Why did a coherer need to be tapped after receiving a radio signal?

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Guglielmo Marconi Italian physicist Guglielmo Marconi, shown here about 1908, invented a successful wireless telegraph (that is, a radio) in 1896.

A coherer was a device used in early radio technology to detect electromagnetic waves. It consisted of a tube filled with metal filings that would coalesce, or "cohere," when a radio-frequency voltage was applied to the ends of the tube.1

This cohesion allowed an electrical current to pass through the device, which in turn triggered a relay to reproduce Morse signals. Because the filings remained stuck together after the signal passed, the coherer had to be mechanically tapped to separate them. This action reset the device, effectively "de-cohering" the filings so they could return to their original state and react to the next incoming radio signal.1