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.
Guglielmo Marconi patented a system of wireless telegraphy in the UK in 1896. That year, having demonstrated his system to the Navy, Army and representatives of the Post Office in trials on Salisbury Plain, he arranged a demonstration to accompany a public lecture on telegraphy by William Preece, chief engineer to the General Post Office. This was held in Toynbee Hall, the educational and charitable institution in London’s East End. Preece operated a transmitter and whenever he created an electric spark, a bell rang on this coherer box Marconi took to any part of the lecture room. There was no visible connection between the two. The demonstration caused a sensation and made Marconi a celebrity.