correspondingly reduced. By installing the coherer in an electrical circuit capable of measuring the differences in conductivity between the rest state and the radio wave stimulation state, it becomes possible to detect the presence of radio waves.
The implications for such an advancement were immediately apparent, and the coherer and its applications were researched and developed by a number of famous pioneers in the field of radio and electronics, such as Marconi and Tesla among others. The first and most significant application for the device was in wireless telegraphy. The pulses of incoming radio energy could be converted by the coherer into an output in Morse code form by a system of actual sounds or very simple printouts on paper which could then be converted into readable text by a telegrapher skilled in Morse code.
It was quickly discovered that the particles in the coherer, once exposed to radio waves, did not revert to their original state when the radio pulse stopped. For this reason, wireless telegraphs using this device were designed with a second device called a decoherer, which automatically reset the coherer tube after every pulse by mechanically impacting it to dislodge the particles. This allowed for rapid and automatic transmission of Morse code messages without wires.
For more than a century after its invention, it was not known precisely why a coherer works the way it does. A prevailing theory purported that the presence of the radio waves, a form of electromagnetic radiation, caused welding between the particles on a molecular level, increasing conductivity. This was an example of scientists being unable to quantitatively prove the exact mechanism behind an observed phenomena but applying its function anyway. Experiments by modern researchers have shown that this explanation is most likely correct.