File:Tapping coherer.jpg

English: Drawing of a Branly coherer radio detector with a "tapper" or decoherer mechanism. Coherers were used in the first radio receivers during the wireless telegraphy era from about 1900 to 1910. The Branly coherer was a tube with two silver "plugs" or electrodes, with loose metal filings in the space between. When a radio signal from an antenna was applied across the electrodes, it caused the resistance of the filings to decrease, making the device conductive. The coherer was also connected in a DC circuit powered by a battery, with an earphone or a Morse paper tape recorder. When the coherer turned "on", it made a sound in the earphone or a mark on a paper tape, recording the code symbol received. However, the coherer remained in the conductive state after the radio signal disappeared. To prepare it to receive the next radio signal, the filings had to be mechanically disturbed to return the device to the nonconductive "off" state. This was done by a "tapper" or decoherer consisting of an electromagnet connected in the DC circuit. When the coherer turned on, the electromagnet attracted the arm with the metal ball to tap the coherer tube, returning the coherer to the nonconductive state.