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Nature's Miracles Volume 3: Electricity and Magnetism
Reading a book by Elisha Gray, a contender for being the inventor of the telephone. He mentioned the use of a coherer, a primitive detector of "ether waves," as they called them before Einstein. The coherer was built of two contacts with metal filing between. The resistance decreased after receiving a signal and stayed low until the detector was tapped to be reset. It is apparently not completely clear to this day how it worked, but probably when radio waves passed through the conductors it caused some kind of micro welding in the filings that made the tube more conductive.