Coherer

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. Invented around 1890 by French scientist Édouard Branly, it consists of a tube or capsule containing two electrodes spaced a small distance apart, with metal filings in the space between them. When a radio frequency signal is applied to the device, the initial high resistance of the filings reduces, allowing an electric current to flow through it. The coherer was a key enabling technology for radio, and was the first device used to detect radio signals in practical spark gap transmitter wireless telegraphy. It became the basis for radio reception around 1900, and remained in widespread use for about ten years. It was superseded by more sensitive electrolytic and crystal detectors and became obsolete, although in the 1950s a coherer was briefly used in least one radio-controlled toy.

There are two basic kinds of coherers: the original metal filings type, called the Branly coherer; and a later imperfect junction type.