What is "coherer"

Coherer \Co*her"er\, n. (Elec.) Any device in which an imperfectly conducting contact between pieces of metal or other conductors loosely resting against each other is materially improved in conductivity by the influence of Hertzian waves; -- so called by Sir O. J. Lodge in 1894 on the assumption that the impact of the electic waves caused the loosely connected parts to cohere, or weld together, a condition easily destroyed by tapping. A common form of coherer as used in wireless telegraphy consists of a tube containing filings (usually a pinch of nickel and silver filings in equal parts) between terminal wires or plugs (called

conductor plugs).

n. (context physics English) A detector of radio waves used in very early radio receivers.

The coherer is a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the 20th century. Its use in radio was based on the 1890 findings of French physicist Edouard Branly and adapted by other physicists and inventors over the next ten years. The device consists of a tube or capsule containing two electrodes spaced a small distance apart with loose metal filings in the space between. When a radio frequency signal is applied to the device, the metal particles would cling together or "cohere", reducing the initial high resistance of the device, thereby allowing a much greater direct current to flow through it. In a receiver, the current would activate a bell, or a Morse paper tape recorder to make a record of the received signal. The metal filings in the coherer remained conductive after the signal (pulse) ended so that the coherer had to be "decohered" by tapping it with a clapper, such as a doorbell ringer, each time a signal was received, thereby restoring the coherer to its original state. Coherers remained in widespread use until about 1907, when they were replaced by more sensitive electrolytic and crystal detectors.