Marconi coherer and Admiralty pattern decoherer, unknown maker, British, 1900-1910

The coherer was the first practical device for detecting Hertzian waves. Its most familiar form was a tube containing metal filings or turnings loosely packed between metal plugs. This presented a high resistance to current but changed to low resistance in the presence of electric waves because the filings 'cohered' or stuck together. In order to restore the high-resistance state the tube needed to be given a mechanical tap or jolt. Filings coherers were fitted with clockwork or other arrangements for driving a 'tapper' to provide continual mechanical agitation. Marconi brought the coherer to its most efficient form and it was used with receiving sets in wireless telegraphy equipment used by the Royal Navy.