English: A coherer, a crude radio detector used in the first radio receivers from about 1895 to 1906 during the wireless telegraphy era. This example was built by radio researcher Emile Guarini Foresio. It is a glass tube (right) containing metal powder between two electrodes. When a radio signal is applied between the electrodes, the metal powder coheres (clumps together) causing the tube to conduct electricity. The coherer is attached to a DC circuit (not shown) which makes a mark on a paper tape to record the Morse code radio message. Because the coherer remained in its conducting state after the radio wave stopped, the device includes a "decoherer" or "tapper" (left) which uses an electromagnet-activated arm to tap the coherer, agitating the powder, to return the device to its nonconducting state in preparation to receive another radio signal.