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A coherer (or, sometimes, receiver) was an early form of detector in wireless telegraphy, based around the effect that small particles of metal filings stick together (or 'cohere') when an electric field is present. A coherer circuit consisted of a basic electromagnetic wave detector for various wavelengths and a circuit that obtained signals from modulated radio waves. The coherer then 'decoded' these signals.

A coherer consisted of a glass tube having very fine metallic filings enclosed between two plugs (usually silver) forming a V gap, and is attached to a stem (usually ivory or bone) with a square end. The coherer is usually situated about one-sixteenth of an inch from the de-coherence tapper hammer. On receiving a wireless signal, the metal filings in the coherer become conductive, permitting a weak current to pass through the coherer and the telegraph relay. This brings into action the Morse recorder, which registers a dot or dash as the case may be. The tapper then taps the coherer, causing the filings to de-cohere within the coherer and thus it becomes non-conductive again. The tapper then ceases to function and the coherer is ready for the next wireless signal.

A basic coherer for 'Hertzian waves' (wireless telegraphy) was first developed by a French physicist, Edouard Branly, and was later developed by Oliver Lodge and others. Marconi used a Branly-type coherer is his early wireless telegraphy experiments in Italy.

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