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The coherer effect refers to the operation of the coherer, an early type of radio signal detector invented by Edouard Branly and further developed by other scientists like Oliver Lodge and Guglielmo Marconi in the late 19th century. The coherer was crucial in the development of wireless telegraphy and early radio communication.

Construction: The coherer typically consists of a glass tube filled with loosely packed metal filings, usually made of nickel and silver, placed between two electrodes.

Operation: In its normal state, the metal filings have high resistance to electrical current. When exposed to radio frequency electromagnetic waves, the filings "cohere," or clump together, reducing their resistance significantly and allowing current to pass through more easily.

Detection: The reduced resistance allows the coherer to conduct electricity from a local battery, producing a signal that can be used to activate a relay or other indicating device. This action can be used to ring a bell or produce a mark on paper, indicating the reception of a radio signal.

Decohering: After detecting a signal, the filings need to be reset (decohered) to return to their high-resistance state. This is usually done by tapping or shaking the coherer, which separates the filings again.